{"count":5068,"next":"http://www.astro4edu.org/oae-api/glossary-terms/?page=3","previous":"http://www.astro4edu.org/oae-api/glossary-terms/","results":[{"term_name":"Escape Velocity","term_definition":"Escape velocity is the minimum speed that an object must have to permanently escape from the gravitational field of a celestial body. This is a scalar quantity so should more correctly be referred to as escape speed; however, the term escape velocity is commonly used. The simplest case is that of a body that is spherically symmetric – which is an excellent approximation for describing stars and planets. In this case, the escape velocity at a distance r from the center of a body of mass m, is given by √(2Gm/r), where G is the gravitational constant. At the surface of a spherical body the distance from its center is its radius. This means that the escape velocity at the surface of an approximately spherical celestial body depends on the radius and its mass. In the case of the Sun, it is 617.5 kilometers per second (km/s), and for Earth, 11.2 km/s. It is 2.4 km/s for the Moon, which means that an object on the Moon needs to attain a lower speed to escape the Moon's gravity than an object on Earth would need to leave Earth's gravity.","term_approval_level":"A","language_code":"en","term_number":105,"term_in_english":"Escape Velocity","based_on_current_english_version":null,"linked_terms":[135],"alternate_terms":[],"categories":["Space Exploration"],"category_ids":[10],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/105/"},{"term_name":"Exoplanet","term_definition":"An exoplanet, or extrasolar planet, is a planet located outside the Solar System. Their existence was theorized from the 16th century and observational research was started in the 19th century to find them. The first confirmed exoplanets were discovered in the 1990s. Of these, the first confirmed to be orbiting around a star on the main sequence was the exoplanet Dimidium, indirectly discovered at the Haute-Provence Observatory. This exoplanet is orbiting the star 51 Pegasi, a yellow subgiant and was discovered in 1995. Since then thousands of exoplanets have been identified.","term_approval_level":"A","language_code":"en","term_number":106,"term_in_english":"Exoplanet","based_on_current_english_version":null,"linked_terms":[253,331],"alternate_terms":[],"categories":["Chemistry","Exoplanets & Astrobiology"],"category_ids":[12,6],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/106/"},{"term_name":"Extinction","term_definition":"The reduction in intensity of starlight reaching us is called extinction. This reduction is due to absorption and scattering of light by particles along the path of light. Extinction could be due to Earth's atmosphere (called atmospheric extinction), material in the immediate vicinity of a star (called circumstellar extinction), or due to material between stars in deep space (called interstellar extinction). The atmospheric extinction is mainly due to aerosols and molecules present in Earth's atmosphere such as water, carbon dioxide, and ozone at optical and near-infrared wavelengths. Interstellar extinction is attributed to interstellar matter made up of gas and submicron-sized dust particles. Interstellar dust has a drastic effect on starlight as compared to the gas particles. Extinction is generally higher at shorter wavelengths and vice-versa making astronomical objects appear redder than their true color (reddening).","term_approval_level":"A","language_code":"en","term_number":107,"term_in_english":"Extinction","based_on_current_english_version":null,"linked_terms":[30,85,472],"alternate_terms":[],"categories":["Milky Way and Interstellar Medium"],"category_ids":[7],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/107/"},{"term_name":"Extraterrestrial Intelligence","term_definition":"Extraterrestrial intelligence refers to the possible existence of intelligent entities other than those on Earth.\r\n\r\nSearches for extraterrestrial intelligence have included looking for radio or other signals, but much current research is centered on determining whether the conditions for intelligent life are present elsewhere. Going by the available evidence, the necessary conditions for the emergence of life on Earth – a solid planet, at a distance from its star where liquid water can exist on the planet's surface, atoms like carbon and oxygen, and the conditions to form more complex molecules – should exist on numerous other planets within our Galaxy and beyond. It is not an unreasonable assumption that intelligent life might have arisen at least on some of those planets, although we have no solid basis for estimating the probability of that happening. \r\n\r\nSome astronomers have used radio signals to search for putative messages as part of a Search for Extraterrestrial Intelligence (SETI). The speed of light as a fundamental physical limit means that interstellar journeys would take very long times, limiting the ability for us to come into direct contact with extraterrestrial intelligences.","term_approval_level":"A","language_code":"en","term_number":108,"term_in_english":"Extraterrestrial Intelligence","based_on_current_english_version":null,"linked_terms":[20,109],"alternate_terms":[],"categories":["Exoplanets & Astrobiology"],"category_ids":[6],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/108/"},{"term_name":"Extraterrestrial Life","term_definition":"Extraterrestrial life or alien life refers to forms of life that may occur outside Earth and which did not originate on Earth. \r\n\r\nAlthough not even simple forms of extraterrestrial life have been detected to date, it is likely that life has similarly arisen on some of the billions and billions of exoplanets in our galaxy. Astronomers estimate that, on average, each star in our galaxy has more than one planet. A number of these exoplanets are expected to offer conditions similar to those that allowed life to arise here on Earth, in particular liquid water.  \r\n\r\nJust as on Earth, the presence of certain kinds of life is expected to significantly alter the chemical composition of such a planet's atmosphere, and a major goal of astronomy for the coming decades is to detect the presence of life by studying distant planetary atmospheres. The  Search for Extraterrestrial Intelligence (SETI) project, on the other hand, has long been searching for possible radio signals from intelligent life on other worlds. In our own Solar System, conditions in the ice-covered oceans of Jupiter's moon Europa, and Saturn's moon Enceladus, might allow for life.","term_approval_level":"A","language_code":"en","term_number":109,"term_in_english":"Extraterrestrial Life","based_on_current_english_version":null,"linked_terms":[20,108],"alternate_terms":[],"categories":["Chemistry","Exoplanets & Astrobiology"],"category_ids":[12,6],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/109/"},{"term_name":"F-type Star","term_definition":"A star with spectral type \"F\". Astronomers identify F-type stars by the presence of moderately strong ionized calcium lines and some other atomic metal lines and the weak hydrogen absorption lines in their spectra. They have typical (effective) temperatures between around 6000 kelvins (K) and 7400 K. Compared to other stars, they appear white or yellowish white to human eyes unless interstellar or atmospheric reddening is important. Polaris (the North Star) is an example of an F-type star.","term_approval_level":"A","language_code":"en","term_number":110,"term_in_english":"F-type Star","based_on_current_english_version":null,"linked_terms":[325,331,396,440,514],"alternate_terms":["F star","F-star"],"categories":["Stars"],"category_ids":[2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/110/"},{"term_name":"Frequency","term_definition":"Frequency is the number of oscillations per unit of time, as in cycles/second (or hertz [Hz]). It is a general property of any wave – sound wave, light, or gravitational waves. The frequency and wavelength of a wave are related by the formula frequency = v/wavelength where v is the speed of the wave. When discussing electromagnetic radiation astronomers will generally use either frequency or wavelength (sometimes interchangeably).","term_approval_level":"A","language_code":"en","term_number":112,"term_in_english":"Frequency","based_on_current_english_version":null,"linked_terms":[96,381],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/112/"},{"term_name":"Full Moon","term_definition":"","term_approval_level":"N","language_code":"en","term_number":113,"term_in_english":"Full Moon","based_on_current_english_version":null,"linked_terms":[],"alternate_terms":[],"override_term_number":182,"categories":[],"category_ids":[],"override_url":"https://astro4edu.org/resources/glossary/term/182/","url":"https://astro4edu.org/resources/glossary/term/113/"},{"term_name":"Fusion","term_definition":"","term_approval_level":"A","language_code":"en","term_number":114,"term_in_english":"Fusion","based_on_current_english_version":null,"linked_terms":[],"alternate_terms":[],"override_term_number":221,"categories":["Stars"],"category_ids":[2],"override_url":"https://astro4edu.org/resources/glossary/term/221/","url":"https://astro4edu.org/resources/glossary/term/114/"},{"term_name":"Galactic Bulge","term_definition":"In the Milky Way the galactic bulge is the region around the galactic center, where stars are arranged in a less-flattened volume than in the surrounding disk-like region. The Milky Way's bulge component contains mostly old stars but with an admixture of young stars. Other spiral galaxies contain central bulges of diverse extent.","term_approval_level":"A","language_code":"en","term_number":115,"term_in_english":"Galactic Bulge","based_on_current_english_version":null,"linked_terms":[116,117,118,119],"alternate_terms":[],"categories":["Galaxies","Milky Way and Interstellar Medium"],"category_ids":[8,7],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/115/"},{"term_name":"Galactic Center","term_definition":"The galactic center is the central region of the Milky Way (the galaxy in which the Solar System is located), the region that the Milky Way disk rotates around. The galactic center is part of the galactic bulge and is around 27,000 light years (8 kiloparsecs) from the Solar System, compared to the diameter of the galactic disk of about 100,00 light years (roughly 31 kiloparsecs). It contains the radio source Sagittarius A and the compact radio source Sagittarius A* which is the supermassive black hole at the heart of our Galaxy. This black hole, which has a mass of approximately 4.5 million solar masses, is orbited by several young, massive stars. All this is surrounded by millions of older stars making up the so-called nuclear star cluster.","term_approval_level":"A","language_code":"en","term_number":116,"term_in_english":"Galactic Center","based_on_current_english_version":null,"linked_terms":[115,117,118,119,292,314],"alternate_terms":[],"categories":["Galaxies","Milky Way and Interstellar Medium"],"category_ids":[8,7],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/116/"},{"term_name":"Galactic Disk","term_definition":"The galactic disk is the disk component of the Milky Way that contains stars, gas, and dust in circular coplanar motion around the galactic center. The galactic disk is very thin compared to its diameter of about 100,000 light years. It is sometimes divided into two components: the thin disk which is about 1000 light years thick and the thick disk about 5000 light years thick. Whether the thick disk is a separate component of the galaxy or an extension of the thin disk is a matter of debate amongst astronomers. The thin disk has four spiral arms where the rate of star formation is relatively high. The disk is surrounded by a large galactic halo. \r\n\r\nAlthough the Milky Way is well studied its precise structure is still a matter of some debate, particularly near the core where the large density of stars and extinction due to interstellar material makes study difficult.\r\n\r\nMany other galaxies, including spiral and lenticular galaxies, have their own galactic disks.","term_approval_level":"A","language_code":"en","term_number":117,"term_in_english":"Galactic Disk","based_on_current_english_version":null,"linked_terms":[115,116,118,119,178,336],"alternate_terms":[],"categories":["Galaxies","Milky Way and Interstellar Medium"],"category_ids":[8,7],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/117/"},{"term_name":"Galactic Halo","term_definition":"The galactic halo is a roughly spherical distribution of stars, gas, and dark matter which extends above, below, and beyond the disk of the Milky Way. Stars in the halo are older and have a lower metal content than most of the stars in the disk of the Milky Way. In the region around the Sun only a few percent of the stars are from the halo. Globular clusters of stars are also found in the galactic halo.