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"term_name": "Spectrum",
"term_definition": "A rainbow forms when water droplets split light into elementary colors, from violet, blue, and green, to yellow, orange, and red. Each color corresponds to a range of wavelengths, and the rainbow colors are arranged in order of increasing wavelength from violet to red. This kind of decomposed light, or electromagnetic radiation in general, into different wavelengths is called a spectrum.\r\n\r\nElectromagnetic radiation is a mixture of light particles called \"photons\". Creating a spectrum amounts to sorting photons by energy and documenting how many photons there are in each given energy range. By a basic law of quantum mechanics, this is equivalent to sorting light by frequency – yet another way of documenting a spectrum.\r\n\r\nIf the amount of energy varies smoothly with wavelength (or photon energy, or frequency), the spectrum is called continuous. In contrast, sharp dips or peaks in a spectrum at certain wavelengths are called absorption and emission lines, respectively. Such lines arise due to transitions between different energy levels within atoms or molecules (or even atomic nuclei), either absorbing or emitting radiation at specific wavelengths. For example, in visible light, stars show continuous spectra with absorption lines. The lines carry information about a star's chemical composition. The analysis of spectra is known as spectroscopy; instruments that allow for the recording of spectra are called spectroscopes, spectrometers, or spectrographs.",
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"term_name": "Spiral Galaxy",
"term_definition": "Spiral galaxies are galaxies that have spiral arms: regions of higher density that form as a galaxy rotates where gas and dust are compressed and new stars are born. Most spiral galaxies are disk galaxies, so the names are sometimes used interchangeably. Most spiral galaxies have a central bulge of stars, and many (including the Milky Way) have a central bar. Spiral galaxies are differentiated from elliptical, lenticular, irregular, and dwarf galaxies (though dwarf spiral galaxies also exist).",
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"term_name": "Star",
"term_definition": "A star is a ball of plasma – atomic nuclei separated from their electrons – that is held together by its own gravity, and prevented from collapse by inner pressure that is the consequence of nuclear fusion processes in the star's core regions. Astronomers, in a slight abuse of physical terminology, commonly use \"gas\" and \"plasma\" interchangeably, and hence also refer to stars as balls of gas. In the atmosphere of a star the plasma may only be partially ionized and (depending on the temperature of the star) even contain some atoms.\r\n\r\nThe star closest to Earth is the Sun. \r\n\r\nIn a more general sense, the word \"star\" is taken to include protostars where nuclear fusion has not yet begun, and stellar remnants such as neutron stars or white dwarfs, which are two possibilities (depending on mass) for what stars turn into once they have exhausted the fuel for their nuclear fusion. Such stellar remnants are not simply plasma balls – a white dwarf can crystallize into an unusual kind of solid after cooling down for billions of years, and neutron stars bear a close similarity to gigantic atomic nuclei.\r\n\r\nWhether viewed with the naked eye or with visible-light telescopes, stars are the most obvious objects in the night sky. In the cosmos, they are typically found within galaxies, each star generally accompanied by one or more planets. The study of how stars form and evolve is an important subfield of astrophysics.",
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"term_name": "Star Cluster",
"term_definition": "Star clusters are groups of stars where all the constituent stars are located in the same area of the sky, with similar distance to us, similar chemical composition, and motion, and are often approximately the same age. The stars in the cluster are likely to have been formed from the same parent cloud of gas. The density distribution could either be centrally concentrated and spherical, or have more complex shapes. They are broadly categorized into two types: Open star clusters and globular clusters. Open clusters are younger (few million years to about 4–5 billion years), have memberships of hundreds to thousands of stars, and could have associated gas and dust remnants from the parent cloud. In the Milky Way, open clusters are typically found in the galactic disk. Globular clusters are among the oldest entities with most being more than 10 billion years old. They are centrally concentrated, spherical shaped clusters containing thousands to millions of stars. In the Milky Way, globular clusters are typically found in the galactic halo.",
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"term_name": "Star Formation",
