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"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.",
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"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.",
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"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).",
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"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.",
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"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.",
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"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.",
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"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).",
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"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.",
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"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.",
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"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.",
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"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.",
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"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.",
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"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.",
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"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.",
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"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.",
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"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.",
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"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.",
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"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.",
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"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.",
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"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.",
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"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.",
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"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.",
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"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.",
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"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.",
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"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.",
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"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).",
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"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.",
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"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.",
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"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.",
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"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.",
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"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\".",
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"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.",
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"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.",
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"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.",
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"term_name": "Helium Fusion",
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"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.",
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"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.",
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"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.",
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"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.",
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"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.",
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"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.",
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"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.",
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"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.",
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"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.",
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"term_in_english": "Ice Giant",
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"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.",
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"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.",
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"term_in_english": "Infrared (IR)",
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"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.",
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"term_in_english": "Infrared Astronomy",
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"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.",
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"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.",
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"term_number": 158,
"term_in_english": "International Astronomical Union",
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{
"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.",
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"term_number": 159,
"term_in_english": "Io",
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{
"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",
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"term_number": 160,
"term_in_english": "Ionization",
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"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.",
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"term_number": 162,
"term_in_english": "Ionosphere",
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"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.",
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"cate