\n\nAn invisible halo of dark matter also extends through and around the Milky Way, containing most of the Galaxy's mass. \n\nMost other galaxies also have haloes.","term_approval_level":"A","language_code":"en","term_number":118,"term_in_english":"Galactic Halo","based_on_current_english_version":null,"linked_terms":[115,116,117,119,132,336],"alternate_terms":[],"categories":["Galaxies","Milky Way and Interstellar Medium"],"category_ids":[8,7],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/118/"},{"term_name":"Galaxy","term_definition":"A galaxy is a system of stars and other material components such as dark matter, gas, and dust that is gravitationally bound, and usually separated from its neighbors by hundreds of thousands of light years. Galaxies come in various different shapes and sizes. The smallest galaxies can have a few thousand stars, while the largest can have tens of trillions. \"The Galaxy\" or \"Galaxy\" with a capital letter usually refers to our home galaxy, the Milky Way, which has around 100–400 billion stars.","term_approval_level":"A","language_code":"en","term_number":119,"term_in_english":"Galaxy","based_on_current_english_version":null,"linked_terms":[76,82,85,86,99,115,116,117,118,120,124,138,164,199,330,331,443,452],"alternate_terms":[],"categories":["Galaxies","Milky Way and Interstellar Medium"],"category_ids":[8,7],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/119/"},{"term_name":"Galaxy Cluster","term_definition":"A galaxy cluster is a physical group of galaxies that are gravitationally bound. Galaxy clusters can vary in size and concentration, containing anywhere from hundreds to thousands of galaxies. The Virgo Cluster, which is the nearest galaxy cluster is an example of a large cluster containing thousands of galaxies. In addition to galaxies, clusters also contain plasma and large amounts of dark matter.","term_approval_level":"A","language_code":"en","term_number":120,"term_in_english":"Galaxy Cluster","based_on_current_english_version":null,"linked_terms":[76,119,258],"alternate_terms":["Cluster of Galaxies"],"categories":["Galaxies"],"category_ids":[8],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/120/"},{"term_name":"Galilean Satellites","term_definition":"The Galilean satellites are the four biggest and brightest satellites, or moons, orbiting the planet Jupiter: Io, Europa, Ganymede, and Callisto. Closest to Jupiter is Io, which has hundreds of active volcanoes. The main reason for Io's vulcanism is that Io is getting \"kneaded\" by the tidal effects of Jupiter's gravity. Europa's entire surface is covered with ice. Under the ice is believed to be an ocean of liquid water which is one of the best candidates for harboring life outside of Earth in the Solar System. Ganymede is the largest moon in our Solar System, and also the moon with the largest mass, at twice the mass of Earth's Moon. Callisto is almost exactly the same size as the planet Mercury, but has only about one third of Mercury's mass. \r\n\r\nThe four Galilean moons were discovered by Galileo Galilei in 1610, as one of several discoveries in Galilei's pioneering campaign of using a telescope for astronomical observations. Galilei was able to document that, over time, the four moons orbit Jupiter like a miniature solar system. This was a clear demonstration that astronomical objects can orbit a center other than the Earth; this observation played an important role in the shifting of the scientific consensus from the Earth-centered (geocentric) to the Sun-centered (heliocentric) view of the Solar System.","term_approval_level":"A","language_code":"en","term_number":121,"term_in_english":"Galilean Satellites","based_on_current_english_version":null,"linked_terms":[159,167,204,484],"alternate_terms":[],"categories":["Solar System"],"category_ids":[1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/121/"},{"term_name":"Galilean Telescope","term_definition":"In a refracting telescope, light first encounters a convex lens (converging lens), called the objective lens, which serves to bundle infalling parallel light rays. Such almost perfectly parallel light rays correspond to light we receive from a distant object, such as a star. In order to produce an image that can be observed by eye, those converging rays must be made parallel again. This is the task of an additional optical element: the eyepiece, which is where you put your eye if you observe through a telescope.\r\n\r\nIn a Galilean telescope, named after the model of telescope built by Galileo Galilei in 1609 and used for some of the first systematic astronomical telescope observations, this is achieved by inserting a concave lens (diverging lens) as the eyepiece.\r\n\r\nIn contrast, in a Keplerian telescope, invented by Johannes Kepler in 1611, the converging light rays are allowed to cross, and the resulting divergent light rays are then made parallel using a second convex lens. Compared to a Keplerian telescope, a Galilean telescope provides a visual image that is upright (not inverted), but it has a much narrower field of view than a Keplerian telescope. \r\n\r\nThe wider field of view is why nearly all modern refracting telescopes used by amateur astronomers are Keplerian in design – in the case of particularly high-quality telescopes, Keplerian with additional lenses providing improved image quality. For professional astronomy, the distinction between Keplerian and Galilean is largely irrelevant: professional observations use cameras instead of eye pieces, and most professional telescopes are reflective (mirror) telescopes, not refracting telescopes.","term_approval_level":"A","language_code":"en","term_number":122,"term_in_english":"Galilean Telescope","based_on_current_english_version":null,"linked_terms":[281,451],"alternate_terms":[],"categories":["Telescopes, Instruments and Observatories"],"category_ids":[3],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/122/"},{"term_name":"Gamma Ray","term_definition":"Gamma ray photons are the most energetic photons in the electromagnetic spectrum, expressed by the Greek letter \"γ\". Gamma ray photons generally carry an energy greater than 100 kiloelectronvolts, greater than 50,000 times more energy than photons of visible light, and have frequencies of about 3x10¹⁹ hertz or greater, and wavelengths less than 10 picometers (1 picometer is 10⁻¹² m).\r\n\r\nGamma rays are emitted by the nuclei of some radionuclides after radioactive decay. In astronomy, gamma rays are emitted by the most extreme supernovae as gamma ray bursts, by active galactic nuclei such as blazars, and by solar flares.\r\n\r\nGamma rays emitted by astronomical sources  do not reach the Earth's surface. Therefore, to study gamma rays, it is necessary to place detectors  above the Earth's atmosphere.","term_approval_level":"A","language_code":"en","term_number":123,"term_in_english":"Gamma Ray","based_on_current_english_version":null,"linked_terms":[5,96,311,371,444],"alternate_terms":["gamma radiation"],"categories":["Stars","Telescopes, Instruments and Observatories"],"category_ids":[2,3],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/123/"},{"term_name":"Gas","term_definition":"In physics, gas is a state of matter where molecules or atoms are loosely bound to each other so as to allow for constant, chaotic motion, with atoms and molecules going every which way at different speeds. The air we breathe is a mixture of gases such as molecular nitrogen and oxygen. The mean energy of motion is a measure of the temperature of the gas. The main effect responsible for a gas's pressure (exerted, for instance, upon the walls of a container) is gas particles hitting those walls and recoiling. In astronomy, one may encounter gases as the constituents of interstellar gas clouds, giant molecular clouds, intergalactic gas, or the gas of a planetary atmosphere. In a slight misuse of language, astronomers also refer to plasma (where the atoms in question are ionized) as gas – for example, when they call stars \"balls of gas,\" or refer to gas swirling in an accretion disk.","term_approval_level":"A","language_code":"en","term_number":124,"term_in_english":"Gas","based_on_current_english_version":null,"linked_terms":[426],"alternate_terms":[],"categories":["Milky Way and Interstellar Medium"],"category_ids":[7],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/124/"},{"term_name":"Gas Giant","term_definition":"A gas giant is a giant planet mostly composed of hydrogen and helium which are gases in interstellar and interplanetary space, hence the name. However, most of the hydrogen and helium in gas giants is actually in a liquid state.\r\n\r\nGas giants are thought to have rocky cores which are surrounded by thick layers of hydrogen and helium. In the deepest parts of the planet these gases are compressed into liquid form with the deepest layers thought to contain an ocean of metallic hydrogen. In the outer layers the hydrogen and helium are in gas form. Other elements in the atmosphere can form clouds and rain. In the coolest gas giants the clouds in the upper layer can be made of water or ammonia vapor. In deeper, hotter layers of cooler gas giants and in the outer layers of hotter gas giants the clouds can be made of iron and minerals which are solid at room temperature. \r\n\r\nThe two largest planets in the Solar System, Jupiter and Saturn, are gas giants.","term_approval_level":"A","language_code":"en","term_number":125,"term_in_english":"Gas Giant","based_on_current_english_version":null,"linked_terms":[129,167,294],"alternate_terms":[],"categories":["Exoplanets & Astrobiology","Naked Eye Astronomy","Solar System"],"category_ids":[6,4,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/125/"},{"term_name":"Gemini","term_definition":"Gemini is one of the 13 constellations of the Zodiac and one of the 88 modern constellations as accepted by the International Astronomical Union, but goes back much further – it was already one of the 48 constellations named by the 2nd century astronomer Claudius Ptolemy. The constellation gets its name from its two brightest stars Castor and Pollux, who in Babylonian mythology were twins and minor gods. Gemini is visible in the northern hemisphere during winter, located between the constellations Taurus and Cancer. Various cultures around the world have their own stories associated with this constellation and its stars. Castor and Pollux are located approximately 50 and 30 light years from Earth, respectively. About 80 stars may be seen in Gemini with the naked eye. Some notable deep-sky objects located within the region of this constellation are M35, NGC 2158, NGC 2392, and Abell 21.","term_approval_level":"A","language_code":"en","term_number":126,"term_in_english":"Gemini","based_on_current_english_version":null,"linked_terms":[66,138,158,391],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/126/"},{"term_name":"Geocentric Model","term_definition":"Early models of the Universe were geocentric, placing Earth at the center of the cosmos, with the Moon, Sun, and planets moving around it relative to the \"fixed\" stars. Geocentric models existed in many cultures in antiquity alongside heliocentric models which put the Sun in the center. An influential geocentric model is the Ptolemaic system, named after Claudius Ptolemy, an astronomer from the 2nd century. This became the most prominent model of the cosmos for over a thousand years in Europe, North Africa, and the Middle East. Late in the 16th century a shift began towards a heliocentric model, which is commonly associated with the name of Nicolaus Copernicus. Today, we know that the Solar System is only one of many such systems, and certainly not the center of the Universe. In practice, geocentric descriptions of the sky are still in use, but only as a way of calculating which astronomical objects are visible from a given location at a given time.","term_approval_level":"A","language_code":"en","term_number":127,"term_in_english":"Geocentric Model","based_on_current_english_version":null,"linked_terms":[141],"alternate_terms":[],"categories":["Astronomy and Society","Solar System"],"category_ids":[11,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/127/"},{"term_name":"Geomagnetic Storm","term_definition":"In addition to the Sun's output of electromagnetic radiation, there is a steady flow of charged particles leaving the Sun, known as solar wind. Certain types of solar activity – solar flares, and the more dramatic coronal mass ejections – can suddenly and drastically increase the amount of charged particles leaving the Sun, creating a shock front within the solar wind, travelling outwards. If parts of that shock front reach our home planet they interact with Earth's magnetic field, creating a geomagnetic storm (sometimes also called a solar storm). The consequences range from the harmless – increased and more beautiful polar lights (aurorae) – to harmful interactions that can damage satellites, disturb broadcasts, and in extreme cases disrupt electric power grids.","term_approval_level":"A","language_code":"en","term_number":128,"term_in_english":"Geomagnetic Storm","based_on_current_english_version":null,"linked_terms":[33,184,241,311,315,323,455],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/128/"},{"term_name":"Giant Planet","term_definition":"A giant planet is a large body mostly composed of hydrogen, helium, or more complex molecules such as water, methane, or ammonia. While a terrestrial planet is mostly composed of material with a very high boiling point such as iron or rock, giant planets are thought to have a solid core surrounded by other material. The mass of a giant planet is substantially higher than that of Earth, so its gravity is strong enough to retain the extended gaseous atmosphere made up of light elements like hydrogen and helium. \r\n\r\nGiant planets fall into two categories: gas giants which are mostly made up of hydrogen and helium, and ice giants which are mostly made up of water, methane, and ammonia surrounded by an atmosphere of hydrogen and helium. In both cases the names can be confusing as most of the material in gas giants is not in a gaseous state and ice giants do not contain solid ice, rather material that was frozen in the cold outer Solar System before it was accreted onto the planet.\r\n\r\nThe four largest planets in the Solar System (Jupiter, Saturn, Neptune, and Uranus) are all giant planets.","term_approval_level":"A","language_code":"en","term_number":129,"term_in_english":"Giant Planet","based_on_current_english_version":null,"linked_terms":[125,153,167,212,253,294,375,425],"alternate_terms":[],"categories":["Exoplanets & Astrobiology","Naked Eye Astronomy","Solar System"],"category_ids":[6,4,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/129/"},{"term_name":"Giant Star","term_definition":"Giant stars, giants for short, are stars that are unusually large and luminous, compared with other stars that have the same color. Stars do not start out as giants; instead, they intermittently become giants at various stages of their evolution.\r\n\r\nFor most stars that are not giants, there is a direct relation between their color and their luminosity (that is, the energy they emit over time). Those stars are called \"main sequence stars\", and their energy emissions are powered by the nuclear fusion of hydrogen into helium in their cores.  When the hydrogen supply in the core is exhausted, hydrogen fusion continues in a shell around the core and the star expands to become much more luminous and redder. Eventually the temperature in the center may be high enough that the nuclear fusion of elements heavier than hydrogen becomes possible, providing an additional energy source for the star. In the course of this conversion, such stars expand to a much larger size, cool down to become more reddish, and overall become much more luminous – they become what is known as red giant stars, red giants for short. The Sun, for instance, will be hundreds of times larger and brighter as well as much cooler when it expands to become a red giant star. For stars of different masses, additional, often short-lived phases of evolution sees those stars become blue giants, or even more luminous red or blue supergiants.