"term_definition": "The birth of a star results from gravitational collapse of cold and dense regions called cores within giant molecular clouds, which are mostly found in the spiral arms of galaxies. Star formation involves complex physical processes, occurring at different scales, resulting from the effects of gravity, pressure, radiation, magnetic fields, turbulence, chemistry, etc., some of which are still not well understood. Depending on the mass of the parent cloud and accretion processes during the formation stages, the mass of the star can range from 0.08 to a few hundred solar masses. Most stars do not form in isolation but as part of a cluster of stars. During the formation stages, a protostellar disk builds up around the central star, which eventually provides the building material for planets to form.",
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"term_name": "Stellar Evolution",
"term_definition": "Stellar evolution describes the aging of stars and how they change over their life cycle. Unlike in evolutionary biology, stellar evolution does not refer to changes in traits between different generations of stars.\r\n\r\nStars spend most of their life on the main sequence stage of stellar evolution, fusing hydrogen into helium in their cores and releasing energy. As a star ages and starts to run out of hydrogen in its core, the core will contract and may become hot enough to start helium fusion. Depending on the mass of the star, this can lead to the star evolving into a giant or supergiant. In some giants and supergiants, fusion produces heavier and heavier elements. \r\n\r\nStars with initial masses between a half and eight times the mass of our Sun will end up with cores of carbon, oxygen, and/or neon, while hydrogen and helium fusion continues in shells around the core, providing them with a layered onion-like structure. They will eventually lose their outer layers, which go on to form a planetary nebula, leaving only the core as a small, bright white dwarf. \r\n\r\nStars weighing more than eight solar masses continue fusing heavier elements until the nuclei in their core have fused to iron. Further fusion then can liberate no additional energy. This triggers a supernova explosion, which leaves behind either a very compact neutron star or, for very massive stars, a black hole. \r\n\r\nBoth planetary nebulae and supernova explosions eject matter from stars into the interstellar medium. At certain other phases of their evolution, many stars also eject mass by stellar winds, extreme pulsations, or explosions. The ejected matter has been enriched in heavy elements as a result of the nuclear fusion and, in the case of an explosion, nuclear reactions during the explosion itself. This enriched material may be incorporated into future generations of stars. \r\n\r\nThe evolution of stars throughout all these phases can be altered by interaction with a companion in a multiple star system.",
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"term_name": "Stellar Population",
"term_definition": "Stellar population is a term for a group of stars. More generally a stellar population is any population of stars, for instance the stellar population of a galaxy.\r\n\r\nMore specifically this term is often used to refer to different populations of stars that make up a star cluster or galaxy. These populations may have different ages or metallicity, and may have different origins.\r\n\r\nStars in the Milky Way are often divided into Population I stars (the younger stars in the galactic disk with higher metallicities) and Population II stars (the older stars in the galactic halo with lower metallicities). Population III stars are a theorized population of stars with very low metallicity. Population III stars are thought to represent the first generation of stars to have formed after the Big Bang.",
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"term_name": "Stellar Remnants",
"term_definition": "\"Stellar remnants\" is the collective term for white dwarfs, neutron stars, and stellar-mass black holes. These represent the final stage of stellar evolution after a star has both finished hydrogen burning on the main sequence and evolved through the giant phase. Stellar remnants are very compact compared to stars. White dwarfs (the largest type of stellar remnant) have approximately a solar mass of material in an object the size of Earth. Stellar remnants do not generate heat from nuclear fusion in their cores. In close binary systems stellar remnants can be the source of novae, Type Ia supernovae, or (if two stellar remnants spiral towards each other and collide) bursts of gravitational waves.",
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"term_name": "Stellar Structure",
"term_definition": "Simply speaking, stars are balls of gas (or better: plasma), held together by their own gravity, and kept from collapsing by their internal pressure. Models of stellar structure describe the details of how the conditions in a star's core lead to different kinds of nuclear fusion reactions, how the energy set free in those reactions is transported outward by radiation, conduction, or convection, and how the balance between gravity and pressure in the different segments of the star is maintained. In this way, stellar structure models also link the mass, luminosity, composition, and expected lifetime of a star. They also describe the role of stellar rotation, stellar evolution (how stars change over time), and the physics of stellar remnants.",