\r\n\r\nGiants are classified as luminosity class III, brighter than subgiants (class IV) but fainter that bright giants (class II) and supergiants (class I).\r\n\r\nOverall, giant stars are rare. This is due to the relatively short duration of the giant phase (for a star like the Sun a few hundred million years vs ten billion on the main sequence). But given their high luminosity, they are significantly over-represented among the stars that are visible to the naked eye in the night sky.\r\n\r\nExamples of red giants include Arcturus, in the constellation Boötes, and Mira, in Cetus.","term_approval_level":"A","language_code":"en","term_number":130,"term_in_english":"Giant Star","based_on_current_english_version":null,"linked_terms":[221,277,331],"alternate_terms":[],"categories":["Naked Eye Astronomy","Stars"],"category_ids":[4,2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/130/"},{"term_name":"Global Warming","term_definition":"Global warming is a warming of a planet's atmosphere by greenhouse gases, such as water molecules, carbon dioxide, methane, etc. This is caused by an increase in the greenhouse effect where more infrared radiation is trapped by the atmosphere, thus increasing the global mean temperature of a planet. The sources of greenhouse gases can be natural or (on Earth) additionally caused by human industrial activity. Global warming on Earth will have significant very long-term effects on the planet, including short- and medium-term changes in local weather patterns, habitat destruction, and sea level rise.","term_approval_level":"A","language_code":"en","term_number":131,"term_in_english":"Global Warming","based_on_current_english_version":null,"linked_terms":[137],"alternate_terms":["climate change"],"categories":["Astronomy and Society"],"category_ids":[11],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/131/"},{"term_name":"Globular Cluster","term_definition":"A globular cluster is a massive, spherically-shaped and densely-packed group of stars that is gravitationally bound and stable over billions of years. In the Milky Way, globular clusters are made up of only very old stars (in general, ages of 11–13 billion years). Globular clusters are some of the oldest objects in the Milky Way. For most globular clusters the constituent stars all formed at approximately the same time although a few globular clusters show evidence of multiple generations of stars. The Milky Way has approximately 160 globular clusters (mostly residing in its halo), while giant elliptical galaxies can have many thousands.","term_approval_level":"A","language_code":"en","term_number":132,"term_in_english":"Globular Cluster","based_on_current_english_version":null,"linked_terms":[199,332,466],"alternate_terms":[],"categories":["Galaxies","Stars"],"category_ids":[8,2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/132/"},{"term_name":"Gravitational Constant","term_definition":"The gravitational constant is one of the most important constants of the Universe. It was first invoked by Isaac Newton. It is part of Newton's law of gravitational force, that shows that all particles with a mass attract every other particle (that also has a mass) with a force that is directly proportional to the product of the masses of the particles and inversely proportional to the squared distance between the objects. The proportionality constant is the gravitational constant. The value of the gravitational constant has been measured through experiments to be 6.67 × 10⁻¹¹ cubic meters per kilogram per seconds squared (m³ kg⁻¹ s⁻²).","term_approval_level":"A","language_code":"en","term_number":133,"term_in_english":"Gravitational Constant","based_on_current_english_version":null,"linked_terms":[135,190],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/133/"},{"term_name":"Gravitational Lens","term_definition":"Objects with mass can bend the path followed by light that passes nearby their gravitational influence. This effect, predicted by Einstein's theory of general relativity, was first observed during the solar eclipse of 1919 when the bending of starlight from several stars near the Sun was measured. Gravitational lensing is most apparent for very massive objects such as galaxies or galaxy clusters. If observers on Earth are looking at a distant object whose light is thus bent (by \"the lens\"), the object will appear distorted. This distortion always implies light (de)magnification, typically allowing us to better see otherwise faint background objects. When the lens has enough mass concentrated in a small angular area, multiple images of the same background object are produced, each having its light reaching the observer at different times. Measuring these so-called \"time delays\" is one of the best ways to determine the value of the Hubble Constant on extragalactic scales. Multiple images from a lens, on the other hand, allow us to precisely determine its mass (using modelling); this is a very useful method for finding the mass of galaxies and, especially, galaxy clusters.","term_approval_level":"A","language_code":"en","term_number":134,"term_in_english":"Gravitational Lens","based_on_current_english_version":null,"linked_terms":[76,119,120,348,413],"alternate_terms":["Gravitational lensing"],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/134/"},{"term_name":"Gravity","term_definition":"Gravity is the mutual attraction of objects with mass. In classical mechanics, any object with a mass always exerts a force of attraction on another object with a mass. This attractive force is what we know as gravity. Einstein's theory of general relativity recasts gravity as a curvature of spacetime rather than a force. However the classical approximation of gravity is still accurate in most scenarios. The more massive an object is, the stronger its gravitational force/distortion of spacetime and hence the stronger the pull on other objects.","term_approval_level":"A","language_code":"en","term_number":135,"term_in_english":"Gravity","based_on_current_english_version":null,"linked_terms":[190],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/135/"},{"term_name":"Great Red Spot","term_definition":"The Great Red Spot is a gigantic anticyclonic storm in the atmosphere of Jupiter located at 22 degrees south of its equator. About 15,000 kilometers (km) long and nearly 12,000 km wide, it is currently a little larger than Earth, although it has reached much larger dimensions in the past. Winds within the Great Red Spot can reach more than 400 kilometers per hour (km/h) (250 miles per hour (mph)). The reason for its red color is currently unknown although there are several competing hypotheses. A large red spot on Jupiter was discovered by the astronomer Giovanni Cassini in 1665 and was observed for half a century. However there is a century-long gap in observations so it is not known if this spot is the same feature as the one seen for the last two hundred years.","term_approval_level":"A","language_code":"en","term_number":136,"term_in_english":"Great Red Spot","based_on_current_english_version":null,"linked_terms":[167],"alternate_terms":[],"categories":["Solar System"],"category_ids":[1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/136/"},{"term_name":"Greenhouse Effect","term_definition":"The greenhouse effect is a warming of the atmosphere due to the emission of certain gases such as water, methane, and carbon dioxide. Visible light from the Sun reaches the surface of a planet and is re-emitted as infrared radiation. Greenhouse gases trap this infrared radiation within the atmosphere and hence the radiation cannot escape to free space; this makes the planet warmer than it would have been without these gases. Without the greenhouse effect, Earth's temperature would be tens of degrees below 0 degrees Celsius. However, the equilibrium temperature that results from the greenhouse effect is very sensitive to the concentration of greenhouse gases in the atmosphere.\r\n\r\nHuman-made emissions of greenhouse gases since the beginning of the Industrial Revolution in the 19th century have led to global warming on Earth due to the greenhouse effect. \r\n\r\nIn certain circumstances, this heating can lead to more greenhouse gases in the atmosphere, causing a runaway greenhouse effect. This is what has happened in the atmosphere of the planet Venus.","term_approval_level":"A","language_code":"en","term_number":137,"term_in_english":"Greenhouse Effect","based_on_current_english_version":null,"linked_terms":[131,155,201,377,378],"alternate_terms":[],"categories":["Exoplanets & Astrobiology","Solar System"],"category_ids":[6,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/137/"},{"term_name":"Greenwich Mean Time Zone (GMT)","term_definition":"The time zone in which the historic Royal Observatory at Greenwich, Great Britain, is located is called the Greenwich Mean Time zone, or alternatively the Western European time zone. Historically, Greenwich Mean Time (GMT) was the mean solar time determined at the Royal Observatory and used as the reference point for naval chronometers carried on ships. Navigators would determine the time of their local noon (the highest point above the horizon reached by the Sun in a given day) by observations using a sextant or similar device and compare with the GMT shown by their chronometer; the difference allowed them to determine their geographic longitude. In the modern system, time in the GMT time zone corresponds to Universal Time Coordinated (UTC), written as \"UTC + 0h\".","term_approval_level":"A","language_code":"en","term_number":138,"term_in_english":"Greenwich Mean Time Zone (GMT)","based_on_current_english_version":null,"linked_terms":[373],"alternate_terms":[],"categories":["Astronomy and Society"],"category_ids":[11],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/138/"},{"term_name":"Habitable Zone","term_definition":"The habitable zone of a star is defined as the region around it where liquid water can exist on the surface of an Earth-like planet. If Earth were much farther away from the Sun, all surface water would freeze; much closer, and all surface water would boil off. In neither case could life as we know it form or survive. Sometimes, the habitable zone concept is extended to include a planet like Venus, with its runaway greenhouse effect, where liquid water could exist even if the planet were farther away from the Sun. The galactic habitable zone is that part of our Galaxy where conditions are suitable for life-bearing planetary systems: there, heavier elements, which Earth-like planets are made of, should be sufficiently common, and life-threatening events like supernovae sufficiently rare.\r\n\r\nIt should be noted that habitable conditions may exist outside of the habitable zone. For example the possibly habitable subsurface ocean on Jupiter's moon Europa.","term_approval_level":"A","language_code":"en","term_number":139,"term_in_english":"Habitable Zone","based_on_current_english_version":null,"linked_terms":[20,109,137,349,468],"alternate_terms":[],"categories":["Exoplanets & Astrobiology","Milky Way and Interstellar Medium"],"category_ids":[6,7],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/139/"},{"term_name":"Halley's Comet","term_definition":"Halley's comet is, arguably, the most famous comet because it is the only short-cycle comet visible to the naked eye from Earth with a period short enough (about 75 years) to potentially allow people to see it twice in their lives. The comet's last visit was in 1986, and it is expected to return in 2061. It is named after the English astronomer Edmond Halley who was the first to calculate its periodicity and predict its next visit. Halley noticed that the comets that appeared in the years 1531, 1607, and 1682 all had very similar orbits and thus were all visits of the same comet to the inner Solar System. He correctly predicted the comet's return in 1758.\r\n\r\nIt has been visited by the Vega and Giotto space missions. These found that the comet's dust consists mainly of silicates, iron, and magnesium, in addition to carbon–hydrogen–oxygen–nitrogen compounds (CHON). The comet's nucleus is made mostly of ice.","term_approval_level":"A","language_code":"en","term_number":140,"term_in_english":"Halley's Comet","based_on_current_english_version":null,"linked_terms":[62,63],"alternate_terms":[],"categories":["Astronomy and Society","Naked Eye Astronomy","Solar System"],"category_ids":[11,4,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/140/"},{"term_name":"Heliocentric Model","term_definition":"The term heliocentric is from the Greek helios, which is the name for the Sun, and kentro meaning center. This model of the Solar System places the Sun at the center and the planets orbit around it, replacing the geocentric (Earth-centered) model. Although the origin of the model is attributed to Copernicus in the 16th century, Aristarchus of Samos developed a heliocentric model in Ancient Greece and astronomers in India, Europe, and the Islamic world discussed such models prior to Copernicus. Observational evidence for the heliocentric model came through the telescopic observations of Venus made by Galileo. The original heliocentric model placed the Sun at the geometric center of the Solar System; this view changed with the mathematical formulations of Kepler using Tycho Brahe’s data, which Newton built on and expanded with his law of gravitation.","term_approval_level":"A","language_code":"en","term_number":141,"term_in_english":"Heliocentric Model","based_on_current_english_version":null,"linked_terms":[127],"alternate_terms":[],"categories":["Solar System","The Sun"],"category_ids":[1,5],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/141/"},{"term_name":"Helium Fusion","term_definition":"","term_approval_level":"N","language_code":"en","term_number":142,"term_in_english":"Helium Fusion","based_on_current_english_version":null,"linked_terms":[],"alternate_terms":[],"override_term_number":221,"categories":["Stars"],"category_ids":[2],"override_url":"https://astro4edu.org/resources/glossary/term/221/","url":"https://astro4edu.org/resources/glossary/term/142/"},{"term_name":"Hertzsprung–Russell (HR) Diagram","term_definition":"The Hertzsprung–Russell (or HR) diagram is a graph of two observational properties of stars: On the one axis, the total power emitted by stars (luminosity), and on the other axis either their effective temperature or spectral type. Where the effective temperature is used, it is shown on a logarithmic scale, increasing from right to left. The HR diagram is named after two scientists: Ejnar Hertzsprung and Henry Norris Russell who first made different versions of this graph in order to understand the properties of stars. The data points corresponding to the so-called \"main sequence stars\" lie on a diagonal band from the upper left to lower right in this graph. Data points corresponding to giant stars lie above and to the right of the main-sequence band. White dwarfs lie below and to the left of the band. \r\n\r\nThe HR diagram can also be a useful framework for representing the evolution of a star over time. Once a star has formed it will be positioned on the main sequence of the HR diagram, and its temperature and luminosity will remain roughly constant for some time. Later, as it evolves, the star's temperature will drop and its luminosity will increase. This means the star's position on the HR diagram moves up and to the right, away from the main sequence towards the giant branch. A star's evolution, specifically its changes in temperature and luminosity, can be represented by a curve in the HR diagram. Thus a star's evolutionary state can be determined from its temperature and luminosity using the HR diagram.","term_approval_level":"A","language_code":"en","term_number":143,"term_in_english":"Hertzsprung-Russell (HR) Diagram","based_on_current_english_version":null,"linked_terms":[88,130,180,186,331,347,386,440,503],"alternate_terms":[],"categories":["Stars"],"category_ids":[2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/143/"},{"term_name":"Horizon","term_definition":"The horizon is the boundary line that separates the sky from Earth´s surface. At any position on Earth, we only see a limited part of the globe. The boundary dividing what we can from what we cannot see is commonly called the horizon. In astronomy, that definition is refined as follows: Our own position on Earth defines a horizontal plane, which is perpendicular to the downwards direction (which we can make visible using a plumb line). The intersection of that plane with the celestial sphere defines our astronomical local horizon. The horizontal coordinate system makes use of the horizontal plane to define positions in the sky. The angle between our sightline to an object and the horizontal plane is called the object's altitude; the angle between the sightline's projection onto that plane and true north is called the object's azimuth.","term_approval_level":"A","language_code":"en","term_number":145,"term_in_english":"Horizon","based_on_current_english_version":null,"linked_terms":[6,36,90],"alternate_terms":[],"categories":["Naked Eye Astronomy"],"category_ids":[4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/145/"},{"term_name":"Horizontal Branch","term_definition":"Stars that are fusing helium to carbon in their cores are called horizontal branch stars. The name arises because these stars lie along a horizontal branch in the Hertzsprung–Russell diagram, displaying a range of \"surface\" temperatures (effective temperatures) but nearly constant luminosity. These are stars that have evolved beyond the red giant phase with variable amounts of mass (outer layers) lost. Main sequence stars with mass up to eight times the mass of the Sun can go through this evolutionary phase.","term_approval_level":"A","language_code":"en","term_number":146,"term_in_english":"Horizontal Branch","based_on_current_english_version":null,"linked_terms":[186,277,334,440],"alternate_terms":[],"categories":["Stars"],"category_ids":[2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/146/"},{"term_name":"Hour Angle","term_definition":"The hour angle is the angle between an object's hour circle and the observer's meridian.\r\n\r\nAs seen from Earth, all the various positions in the sky together form what appears to be a distant sphere with Earth at its center. The points in the sky directly above Earth's equator form the celestial equator on that sphere. The point directly above Earth's geographic North Pole is the celestial North Pole, and that above Earth's South Pole, the celestial South Pole.\r\n\r\nJust like geographers define geographic longitude and latitude on Earth's surface, one can define longitude and latitude on the celestial sphere. The meridian of an observer corresponds to the observer's geographical meridian (the circle whose center is the Earth's center, and which intersects the North Pole, South Pole, and the observer's position), projected onto the celestial sphere. It intersects the north point on the observer's horizon, the zenith, and the south point. The projected meridian that passes through a given celestial object is called that object's hour circle. The hour angle is the angle between the object's hour circle and the observer's meridian. As time passes, the hour angle changes: An hour angle of zero corresponds to the star's highest position (its upper culmination) in the sky. As the star moves towards the western horizon, the hour angle increases. As the hour angle approaches 360 degrees, the star approaches its next upper culmination. Note the time between upper culminations is one sidereal day, this is roughly four minutes shorter than a solar day. Because of this direct connection with time, the hour angle is usually stated in hours, not in degrees, with 360 degrees corresponding to 24 hours.  The hour angle can be used to compute the time until an object's upper culmination. This is useful to astronomers planning their observations: at or near upper culmination, when it is farthest from the horizon, is a particularly good time to observe an object.","term_approval_level":"A","language_code":"en","term_number":147,"term_in_english":"Hour Angle","based_on_current_english_version":null,"linked_terms":[50,51,78,286,490,533],"alternate_terms":[],"categories":["Naked Eye Astronomy"],"category_ids":[4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/147/"},{"term_name":"Hubble Diagram","term_definition":"The original Hubble diagram is a graph of velocity (y-axis) versus distance (x-axis) of galaxies. The graph shows a linear relationship between velocity and distance, providing evidence that distant galaxies are moving away faster than closer galaxies, and overall galaxies seem to be moving away from \"us\". This is used as one line of evidence for an expanding Universe. The slope (gradient) of the line is referred to as the Hubble parameter (H), and the equation of the line is called the Hubble–Lemaître Law. The value of the Hubble parameter in the current era (13.8 billion years after the Big Bang) is called the Hubble constant (H₀). Modern iterations of the Hubble diagram, based on observations of Type Ia supernovae, plot distance modulus (indirect measure of distance using brightness) versus redshift. In fact, the velocity of galaxies in the original Hubble diagram is measured indirectly from the redshift.","term_approval_level":"A","language_code":"en","term_number":148,"term_in_english":"Hubble Diagram","based_on_current_english_version":null,"linked_terms":[471,476],"alternate_terms":[],"categories":["Cosmology","Galaxies"],"category_ids":[9,8],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/148/"},{"term_name":"Hydrogen","term_definition":"Hydrogen is the most abundant and lightest element in the Universe; it has the smallest nuclear charge as it has only one proton. When hydrogen atoms are excited, for instance by radiation from a nearby hot star, they emit light in characteristic narrow regions of the spectrum. These hydrogen emission lines can be used to detect atomic hydrogen: in particular, the hydrogen alpha line, with its rich red color which makes hydrogen clouds show up in splendid red in astronomical images, and the 21 centimeter (cm) hydrogen line which can be used to map out large clouds of gas using radio telescopes.","term_approval_level":"A","language_code":"en","term_number":149,"term_in_english":"Hydrogen","based_on_current_english_version":null,"linked_terms":[263],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/149/"},{"term_name":"Hydrogen Fusion","term_definition":"Nuclear fusion is the umbrella term for all reactions whereby lighter atomic nuclei collide and merge to form one or more heavier atomic nuclei. In astronomy, hydrogen fusion is the nuclear fusion reaction that transforms hydrogen nuclei (each consisting of a single proton) into helium-4 nuclei (each consisting of two protons and two neutrons bound together). The helium-4 nucleus has a mass less than the sum of the masses of the protons and neutrons it is made of. By Einstein's famous formula E=mc², that mass difference corresponds to an energy difference. As the protons and neutrons fuse to form helium-4, the amount of energy corresponding to that difference is released. In this way, hydrogen fusion serves as the energy source for so-called main sequence stars like our Sun. At least for some time, such stars are in an equilibrium state: the amount of energy released by hydrogen fusion in their cores corresponds to the energy those brightly-shining stars emit in the form of light and other kinds of electromagnetic radiation as well as particles. \r\n\r\nHydrogen fusion proceeds via several intermediate steps. For stars with the mass of our Sun or less, it proceeds via the so-called proton–proton chain (pp chain). In the simplest version of that chain of reactions, two hydrogen nuclei (protons) fuse to yield deuterium nuclei (one proton, one neutron each), which then fuse with one additional hydrogen nucleus to yield helium-3 (two protons, one neutron). Two such helium-3 nuclei fuse to yield helium-4 plus two remaining hydrogen nuclei. In stars with more than about 1.3 times the mass of our Sun, an alternative process called the carbon–nitrogen–oxygen (CNO) cycle becomes the dominant way for fusing hydrogen into helium. Scientists on Earth have built machines to create fusion reactions with the hope that in future it can become a viable way of generating energy. Hydrogen fusion occurs not only in stars, but also took place during the early Big Bang phase of our Universe.","term_approval_level":"A","language_code":"en","term_number":150,"term_in_english":"Hydrogen Fusion","based_on_current_english_version":null,"linked_terms":[149,221],"alternate_terms":[],"categories":["Stars","The Sun"],"category_ids":[2,5],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/150/"},{"term_name":"Hypothesis","term_definition":"In science, hypothesis refers to an idea, or prediction a scientist may have. It may have some evidence, but it has not been proven. It is only through experimentation, observations, data, and models, that a hypothesis is gradually refined or completely discounted. Contrary to popular belief a hypothesis is not always the first stage in the development of scientific knowledge. For example, scientists make observations of phenomena or objects, analyze the data, and then propose an explanation or make a prediction of what could be going on: That is their hypothesis. It is only through additional data and observations that the hypothesis is validated, or not. A hypothesis can have a mathematical basis or start with a mathematical formulation that makes a prediction.","term_approval_level":"A","language_code":"en","term_number":152,"term_in_english":"Hypothesis","based_on_current_english_version":null,"linked_terms":[295,355],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/152/"},{"term_name":"Ice Giant","term_definition":"In the Solar System there are four giant planets: Jupiter, Saturn, Uranus, and Neptune. The last two, Uranus and Neptune, are known as ice giants. They have solid rocky cores surrounded by a thick layer of water, ammonia, and methane. These chemicals are in a strange, high-pressure state of matter: not quite solid, not quite liquid. The outer atmosphere of both planets is a thick, puffy layer of hydrogen and helium.\r\n\r\nIn the early Solar System, in the regions far from the Sun, it was cold enough for water, methane, and ammonia to freeze into ices. Here \"ices\" is a general term for frozen chemicals made of molecules. The young planets Uranus and Neptune accreted these ices due to their gravitational pull. Because these ices were the source of such an important component of these two planets, they were named the ice giants.","term_approval_level":"A","language_code":"en","term_number":153,"term_in_english":"Ice Giant","based_on_current_english_version":null,"linked_terms":[129,212,375,425],"alternate_terms":[],"categories":["Exoplanets & Astrobiology","Solar System"],"category_ids":[6,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/153/"},{"term_name":"Impact Craters","term_definition":"An impact crater is a depression on the surface of a planet, moon, or other solid minor body of the Solar System. Impact craters are formed by a high velocity impact of a smaller body (meteorite). The Moon is full of impact craters due to its lack of atmosphere. The thick atmosphere on Earth stops many meteors from ever reaching the ground. An example of an impact crater on Earth Is Meteor Crater in Arizona, USA.","term_approval_level":"A","language_code":"en","term_number":154,"term_in_english":"Impact Craters","based_on_current_english_version":null,"linked_terms":[74],"alternate_terms":[],"categories":["Naked Eye Astronomy","Solar System"],"category_ids":[4,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/154/"},{"term_name":"Infrared (IR)","term_definition":"Infrared light is electromagnetic radiation with wavelengths longer than those of visible light but shorter than microwave and radio waves. Infrared light has wavelengths in the range of 700 nanometers to one millimeter while visible light has wavelengths from around 400–700 nanometers. Infrared light is therefore invisible to the human eye and can only be seen with special cameras. Thermal bodies with temperatures of tens to a few thousand kelvins, such as molecular clouds in space, the human body, or brown dwarfs, have their peak electromagnetic emission in infrared light.","term_approval_level":"A","language_code":"en","term_number":155,"term_in_english":"Infrared (IR)","based_on_current_english_version":null,"linked_terms":[46,96,378,482],"alternate_terms":["Infrared radiation","IR"],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/155/"},{"term_name":"Infrared Astronomy","term_definition":"Infrared astronomy is a branch of astronomy that looks at infrared light. It is more sensitive to cool objects than observations in visible light and can observe very distant galaxies whose light has been redshifted a lot. Infrared observations are less affected by extinction and can thus see deeper into interstellar clouds of gas and dust.