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"term_definition": "The Sun is the star that is closest to the Earth. For astronomers, it is a star of type \"G2V\". This means the Sun is a main sequence star with a typical temperature (\"effective temperature\") of 5800 kelvins (K). Main sequence stars are stable, with energy released by hydrogen fusion in their core balancing the inwards force due to gravity. The Sun appears white to the human eye as it emits lots of light all across the visible spectrum. When lower in the sky, increased atmospheric extinction can make the Sun appear yellow or orange, hence its common depiction as yellow. Stars range from more than 1000 times brighter than the Sun to some 1000 times fainter, but the brighter ones are relatively rare: the Sun is brighter (and heavier) than most (perhaps some 85%) of the stars in the Galaxy.\r\n\r\nFor astronomers, the Sun is interesting because of its proximity, which means that the surface can be resolved in greater detail, allowing structures and phenomena to be studied. For example, detailed studies of solar activity, which is related to the Sun's magnetic fields, can include: sunspots (cooler areas), flares (short-lived bright flashes), and even coronal mass ejections (electrically charged particles flung away from the Sun). Physicists have also detected elementary particles known as neutrinos from the Sun's core; this is direct evidence for nuclear fusion processes. The element helium was first detected in the solar spectrum, hence the name helium, which comes from Helios (in Greek mythology the Sun god).",
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"term_name": "Sun Path (Day Arc)",
"term_definition": "The Sun's path or day arc traces the apparent motion of the Sun in the sky, as seen by an observer at a fixed position on Earth. Each day, the path traces out an arc, starting with sunrise in the eastern part of the sky and ending with sunset in the western part. Only at the spring and autumn equinoxes does the Sun rise exactly due east and set exactly due west. At the winter solstice, the arc is shortest and lowest in the sky. From then on, the length of that arc, and with it the length of the day, increases, and each successive arc is higher in the sky than the day before, reaching maximum height, and maximum day length, at the summer solstice. This process then repeats in the opposite order, again reaching the shortest, lowest arc at winter solstice. \r\n\r\nNorth of the Arctic Circle and south of the Antarctic Circle the Sun does not rise for extended periods in winter or set for extended periods in summer. Hence, they can have nights that last several months in the winter and days that last several months in the summer. During one of these polar days the Sun travels around the sky in a circle, moving from a high point at midday to a low point at midnight without ever crossing the horizon.\r\n\r\nThe set of solar arcs can be captured in a long-term exposure image called a solargraph.",
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"term_name": "Sundial",
"term_definition": "The Sun's position in the sky has long been used to define time, with local noon at the point of the Sun's greatest height above the horizon and the day defined in the narrow sense as the period between sunrise and sunset, and in the broader sense as the length of time between one local noon and the next. A sundial is a device that projects the direction of the Sun onto a surface with markings, usually by tracking the shadow of an elongated pointer known as the \"gnomon\". When that shadow points at the marking designated \"12\", for instance, one would read off the time as 12 o'clock. Simple sundials only show apparent solar time, which is directly linked to the Sun's sky position. More elaborate versions have provisions for allowing the determination of mean solar time.",
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"term_name": "Sunspot",
"term_definition": "A sunspot is a temporary, cool region, caused by a strong magnetic field in the Sun's photosphere. Sunspots are areas where tubes of magnetic flux emerge from deeper in the Sun. The high magnetic field increases the magnetic pressure in these regions. To stay at the same pressure as their surroundings, the gas and plasma pressure in the sunspot must drop, making it cooler than its surroundings. As they are cooler than the surrounding photosphere, sunspots can be seen through a telescope as dark patches/blotches on the surface of the Sun. Sunspots range in size from tens of kilometers across to over a hundred thousand kilometers across. They can persist for timescales between a few days and a few months. The number and location of sunspots on the Sun varies over the solar cycle. Other stars are also thought to have spots caused by their magnetic fields.",
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"term_name": "Sunspot Cycle",
"term_definition": "An approximately 11-year cyclic variation in the number of sunspots that form on the Sun, corresponding to a variation in the solar activity. A new sunspot cycle begins after a period of solar minimum, when the Sun has few to no sunspots. At the beginning of a new sunspot cycle, sunspots form at latitudes around +/- 30 degrees (north or south) from the Sun's equator. As the cycle continues, new sunspots form at the latitudes closer to the solar equator. After a while the number of sunspots forming decreases as the cycle moves past solar maximum to solar minimum, which indicates the end of the cycle.",