\r\n\r\nMolecules in Earth's atmosphere absorb much of the infrared light coming from space and thus infrared astronomy on the ground is done mostly in wavelength ranges where this absorption is lower. Both Earth and its atmosphere radiate in the infrared so special techniques are required to remove this background radiation. For infrared radiation with longer wavelengths, this background plus atmospheric absorption makes observing on the ground almost impossible. As a result many infrared observations are carried out using space telescopes. However, for the longest infrared wavelengths ground-based observations are possible from very dry sites. This is typically called submillimeter astronomy.","term_approval_level":"A","language_code":"en","term_number":156,"term_in_english":"Infrared Astronomy","based_on_current_english_version":null,"linked_terms":[96,155,157,471,472,482],"alternate_terms":[],"categories":["Telescopes, Instruments and Observatories"],"category_ids":[3],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/156/"},{"term_name":"Infrared Telescope","term_definition":"An infrared telescope observes infrared light and is used for infrared astronomy. Infrared telescopes can be situated on the ground or in space. Observatories situated on Earth are limited in what they can observe by atmospheric absorption and infrared radiation emitted by the Earth's atmosphere, the telescope itself, and its surroundings. Space infrared telescopes do not need to deal with atmospheric absorption or infrared radiation from their immediate surroundings and can be shielded from the Sun and cooled, reducing the infrared radiation emitted by the telescope.","term_approval_level":"A","language_code":"en","term_number":157,"term_in_english":"Infrared Telescope","based_on_current_english_version":null,"linked_terms":[155,156],"alternate_terms":[],"categories":["Telescopes, Instruments and Observatories"],"category_ids":[3],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/157/"},{"term_name":"International Astronomical Union","term_definition":"The International Astronomical Union (IAU) is a society of professional astronomers from around the world who are currently working in some aspect of astronomy research, education, or outreach. It was founded in 1919 and it works to develop outreach and educational activities for the public, alongside promoting active astronomy research through its scientific meetings.","term_approval_level":"A","language_code":"en","term_number":158,"term_in_english":"International Astronomical Union","based_on_current_english_version":null,"linked_terms":[],"alternate_terms":[],"categories":["Astronomy and Society"],"category_ids":[11],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/158/"},{"term_name":"Io","term_definition":"Io is a moon or satellite orbiting around Jupiter, discovered by Galileo Galilei in 1610. It is the closest of the four largest satellites of Jupiter. Io orbits around Jupiter every 42.5 hours, at a distance of 422,000 kilometers from the planet. Io has many active volcanoes on its surface as a consequence of strong tidal forces experienced by the satellite while revolving around Jupiter.","term_approval_level":"A","language_code":"en","term_number":159,"term_in_english":"Io","based_on_current_english_version":null,"linked_terms":[121,167],"alternate_terms":[],"categories":["Solar System"],"category_ids":[1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/159/"},{"term_name":"Ionization","term_definition":"Ionization is the process of subtracting electrons from or adding electrons to previously neutral atoms, thereby changing a gas of neutral atoms (or molecules) to one made up of charged ions, i.e. a plasma.","term_approval_level":"A","language_code":"en","term_number":160,"term_in_english":"Ionization","based_on_current_english_version":null,"linked_terms":[258,441],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/160/"},{"term_name":"Ionosphere","term_definition":"The ionosphere is a part of the atmosphere (above about 80 kilometers). It contains a high concentration of ions due to the high-energy part of the radiation the Earth receives from the Sun, which breaks the molecules into atoms, and then atoms into ions and electrons. The presence of the free electrons makes this region of the atmosphere a good conductor of electricity. As such, the ionosphere is able to reflect radio waves at certain wavelengths.","term_approval_level":"A","language_code":"en","term_number":162,"term_in_english":"Ionosphere","based_on_current_english_version":null,"linked_terms":[29,441],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/162/"},{"term_name":"Iron Meteorite","term_definition":"Iron meteorites are meteorites with an abundant amount of iron and nickel. They are thought to be part of the core of asteroids and are very dense and very heavy. Almost 60% of the meteorites found on Earth are iron meteorites even though they make up only 5% of the meteorites that hit Earth's surface. One of the reasons for this is that they are stronger and do not weather as easily as stony meteorites.","term_approval_level":"A","language_code":"en","term_number":163,"term_in_english":"Iron Meteorite","based_on_current_english_version":null,"linked_terms":[196,302],"alternate_terms":[],"categories":["Naked Eye Astronomy","Solar System"],"category_ids":[4,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/163/"},{"term_name":"Irregular Galaxy","term_definition":"An irregular galaxy is a type of galaxy with little or no symmetry (it has a distorted morphology or shape). They are typically smaller than spiral and elliptical galaxies, and they often contain significant amounts of star-forming gas. The Large and Small Magellanic Clouds are irregular galaxies, relatively close (approximately 160,000 light years, and 200,000 light years) to our Milky Way galaxy, that can be observed from Earth's southern hemisphere with the unaided eye.","term_approval_level":"A","language_code":"en","term_number":164,"term_in_english":"Irregular Galaxy","based_on_current_english_version":null,"linked_terms":[99,119,330],"alternate_terms":[],"categories":["Galaxies"],"category_ids":[8],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/164/"},{"term_name":"Isotope","term_definition":"Atomic nuclei consist of neutrons and protons. The number of protons in a nucleus is the so-called \"atomic number\", which determines the corresponding chemical element: Atoms with a single proton are hydrogen atoms, those with six protons are carbon, and so on. Atomic nuclei that have the same number of protons, but different numbers of neutrons, are called isotopes of the chemical element in question. Ordinary hydrogen nuclei only have a single proton, and no neutron at all. A nucleus with one proton and one neutron is still hydrogen, but \"heavy hydrogen\", also called deuterium. Typically, only a few isotopes of a given element are stable. The others will decay radioactively into more stable nuclei.","term_approval_level":"A","language_code":"en","term_number":165,"term_in_english":"Isotope","based_on_current_english_version":null,"linked_terms":[31,213,224,263],"alternate_terms":[],"categories":["Chemistry"],"category_ids":[12],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/165/"},{"term_name":"Jupiter","term_definition":"Jupiter is the largest planet in the Solar System and the fifth major planet from the Sun. It is a gas giant with a radius of 71,300 kilometers (km), about 11 times the radius of the Earth. The mass of Jupiter (318 times the mass of the Earth) is greater than all the other planets and the smaller bodies in the Solar System put together. \r\n\r\nIts typical distance from the Sun is 778 million km, about five astronomical units (Earth–Sun distances), taking a little under 12 years to complete one orbit. As of 2023, astronomers have detected more than 90 moons or natural satellites orbiting Jupiter.\r\n\r\nIt is visible with the naked eye. Its name in English derives from the Roman king of the gods. Observed in a small telescope we can see cloud belts of different colors and a giant red circular storm region (the so-called Great Red Spot). A few space probes have been sent to Jupiter over the past decades, and in 2016 the NASA spacecraft Juno started exploring Jupiter and its moons in much greater detail.","term_approval_level":"A","language_code":"en","term_number":167,"term_in_english":"Jupiter","based_on_current_english_version":null,"linked_terms":[26,121,125,129,136,234,253,314],"alternate_terms":[],"categories":["Naked Eye Astronomy","Solar System"],"category_ids":[4,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/167/"},{"term_name":"K-type Star","term_definition":"A star with spectral type \"K\". Astronomers identify K-type stars by the presence of  very weak hydrogen lines but strong lines from iron and manganese atoms in their spectra. They have typical (effective) temperatures between around 3700 kelvins (K) and 5200 K. Compared to other stars, they appear orange-white to human eyes unless interstellar or atmospheric reddening is important. Examples of K-type stars are Aldebaran, in Taurus, and Pollux, in Gemini.","term_approval_level":"A","language_code":"en","term_number":168,"term_in_english":"K-type Star","based_on_current_english_version":null,"linked_terms":[325,331,396,440,514],"alternate_terms":["K star","K-star"],"categories":["Stars"],"category_ids":[2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/168/"},{"term_name":"Kepler's Laws","term_definition":"The three laws formulated by Johannes Kepler in the beginning of the 1600s were the first to describe the orbits of the planets as not perfectly circular. The first law states that the planets orbit the Sun in an elliptical shape with the Sun at one of the focal points. The second law says that the area covered by a line between the planet and the Sun is the same in a given time interval of the orbit. According to the third law, the square of the time (T²) a planet takes to move around the Sun is proportional to the cube of its semi-major axis (a³, semi-major axis is a length which characterizes the size of the planet's orbit around the Sun). Kepler found these three laws by studying the observations of Mars carried out by his mentor Tycho Brahe. He used the laws to make the most precise calculation of the orbits of the planets known to his time.","term_approval_level":"A","language_code":"en","term_number":169,"term_in_english":"Kepler's Laws","based_on_current_english_version":null,"linked_terms":[98,232,253,314],"alternate_terms":[],"categories":["Exoplanets & Astrobiology","Solar System"],"category_ids":[6,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/169/"},{"term_name":"Kuiper Belt","term_definition":"The Kuiper Belt is a band of small, icy objects in the outer Solar System mostly lying beyond the orbit of Neptune. Most objects are found at distances of 40–48 astronomical units from the Sun.\n\nObjects in the Kuiper Belt are mostly small although several dwarf planets can be found there, including Pluto. Unlike the small bodies and dwarf planets in the asteroid belt, the objects in the Kuiper Belt are mostly made of frozen water, methane, and ammonia.","term_approval_level":"A","language_code":"en","term_number":170,"term_in_english":"Kuiper Belt","based_on_current_english_version":null,"linked_terms":[212,259,314,465],"alternate_terms":["Edgeworth-Kuiper Belt"],"categories":["Solar System"],"category_ids":[1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/170/"},{"term_name":"Latitude","term_definition":"Earth is a sphere. To define locations on Earth two sets of imaginary lines are drawn onto the surface of the sphere: Lines of latitude are circles which run around Earth parallel to the equator. The equator has a latitude of 0 degrees. Latitudes in the northern hemisphere are positive; latitudes in the southern hemisphere are negative. The North and South Poles have the highest/lowest latitudes. The North Pole is at +90 degrees and the South Pole at -90 degrees. There are also lines running in great circles through the poles. These are lines of longitude.","term_approval_level":"A","language_code":"en","term_number":171,"term_in_english":"Latitude","based_on_current_english_version":null,"linked_terms":[102,179,424,483,496,497],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/171/"},{"term_name":"Leo","term_definition":"Leo, \"the Lion\", is a constellation in the Zodiac, i.e. the stars that make up this constellation are in the part of the sky that intersects with the ecliptic (the plane defined by the Earth's path around the Sun). Hence, from our point of view here on Earth, we can regularly find the Sun, and also the other planets in our Solar System, in this constellation – in the case of the Sun from August 11 to September 17. (Of course if the Sun is there, we cannot see the constellation's stars.) In the night sky, Leo is easiest to observe in spring. It is one of the 88 modern constellations defined by the International Astronomical Union, and also one of the 48 classical constellations named by 2nd century astronomer Claudius Ptolemy. The brightest of Leo's numerous bright stars is called Regulus.","term_approval_level":"A","language_code":"en","term_number":172,"term_in_english":"Leo","based_on_current_english_version":null,"linked_terms":[66,92,158,391],"alternate_terms":[],"categories":["Naked Eye Astronomy"],"category_ids":[4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/172/"},{"term_name":"Libra","term_definition":"Libra, \"the balance\", is a constellation in the Zodiac, i.e. the stars that make up this constellation are in the part of the sky that intersects with the ecliptic – the plane defined by the Earth's orbit around the Sun. Hence, from our point of view here on Earth, we can regularly find the Sun, and also the other planets in the Solar System, in this constellation – in the case of the Sun from late October to late November. (Of course if the Sun is there, we cannot see the constellation's stars.) Libra is one of the 88 modern constellations defined by the International Astronomical Union, and also one of the 48 classical constellations named by 2nd century astronomer Claudius Ptolemy.","term_approval_level":"A","language_code":"en","term_number":173,"term_in_english":"Libra","based_on_current_english_version":null,"linked_terms":[66,92,158,391],"alternate_terms":[],"categories":["Naked Eye Astronomy"],"category_ids":[4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/173/"},{"term_name":"Light","term_definition":"Light is electromagnetic radiation. In common use, light typically refers to electromagnetic radiation with a wavelength that can be seen with the naked human eye. The wavelength of light that can be seen by humans is broadly in the range of 380–750 nanometers (nm), although most humans have very little sensitivity to light with wavelengths shorter than 400 nm. This is a narrow part of the electromagnetic spectrum that covers a wide range of wavelengths from gamma (the shortest) to radio waves (the longest). More broadly the term light is sometimes applied to any electromagnetic radiation.\r\n\r\nThe basic properties of light are intensity, direction of propagation, frequency, spectrum, and polarization. Its speed in a vacuum is defined to be exactly 299,792,458 meters per second, and this is one of the fundamental constants of nature. The color of light depends on its wavelength. Violet light has the shortest wavelength in the visible spectrum; red has the longest. Light has multiple sources, natural and artificial; the Sun is the Earth's main source of light. Light is emitted and absorbed in small \"packets\" called photons that have properties of both waves and particles. This latter phenomenon is called the wave–particle duality.","term_approval_level":"A","language_code":"en","term_number":175,"term_in_english":"Light","based_on_current_english_version":null,"linked_terms":[96,378],"alternate_terms":[],"categories":["Naked Eye Astronomy"],"category_ids":[4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/175/"},{"term_name":"Light Curve","term_definition":"A light curve is a graph of the brightness, magnitude, or color of an object over time. Light curves are used to study a range of astronomical objects, e.g. variable stars, binary systems, exoplanets, X-ray binaries, or supernovae. Variations seen in the light curve help to classify the object and provide information such as the timescale or period of variability, which can be used to infer important information about it, such as the nature of the object, the source of energy input, or the types of physical processes that operate on it.","term_approval_level":"A","language_code":"en","term_number":176,"term_in_english":"Light Curve","based_on_current_english_version":null,"linked_terms":[367,485],"alternate_terms":[],"categories":["Exoplanets & Astrobiology","Stars"],"category_ids":[6,2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/176/"},{"term_name":"Light Pollution","term_definition":"Light pollution is the presence of excessive or defective (emitted in a direction above the horizon) artificial lighting, such as streetlights, that causes the brightening of the night sky. This is inappropriate as it disrupts the observation of stars, planets, and other astronomical objects, and changes ecosystems and many natural cycles that affect living beings. Furthermore, light pollution is also an inefficient use of finances and resources.","term_approval_level":"A","language_code":"en","term_number":177,"term_in_english":"Light Pollution","based_on_current_english_version":null,"linked_terms":[462],"alternate_terms":[],"categories":["Astronomy and Society","Naked Eye Astronomy","Telescopes, Instruments and Observatories"],"category_ids":[11,4,3],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/177/"},{"term_name":"Light Year","term_definition":"A light year is a unit of length sometimes used in astronomy to express large astronomical distances like distances to stars or between galaxies. It is defined as the distance that light travels in vacuum in one year: 9.46 trillion kilometers (km), that is 9.5x10¹² km. By using the unit \"light year\", the large distances become numerically more manageable: the nearest star to the Sun is 40 trillion km away or more simply 4.25 light years. Astronomers also use the units light minutes or light hours: light takes about eight minutes to get from the Sun to Earth and about four hours to get to Neptune. Light year is a rarely used term in astronomical research where the parsec (approximately 3.26 light years) is the preferred measure of distance.","term_approval_level":"A","language_code":"en","term_number":178,"term_in_english":"Light-Year","based_on_current_english_version":null,"linked_terms":[238],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/178/"},{"term_name":"Longitude","term_definition":"Earth is a sphere. To define locations on Earth two sets of imaginary lines are drawn onto the surface of the sphere. Lines of longitude are great circles which run around Earth and through both the North and South Poles. Whilst the equator provides a natural reference point for 0 degrees latitude, the line of 0 degrees longitude had to be agreed on. The recognized line of 0 degrees longitude runs through Greenwich in London, UK, and is also referred to as the Prime Meridian or Greenwich Meridian. The antemeridian is halfway around the world at 180 degrees and is the basis for the International Date Line. The full distance around Earth is 360 degrees. There are also lines running in small circles around Earth, parallel to the equator. These are lines of latitude.","term_approval_level":"A","language_code":"en","term_number":179,"term_in_english":"Longitude","based_on_current_english_version":null,"linked_terms":[102,171,496,497],"alternate_terms":[],"categories":["Naked Eye Astronomy"],"category_ids":[4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/179/"},{"term_name":"Luminosity","term_definition":"Luminosity is the energy output per second from some emitting body such as a star. The most common use is for electromagnetic radiation, in which case the range of wavelengths or frequencies needs to be specified. The radiative energy per second at a specific frequency or wavelength is called \"luminosity density\". The luminosity across all electromagnetic wavelengths is referred to as \"bolometric luminosity\".\r\n\r\nOther forms of luminosity include neutrino emission or material outflows such as jets. \r\n\r\nIn astronomical contexts, luminosities are often expressed as multiples of the Sun's luminosity, known as solar luminosities.","term_approval_level":"A","language_code":"en","term_number":180,"term_in_english":"Luminosity","based_on_current_english_version":null,"linked_terms":[45,96,112,382],"alternate_terms":[],"categories":["Stars"],"category_ids":[2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/180/"},{"term_name":"Lunar Eclipse","term_definition":"A lunar eclipse occurs when the Moon passes into the shadow of Earth. This can only happen when the Sun, Earth, and Moon are very closely aligned, with Earth directly between the Sun and the Moon. As such, a lunar eclipse can only occur on the night of a full moon. A total lunar eclipse occurs when the Moon is fully within the shadow of Earth. A partial lunar eclipse occurs when the Moon is only partially covered by the shadow of Earth. The type and length of a lunar eclipse depends on the precise location of the Moon in its orbit around Earth at the time of the eclipse.","term_approval_level":"A","language_code":"en","term_number":181,"term_in_english":"Lunar Eclipse","based_on_current_english_version":null,"linked_terms":[91,475],"alternate_terms":[],"categories":["Astronomy and Society","Naked Eye Astronomy","Solar System"],"category_ids":[11,4,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/181/"},{"term_name":"Lunar Phase","term_definition":"Lunar phase refers to the Moon's position in its orbit around the Earth. The changing position the Moon causes the changing shape of the illuminated portion of the Moon visible from Earth over the course of one lunar month. Half of the Moon is always, apart from during lunar eclipses, illuminated by the Sun. On Earth we see different parts of the Moon illuminated as it moves in its orbit around us. The lunar month starts and ends at the same phase. At a phase of 0 degrees, called \"new moon,\" the Moon is as close to the Sun as it will be on that orbit. At that phase, the illuminated side of the Moon faces away from Earth, and the Moon appears dark. The size of the illuminated part of the Moon gradually increases (waxing phase) becoming a crescent. The first quarter phase (when half of the moon appears to be illuminated, this is popularly known as half moon) occurs at 90 degrees from the start point. The illuminated portion of the Moon continues to increase, becoming gibbous (convex-shaped, or bulging-shaped). Full moon occurs at 180 degrees. After this point, the shape gradually starts decreasing (waning phase), resulting in a gibbous moon, the last quarter phase (when half of the moon appears to be illuminated, this is popularly known as half moon) at 270 degrees from the start, the crescent moon, and ending as a new moon at 360 degrees. Even though half the Moon appears illuminated at phases 90 and 270 degrees, the opposite sides are the ones illuminated.","term_approval_level":"A","language_code":"en","term_number":182,"term_in_english":"Lunar Phase","based_on_current_english_version":null,"linked_terms":[202,247],"alternate_terms":["moon phase"],"categories":["Astronomy and Society","Naked Eye Astronomy","Solar System"],"category_ids":[11,4,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/182/"},{"term_name":"Magnetic Poles","term_definition":"Earth has a comparatively simple magnetic field, approximately what physicists call a dipole field, not dissimilar from that of a toy bar magnet. If you suspend a compass needle so that it can move freely in all directions, you can trace the direction of Earth's magnetic field. Near the equator, the field direction is almost horizontal, but as you come closer to Earth's North and South Poles, the direction turns downward. At the so-called magnetic North and South Poles, the compass needle is vertical. The magnetic poles of Earth do not coincide with Earth's geographic poles.  They also move around – currently about 50 kilometers per year – an effect that navigators using a compass need to take into account.","term_approval_level":"A","language_code":"en","term_number":184,"term_in_english":"Magnetic Poles","based_on_current_english_version":null,"linked_terms":[90,296,455,497],"alternate_terms":[],"categories":["The Sun"],"category_ids":[5],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/184/"},{"term_name":"Magnitude","term_definition":"In astronomy, the magnitude is a measure of how bright  a celestial object is. The magnitude system used in astronomy originated in antiquity as a ranking of stars from the brightest to the least bright. That is why a smaller (or more negative) magnitude value means that the object is brighter, and a larger number means a fainter object. Hence a star with magnitude -1 is brighter than a star with magnitude 0 which in turn is brighter than a star of magnitude 1.\r\n\r\nMagnitude has a logarithmic scale, in which a magnitude difference of five corresponds to a factor of 100 difference in the amount of energy received: a star with magnitude 10  is a hundred times dimmer than a star with magnitude 5. \r\n\r\nThere are different types of magnitude: apparent magnitude measures the apparent brightness of an object, which depends both on the object's luminosity – how much light the object emits – and on its distance from Earth. \r\n\r\nIn contrast, the absolute magnitude is the value we would obtain if the object were at a standard distance of 10 parsecs (32.6 light years) from Earth. (For reflecting objects such as asteroids, there is a different definition.) \r\n\r\nIn practice, magnitude is specified for observations through a specific filter, corresponding to an object's brightness in a given wavelength range of light. Numerous \"photometric systems\" for specifying filters, and corresponding magnitudes, exist.  In contrast, the bolometric magnitude is a direct measure of the luminosity of an object: the total electromagnetic energy emitted in unit time. Visual magnitudes correspond to the brightness as perceived by the human eye.","term_approval_level":"A","language_code":"en","term_number":185,"term_in_english":"Magnitude","based_on_current_english_version":null,"linked_terms":[2,15,45,180,455],"alternate_terms":[],"categories":["Naked Eye Astronomy"],"category_ids":[4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/185/"},{"term_name":"Main Sequence","term_definition":"The main sequence is a long, thin grouping of stars on the Hertzsprung–Russell diagram. This sequence is home to stars in the main hydrogen fusion phase of their evolution. For all but the least massive stars, after a star has finished core hydrogen fusion it moves off the main sequence and begins to evolve into the giant phase. Stars on the main sequence are often referred to as dwarfs to differentiate them from giants. Hot stars on the main sequence are brighter than cool stars on the main sequence. The hottest stars fuse hydrogen quickly so often only spend a few million years on the main sequence. Stars like the Sun will spend around ten billion years on the main sequence with cooler stars maintaining stable hydrogen fusion for even longer.","term_approval_level":"A","language_code":"en","term_number":186,"term_in_english":"Main Sequence","based_on_current_english_version":null,"linked_terms":[88,130,143,150,334],"alternate_terms":[],"categories":["Stars","The Sun"],"category_ids":[2,5],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/186/"},{"term_name":"Major Axis","term_definition":"","term_approval_level":"N","language_code":"en","term_number":187,"term_in_english":"Major Axis","based_on_current_english_version":null,"linked_terms":[],"alternate_terms":[],"override_term_number":98,"categories":[],"category_ids":[],"override_url":"https://astro4edu.org/resources/glossary/term/98/","url":"https://astro4edu.org/resources/glossary/term/187/"},{"term_name":"Mars","term_definition":"Mars is the fourth planet from the Sun in the Solar System. It is a rocky, terrestrial planet with a radius of a little under 3400 kilometers (km), just over one half of the Earth's radius. Mars has a very thin atmosphere, a large canyon system, and the tallest mountain in the Solar System: an extinct volcano called Olympus Mons. It is thought to have hosted liquid water earlier in its existence.\r\n\r\nIts typical distance from the Sun is about 228 million km or 1.52 astronomical units (Earth–Sun distances). It takes 687 days to complete one orbit of the Sun. Mars has two small moons, Phobos and Deimos.