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"term_name": "Supergiant Star",
"term_definition": "Supergiants are the largest and most luminous stars. They can be several hundred times larger than the Sun and many thousand times more luminous. They occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between −3 and −8. The temperature range of supergiant stars spans from about 3400 kelvins (K) to over 20,000 K. They are either massive stars or are in a very late phase of stellar evolution. Supergiant stars can be identified on the basis of their spectra, with distinctive lines sensitive to high luminosity and low surface gravity: these spectral lines are narrow compared to the width of lines in smaller stars. Typical examples of supergiant stars are Betelgeuse in Orion and the Cepheid variables.",
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"term_name": "Supermassive Black Hole",
"term_definition": "As the term suggests supermassive black holes (SMBH) are the largest class of black holes, with masses ranging from millions to billions of times the mass of the Sun. Observational data suggests that all large galaxies seem to harbor a SMBH at their center. The Milky Way has a SMBH known as Sagittarius A* with a mass of around 4.5 million times that of the Sun, and a diameter of around 40 million kilometers. The large amount of mass in a small volume leads to black holes having immense gravitational fields (deep gravitational potential wells). Since 2019, scientists have used data from a network of radio telescopes located around the world to build an image of the event horizons around SMBHs. As of early 2023 two SMBHs have been imaged in this way: Sagittarius A* and the SMBH (6.5 billion times more massive than the Sun) at the center of the galaxy M87, located just over 50 million light years away.",
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"term_name": "Supernova",
"term_definition": "A supernova is a massive stellar explosion. Supernovae briefly become by far the most luminous object in their galaxy before fading over the course of a few years. There are two main pathways that lead to supernovae. The first (Type Ia) involves a white dwarf accreting matter from a binary companion star. Once the white dwarf is destabilized, either by achieving a mass of over 1.4 solar masses (known as the Chandrasekhar limit) or by accumulating enough helium on its surface, it explodes, leaving no remnant. The other main pathway that forms a supernova (Type II) is the evolution of a star with a mass greater than eight solar masses. At the end of such a star's evolution it explodes, resulting in a neutron star or (for the most massive stars) a stellar-mass black hole. \r\n\r\nSupernovae are the source of many of the chemical elements, especially those heavier than magnesium.",
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"term_name": "T-Tauri Star",
"term_definition": "T-Tauri stars are a class of low mass young variable stars. They are named after the prototype, T-Tauri, a variable star in the Taurus star-forming region. T-Tauri variables are found in groups of young stars. Their \"surface\" temperatures (effective temperatures) are similar to those of F, G, K, and M spectral type main sequence stars of the same mass, but they are significantly more luminous because their diameter is larger. These contracting young stars have not started fusing hydrogen and are powered by the release of gravitational energy during their contraction. T-Tauri stars can be identified by their irregular optical variability and spectrum. They emit intense and variable X-ray and radio waves. Brightness variability is also caused by clumps in the circumstellar disk surrounding T-Tauri stars.",
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"term_name": "Taurus",
"term_definition": "Taurus, \"the bull\", is a constellation in the Zodiac so it's close to the ecliptic – the intersection of the celestial sphere with the plane defined by the orbit of the Earth around the Sun. Hence, from our point of view here on Earth, we can regularly find the planets, and also the Sun, in this constellation – in the case of the Sun from May 14 to June 21. (Of course if the Sun is there, we cannot see the constellation's stars.) Taurus 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. In the northern hemisphere, it's prominently visible in the night sky in winter. Its brightest star is the reddish Aldebaran.",
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"term_name": "Temperature",
"term_definition": "In a physical system that has \"settled down\" over time, reaching what is called thermal equilibrium, each way the system can change (each \"degree of freedom\") has, on average, the same energy. The system's temperature is a measure of how large that average energy is.\r\n\r\nTemperature is important in all disciplines of natural science as well as most aspects of everyday life. The most common temperature scales are the Celsius scale (former name centigrade, denoted as °C), the Fahrenheit scale (denoted as °F), and the kelvin scale (denoted as K). The latter is predominantly used for scientific purposes. The lowest theoretical temperature is absolute zero (0 K = -273.15 °C), at which no more thermal energy can be extracted from a body. In the nucleus of stars, the temperature exceeds 10 million K, while in cold molecular clouds of interstellar gas it can be as cold as 10 K.",