\r\n\r\nIt is named after the Roman god of war. It is often called the \"red planet\" due to its reddish rusty color. Scientists have sent many landers to Mars over the years to study its composition and atmosphere.","term_approval_level":"A","language_code":"en","term_number":189,"term_in_english":"Mars","based_on_current_english_version":null,"linked_terms":[20,314,354],"alternate_terms":[],"categories":["Naked Eye Astronomy","Solar System"],"category_ids":[4,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/189/"},{"term_name":"Mass","term_definition":"Mass is the amount of matter a body contains. The number, type, and density of atoms that make up the body combine to determine the mass of that body. In everyday speech mass and weight are often used interchangeably but in physics they are not the same. Mass remains the same regardless of location while weight depends on the local gravitational field.\r\n\r\nThere are two concepts of mass: \"gravitational mass\" and \"inertial mass\". \"Inertial mass\" is the property of matter that causes it to resist acceleration; it is the mass in Newton's second law. We see that acceleration is inversely proportional to mass. Hence the more massive a body the less it accelerates for a given force.\r\n\r\n\"Gravitational mass\" is the property of matter that causes it to exert and experience the gravitational force. The more massive a body, the greater force due to gravity it feels.\r\n\r\nAlbert Einstein asserted that these two mass concepts are identical via his \"principle of equivalence\". This is a fundamental concept in physics.\r\n\r\nThe kilogram (kg) is the internationally recognized unit of mass measurement (other units can be used such as: grams, milligrams, ton, ounce, and pound), and mass is represented by the symbol m. Mass is a scalar quantity and is a form of energy. As such, it can be transformed into other forms of energy, such as during nuclear reactions, and it is not strictly conserved. Massless particles, such as photons, the particles that carry electromagnetic radiation, exist in nature.","term_approval_level":"A","language_code":"en","term_number":190,"term_in_english":"Mass","based_on_current_english_version":null,"linked_terms":[76,96,216,220,221,250],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/190/"},{"term_name":"Matter","term_definition":"Matter is anything that occupies space and has a mass. The most familiar states of matter are solid, liquid, gas, and plasma. The heating or cooling of a substance changes its state of matter from one form to another. The quantity of matter is measured in mass units, e.g. kilograms (kg).","term_approval_level":"A","language_code":"en","term_number":191,"term_in_english":"Matter","based_on_current_english_version":null,"linked_terms":[],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/191/"},{"term_name":"Mercury","term_definition":"Mercury is the closest planet to the Sun and the smallest of the eight major planets in the Solar System. It is a rocky, terrestrial planet with a radius of about 2500 kilometers (km), which is slightly larger than the Earth's Moon. It has a mass of 0.055 times the mass of the Earth. Due to its close distance, the Sun is seven times brighter seen from Mercury, and Mercury's surface is much affected by the solar wind. Its very thin exosphere is made of material produced by this interaction, plus matter blasted off the surface due to frequent falling objects. The thin atmosphere cannot keep the planet's temperature, so the surface is extremely cold (-180 degrees Celsius) during the night, and extremely hot (400 degrees Celsius) during the day, and thus very dry.\r\n\r\nIts typical distance from the Sun is about 58 million km, about 0.39 astronomical units (Earth–Sun distances), taking a little under 88 days to complete one orbit. Mercury has no moons known to orbit it.\r\n\r\nAs Mercury orbits the Sun closer than the Earth, it always appears close to the Sun in the sky. Mercury is named after the speedy Roman messenger god, derived from its fast motion across the sky. Two space probes (Mariner 10 and MESSENGER) have visited Mercury, with the BepiColombo mission due to arrive in the mid-2020s.","term_approval_level":"A","language_code":"en","term_number":192,"term_in_english":"Mercury","based_on_current_english_version":null,"linked_terms":[26,314,354,484],"alternate_terms":[],"categories":["Naked Eye Astronomy","Solar System"],"category_ids":[4,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/192/"},{"term_name":"Meridian","term_definition":"Lines of longitude are also called meridians. The Prime Meridian is defined as 0 degrees longitude. It runs through Greenwich in London, UK and is also referred to as the Greenwich Meridian. The antemeridian is halfway around the world at 180 degrees and is the basis for the International Date Line.","term_approval_level":"A","language_code":"en","term_number":193,"term_in_english":"Meridian","based_on_current_english_version":null,"linked_terms":[179],"alternate_terms":[],"categories":["Naked Eye Astronomy"],"category_ids":[4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/193/"},{"term_name":"Messier Object","term_definition":"A Messier object is one of the 110 objects first cataloged by Charles Messier and Pierre Méchain in 1781. Messier and Méchain were looking for comets, which appear fuzzy and extended but found many fuzzy, extended objects that did not appear to move. They cataloged these objects so as not to waste observing time on them if they observed them again. These fuzzy, extended objects beyond our Solar System became known as \"nebulae\".\r\n\r\nIn its present form, the catalog contains 55 star clusters, 39 galaxies, 11 true nebulae, and five star groupings. These objects are favorite targets for amateur astronomers to observe. One astronomer's trash is another's treasure.\r\n\r\nMessier Objects are often referred to by their catalog number prefixed by the letter \"M\". So the spiral galaxy Messier 101 is often referred to as M101.","term_approval_level":"A","language_code":"en","term_number":194,"term_in_english":"Messier Object","based_on_current_english_version":null,"linked_terms":[119,211,332],"alternate_terms":[],"categories":["Galaxies"],"category_ids":[8],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/194/"},{"term_name":"Meteorite","term_definition":"A meteorite is a solid cosmic body between 30 micrometers and 1 meter in size that falls to the surface of the Earth or another celestial body after passing through that body's atmosphere. An object is only described as a meteorite after it has passed through the Earth or other celestial body's atmosphere. While in interplanetary space and while it is passing through the atmosphere it is known as a meteoroid. The shock wave the meteoroid causes in the atmosphere emits light and may be observed as a meteor. \r\n\r\nMeteorites are usually made of stone, iron-stone, or iron, with stone being the most common when we look at types of meteorites that are linked to the sighting of a meteor. However, stony meteorites often look like terrestrial rocks so can be overlooked and iron meteorites are the most common type of meteorite found on the ground. Most meteorites are found in Antarctica or deserts as this is where they are easiest to spot.\r\n\r\nMost of the found meteorites range from a few grams to several kilograms in mass. The largest known one is Hoba, which is more than 60 tons and lies in Namibia. Some meteorites can be large enough to produce an impact crater. Meteorites are named after the area where they fell.","term_approval_level":"A","language_code":"en","term_number":196,"term_in_english":"Meteorite","based_on_current_english_version":null,"linked_terms":[163,197,302],"alternate_terms":[],"categories":["Chemistry","Naked Eye Astronomy","Solar System"],"category_ids":[12,4,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/196/"},{"term_name":"Meteoroid","term_definition":"A meteoroid is a fragment of an asteroid or comet with a size ranging from a few millimeters to several tens of meters. Meteoroids can fall towards any celestial object, with or without an atmosphere, at very high speed. If the celestial body has an atmosphere the meteoroid is slowed down by collisions with atmospheric molecules. We observe meteoroids in Earth's atmosphere as meteors. If the meteoroid is not completely destroyed in the atmosphere (or if the celestial body has no atmosphere) it falls to the surface of that body, and then it is called a meteorite.","term_approval_level":"A","language_code":"en","term_number":197,"term_in_english":"Meteoroid","based_on_current_english_version":null,"linked_terms":[196,302],"alternate_terms":[],"categories":["Naked Eye Astronomy","Solar System"],"category_ids":[4,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/197/"},{"term_name":"Microwave Radiation","term_definition":"Microwave radiation is the region of the electromagnetic spectrum with wavelengths of approximately 1 millimeter to 1 meter. This is the shortest wavelength end of the spectrum of radio waves. Compared to visible light, microwaves have long wavelengths which means  that they carry less energy per photon and thus cannot produce ionization. Microwave's long wavelengths mean they are unhindered by dust, allowing objects embedded in dusty environments to be studied.","term_approval_level":"A","language_code":"en","term_number":198,"term_in_english":"Microwave Radiation","based_on_current_english_version":null,"linked_terms":[69,96,274],"alternate_terms":["millimeter radiation"],"categories":["Telescopes, Instruments and Observatories"],"category_ids":[3],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/198/"},{"term_name":"Milky Way","term_definition":"The Milky Way is the galaxy in which the Solar System resides. It is an aggregate of about 100–400 billion stars. The Solar System is located about 26,600 light years from the center of the Milky Way. In the night sky we can see it as a faint band extending across the sky with its center in the Sagittarius constellation.\r\n\r\nThe Milky Way is a relatively large barred spiral galaxy, and the distribution of stars in it extends to about 100,000 light years along the disk with a thickness of about 1000 light years. The galactic disk formed 8–10 billion years ago.\r\n\r\nThis disk is surrounded by a much more sparse halo of stars, including globular clusters of stars. These globular clusters are some of the oldest objects in the Galaxy, with ages of about 12.5 billion years. In addition to stars, the Milky Way is comprised of the gas and dust of the interstellar medium and dark matter. While the interstellar medium is limited mostly to the disk, the surrounding dark matter halo extends to much larger distances than the stellar halo. \r\n\r\nThe center of the Milky Way hosts a supermassive black hole which is about 4 million times the mass of the Sun. Surrounding the galactic center is a bulge of mostly older stars that is elongated in one direction, forming a bar.","term_approval_level":"A","language_code":"en","term_number":199,"term_in_english":"Milky Way","based_on_current_english_version":null,"linked_terms":[76,115,116,117,118,119,138,314,450,474],"alternate_terms":[],"categories":["Galaxies","Milky Way and Interstellar Medium","Naked Eye Astronomy"],"category_ids":[8,7,4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/199/"},{"term_name":"Minor Planet","term_definition":"","term_approval_level":"N","language_code":"en","term_number":200,"term_in_english":"Minor Planet","based_on_current_english_version":null,"linked_terms":[17,62,87,253,314],"alternate_terms":[],"override_term_number":513,"categories":["Solar System"],"category_ids":[1],"override_url":"https://astro4edu.org/resources/glossary/term/513/","url":"https://astro4edu.org/resources/glossary/term/200/"},{"term_name":"Molecule","term_definition":"A molecule is a group of two or more atoms which are bound together by what are known as chemical bonds and carries zero net electric charge. In chemistry, molecules are limited to atoms bound together by covalent bonds, but in astronomy ionic compounds are sometimes referred to as \"molecules\". \r\n\r\nMolecules are present in conditions that range from the atmospheres of solar type and cooler stars and brown dwarfs; to the atmospheres, oceans, and icy regions of planets and moons; to icy material on comets and asteroids; as well as the colder parts of the interstellar medium. In order to form new stars it takes interstellar molecular clouds made mostly of hydrogen molecules (H2). A new star is formed as part of such a cloud contracts under its own gravity. \r\n\r\nMolecules can be detected in space because under the right circumstances, as they rotate or vibrate, they absorb and emit electromagnetic radiation in narrow wavelength regions, typically in the radio or infrared. These \"molecular lines\" form patterns that allow for the identification of a molecule.","term_approval_level":"A","language_code":"en","term_number":201,"term_in_english":"Molecule","based_on_current_english_version":null,"linked_terms":[31,96,101],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/201/"},{"term_name":"Month","term_definition":"In general terms a month is an interval of time that is linked to the Moon’s motion around Earth. There are different types of months, and each is linked to different aspects of the Moon’s orbit and its motion on the celestial sphere. The various types of months include synodic (based on the cycle of phases); sidereal (based on relative position to the stars); anomalistic (based on the apparent size, which is related to the elliptical orbit of the Moon, and therefore its distance from Earth); and draconic (based on the Moon’s motion on the celestial sphere). Each of these months has a different time period varying between 27 and 29 days. Calendars of various cultures around the world are aligned to the various types of months, and the religious festivals mark various cycles of the Moon.","term_approval_level":"A","language_code":"en","term_number":202,"term_in_english":"Month","based_on_current_english_version":null,"linked_terms":[47,182,203],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/202/"},{"term_name":"Moon","term_definition":"The Moon is a celestial body that is not luminous on its own, but reflects the sunlight falling on it. This produces the Moon's characteristic phases. The Moon is the only major natural satellite of Earth, and ranks fifth among the natural satellites of the Solar System in terms of size and mass. \"Moon\" is capitalized to distinguish it from other natural satellites, or moons, in the Solar System and beyond. Compared with other Solar System moons, the Moon has the largest size relative to the size of the planet it orbits. The Moon follows an elliptical orbit around Earth, at an average distance from Earth of 384,000 kilometers (km). It has no atmosphere and is composed of similar materials to the Earth with an iron-rich core and rocky outer layers. The similarity is no accident: To the best of our knowledge, the Moon formed from the debris of the collision between Earth and a Mars-sized planet around 4.5 billion years ago; most of its material stems from the original Earth's mantle. The surface of the Moon has dark areas known as mare, lighter highlands, and is pockmarked with craters. The surface area of the Moon is 3.79 x 10⁷ square kilometers, its volume is 2.20 x 10¹⁰ cubic kilometers, and its mass is 7.35 x 10²² kilograms (kg). The exact value of the Moon's orbital period around Earth depends on the frame of reference: relative to the distant stars, it completes one orbit every 27.3 days (\"sidereal period\"). For an observer on Earth, the time between two new moons is 29.5 days (\"synodic period\").","term_approval_level":"A","language_code":"en","term_number":203,"term_in_english":"Moon","based_on_current_english_version":null,"linked_terms":[202,204,484],"alternate_terms":[],"categories":["Naked Eye Astronomy","Solar System"],"category_ids":[4,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/203/"},{"term_name":"Moons","term_definition":"Moons are celestial bodies that orbit planets, dwarf planets, or smaller objects such as asteroids. The Earth has one moon, called the Moon. Most other Solar System planets have moons, although Mercury and Venus do not. The dwarf planet Pluto has several moons as do a small number of other dwarf planets and asteroids. Moons are natural satellites; artificial satellites such as those used for communication or scientific purposes are not moons. \r\n\r\nMany moons formed in orbit around the planet, dwarf planet, or other body that they orbit. It is thought that the Moon formed orbiting the Earth from material ejected from a major collision between the Earth and a planetoid in an early stage of the Solar System's formation. Many other (mostly smaller) moons are asteroids which were captured by the gravity of the object they orbit.","term_approval_level":"A","language_code":"en","term_number":204,"term_in_english":"Moons","based_on_current_english_version":null,"linked_terms":[17,87,203,293,412],"alternate_terms":["natural satellite"],"categories":["Exoplanets & Astrobiology","Solar System"],"category_ids":[6,1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/204/"},{"term_name":"Near-Earth Objects","term_definition":"In addition to the Sun and planets, our Solar System contains numerous smaller bodies, notably asteroids and comets. The collision of an asteroid or comet with Earth can have disastrous consequences. So far, none of the objects we know is on a collision course with Earth, but there are a number of objects it makes sense to keep an eye on, nonetheless. Any asteroid or comet that, on its orbit, comes closer to the Sun than 1.3 times the Earth–Sun distance (in astronomical terms: 1.3 astronomical units) is called a Near-Earth Object (NEO). Most NEOs are Near-Earth Asteroids (NEAs). \r\n\r\nA NEO is called a Potentially Hazardous Object (PHO) if it has the following properties: For one, it needs to come closer to Earth's orbit than 5% of the Earth–Sun distance (closer than 0.05 astronomical units). In addition such an object needs to have a certain minimum size – otherwise, it would pose no danger to Earth. Size is difficult to measure for smaller objects in the Solar System, so astronomers instead use a minimal value for such an object's absolute brightness – after all, the larger an object is, the brighter it is likely to be. To qualify as a PHO, an object needs to have an absolute magnitude of 22.0 or brighter (using the magnitude system as astronomy's standard way of measuring brightness). Most PHOs are Potentially Hazardous Asteroids (PHAs).","term_approval_level":"A","language_code":"en","term_number":207,"term_in_english":"Near Earth Objects","based_on_current_english_version":null,"linked_terms":[17,62,314],"alternate_terms":["NEO","NEA","Near-Earth Asteroid","PHA","Potentially Hazardous Asteroid"],"categories":["Solar System"],"category_ids":[1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/207/"},{"term_name":"Names of Stars","term_definition":"Stars have been given different names by many different cultures over time. Most of these names are for bright stars visible with the naked eye.\r\n\r\nMost faint stars do not have names and are referred to either by a number or Greek letter followed by the constellation they belong to, or by a catalog number or coordinate. Variable stars are commonly referred to by two Latin letters followed by the name of their constellation.\r\n\r\nA small number of fainter stars have been named after the astronomer who first observed that star or who led an important study of it.\r\n\r\nStars can often have multiple names from different cultures as well as names based on catalogs or their constellation.\r\n\r\nStars in multiple systems have an uppercase Latin letter at the end of their name to distinguish them from stars in the same system. Exoplanets have lowercase Latin letters added to the end of the name of their host star. Some multiple star systems can also use lowercase latin letters for components. For example the star Mizar, which appears as one star in the Big Dipper asterism, is actually a multiple system of four stars with the names Mizar Aa, Ab, Ba and Bb.\r\n\r\nThe International Astronomical Union (IAU) is the official body for assigning naming designations to celestial bodies and their surface features. The IAU has so far promoted two international public campaigns called NameExoWorlds, the first one in 2015 and the second one in 2019, resulting in exoplanets and their stars officially named after popular names suggested by the public and recognized by the IAU.","term_approval_level":"A","language_code":"en","term_number":208,"term_in_english":"Names of Stars","based_on_current_english_version":null,"linked_terms":[66,106,158,331,485],"alternate_terms":["Star names"],"categories":["Astronomy and Society","Naked Eye Astronomy","Stars"],"category_ids":[11,4,2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/208/"},{"term_name":"Nanometer","term_definition":"One nanometer is one-billionth of a meter. Its symbol is nm. A human hair is approximately 80,000–100,000 nm wide, about the same as the thickness of a piece of paper. A strand of human DNA is about 2.5 nm in diameter. A water molecule is less than 1 nm across. The wavelength of visible light can be measured in nm. Typically, the human eye can detect light with wavelengths between about 380 nm (violet) to 750 nm (red).","term_approval_level":"A","language_code":"en","term_number":209,"term_in_english":"Nanometer","based_on_current_english_version":null,"linked_terms":[],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/209/"},{"term_name":"Navigation","term_definition":"Navigation is the process of finding out where on Earth or where in space you are, your intended direction of travel, and how to get to where you want to go. Historically, the Sun and stars played an important role in navigation. At the simplest level, in a clear night in the northern hemisphere you can use the North Star to determine where north is and choose your direction of travel accordingly. In the southern hemisphere the Southern Cross points towards south. Prior to the development of modern navigational equipment, \"celestial navigation\" was essential for navigation. Navigators would determine the angle of the Sun above the horizon at noon to deduce their latitude, and the comparison of the time of local noon (highest sky position of the Sun) with a clock showing Greenwich Mean Time to determine their longitude. Modern navigation mostly relies on signals from GPS and similar satellite systems.","term_approval_level":"A","language_code":"en","term_number":210,"term_in_english":"Navigation","based_on_current_english_version":null,"linked_terms":[171,179,318],"alternate_terms":[],"categories":["Astronomy and Society","Naked Eye Astronomy"],"category_ids":[11,4],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/210/"},{"term_name":"Nebula","term_definition":"A nebula is a distant celestial object that has the appearance of a cloud. Usually, a nebula consists of interstellar gas and dust. Historically, the term nebulae included any extended fuzzy-looking object, including what we today recognize as galaxies – distant star systems like our own Milky Way galaxy.  Today, the term nebula is restricted to gas and dust clouds that are part of the interstellar medium – the gas and dust between stars within a galaxy. The category includes a number of different kinds of object: Molecular clouds are comparatively cold and dark, and predominantly consist of molecular hydrogen; inside clouds like this is where new stars are formed. Giant molecular clouds can contain up to a few millions of solar masses' worth of hydrogen gas. Young stars frequently emit narrow jets of ionized gas; when those jets excite the surrounding gas, the result is a type of nebula called a Herbig–Haro object. When massive stars have formed, their intense radiation makes the surrounding gas emit characteristic reddish light; the results are nebulae of hot and ionized hydrogen gas which are called HII regions. Other types of nebula are associated with the death of stars: low-mass stars leave behind expanding shells of gas that are (somewhat confusingly) called planetary nebulae. When a high-mass star explodes as a supernova, the ejected gas forms a type of nebula called a supernova remnant.","term_approval_level":"A","language_code":"en","term_number":211,"term_in_english":"Nebula","based_on_current_english_version":null,"linked_terms":[85,119,124,256,349,450],"alternate_terms":[],"categories":["Chemistry","Galaxies","Milky Way and Interstellar Medium"],"category_ids":[12,8,7],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/211/"},{"term_name":"Neptune","term_definition":"Neptune is the eighth, and farthest, major planet from the Sun. Like its neighbor Uranus, Neptune is an ice giant. Neptune's radius is just under 25 thousand kilometers (km), a little less than four times Earth's radius. Neptune has a solid rock core surrounded by a layer of high-pressure water, methane, and ammonia. In the early outer Solar System these chemicals were frozen and accreted onto the young Neptune, hence the name \"ice giant\". Neptune's outer atmosphere is a thick, puffy layer of hydrogen and helium. \r\n\r\nIts typical distance from the Sun is about 4.5 billion km, around 30 astronomical units (Earth–Sun distances). Neptune has at least 14 moons and a faint ring system. As the outermost major planet, Neptune's gravity plays a pivotal role in shaping the orbits of smaller bodies in the Kuiper Belt.\r\n\r\nNeptune is not visible to the naked eye. It was first identified due to the effect its gravity has on the orbit of Uranus. The mathematicians John Couch Adams and Urbain Le Verrier both predicted Neptune's existence and location in the early 1840s. Based on Le Verrier's calculations, Johann Gottfried Galle first identified Neptune in 1846. Neptune is named after the Roman god of the sea.","term_approval_level":"A","language_code":"en","term_number":212,"term_in_english":"Neptune","based_on_current_english_version":null,"linked_terms":[26,129,153,170,234,314,375],"alternate_terms":[],"categories":["Solar System"],"category_ids":[1],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/212/"},{"term_name":"Neutron","term_definition":"A neutron is a subatomic particle with no electric charge. All the matter that we see around us is made up of atoms, and all atoms have the same basic structure: a tiny, dense nucleus of protons and neutrons that carries almost all of the atom's mass, surrounded by electrons. The number of protons is the \"atomic number\" of a nucleus, and each atom with a specific atomic number corresponds to a specific chemical element, whereas the number of neutrons determines which isotope of an element the nucleus represents. \r\n\r\nWhen a star having a mass of approximately eight or more times that of our Sun nears the end of its life, its core collapses triggering a supernova explosion during which most of the protons of the atomic nuclei in its core capture electrons, transforming into neutrons and producing a neutron star, or if there are more than about three solar masses remaining in the imploding core, a black hole.","term_approval_level":"A","language_code":"en","term_number":213,"term_in_english":"Neutron","based_on_current_english_version":null,"linked_terms":[31,165,214,224,263,441],"alternate_terms":[],"categories":[],"category_ids":[],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/213/"},{"term_name":"Neutron Star","term_definition":"A neutron star is a very dense and compact stellar remnant that is left following the collapse of a massive star's core. Stars with masses of about eight solar masses or more end their stellar evolution with their cores collapsing, triggering a supernova explosion. The collapsed core has a density larger than that of most atomic nuclei and is comprised primarily of neutrons. This latter point is due to protons and electrons combining to form neutrons in the extremely hot and dense collapsed core of the massive star. The lower mass limit of a neutron star is 1.4 solar masses, and the upper limit is about 3 solar masses – above this the object would collapse to a black hole. Highly magnetic neutron stars are known as magnetars. The vast majority of known neutron stars are observed as radio pulsars.","term_approval_level":"A","language_code":"en","term_number":214,"term_in_english":"Neutron Star","based_on_current_english_version":null,"linked_terms":[43,213,224,266,312,349,386,526],"alternate_terms":[],"categories":["Stars"],"category_ids":[2],"override_url":null,"url":"https://astro4edu.org/resources/glossary/term/214/"},{"term_name":"New Moon","term_definition":"","term_approval_level":"N","language_code":"en","term_number":215,"term_in_english":"New Moon","based_on_current_english_version":null,"linked_terms":[],"alternate_terms":[],"override_term_number":182,"categories":[],"category_ids":[],"override_url":"https://astro4edu.org/resources/glossary/term/182/","url":"https://astro4edu.org/resources/glossary/term/215/"}]}