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"term_name": "Terrestrial Planet",
"term_definition": "A terrestrial planet is one that is made up mostly of material such as rock and iron. Terrestrial planets lack the puffy atmosphere of hydrogen and helium found in giant gas planets, instead having much smaller atmospheres or no atmosphere at all. Terrestrial planets generally have smaller masses than giant planets and are smaller in size.\r\n\r\nIn the Solar System the terrestrial planets are Mercury, Venus, Earth, and Mars.\r\n\r\nOne of the main focuses of exoplanet astronomy has been the search for terrestrial planets of similar size and composition to Earth which lie in their star's habitable zone.",
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"term_name": "Theory of General Relativity",
"term_definition": "(The theory of) general relativity, published in 1915, is Albert Einstein's theory linking space, time, and gravity. In that theory, gravity is not an ordinary force. Instead, a mass or other source of gravity will distort space and time in its vicinity. This distortion changes how bodies in free fall move. For instance, when a planet orbits the Sun, it's not because of an attractive force – it's because the planet is taking the straightest possible path through space and time (or more precisely: through \"spacetime\", since in Einstein's theory, there is no unique way of separating space from time). The core equations of general relativity, known as Einstein's equations, provide a direct link between spacetime geometry and the mass and similar properties of the matter contained in that spacetime (specifically the energy density or, equivalently, the mass density, and also pressure). Paraphrasing John Wheeler, by these equations, matter tells spacetime how to curve, while spacetime tells matter how to move. Einstein's theory predicts corrections to the orbits of astronomical objects, which can be observed both in the Solar System – most prominently in the orbit of Mercury – and much more clearly in binary neutron stars, where two very compact objects orbit each other. Its predictions for the influence of gravity on clocks play a role in satellite navigation systems. The theory also predicts new phenomena, which have become an integral part of astrophysics: the deflection of light by mass, observable as the so-called gravitational lensing effect; black holes as the ultra-compact end states of certain stars and central ingredients of galaxies, and gravitational waves as a way of gaining information about, among other things, merging black holes. General relativity is also at the foundation of the cosmological models for expanding universes.",
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"term_name": "Thermal Radiation",
"term_definition": "All objects emit electromagnetic radiation in a way directly related to their temperature. This radiation is called thermal radiation. For high enough temperature, some of the thermal radiation is visible to the naked eye, e.g.: a hot stove-top, a heating element, or a poker in the fire, glowing red; molten metal glowing yellow-white; the blueish-white glow during certain welding processes, so bright welders require eye protection. For cooler objects, we do not see the glow, but these objects emit thermal radiation in the infrared range or as radio waves.\r\n\r\nThe simplest form of thermal radiation is \"blackbody radiation\": Thermal radiation emitted by an ideal object that absorbs all incoming radiation, regardless of wavelength. The spectrum of that radiation (the information about how the radiation energy is distributed between the different wavelengths) is governed by a mathematical function called the Planck curve, which only depends on the object's temperature. The Planck curve also specifies that the total energy output of thermal radiation steeply increases with temperature.\r\n\r\nThe radiation we receive from stars is thermal radiation that closely follows the Planck curve. This, in turn, makes it possible to assign an \"effective temperature\" to each star. The Sun, with a temperature of 5500 degrees Celsius (5780 kelvins) shines in a range of colors that add up to what we define as white. Stars with a lower effective temperature appear reddish, such as red giants or red dwarfs. Hotter stars appear blueish and are typically very bright. At temperatures of tens of thousands or even millions of degrees, most thermal radiation is emitted in the ultraviolet, X-ray, or even gamma ray regions.\r\n\r\nThermal radiation is related to the concept of thermal equilibrium. Objects are in thermal equilibrium if they have the same temperature. Thermal radiation is a consequence of objects striving towards thermal equilibrium with the electromagnetic fields permeating all of space.",
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"term_name": "Tide",
"term_definition": "On Earth, the gravitational force is almost constant, pointing what we call downwards. But over larger distances, gravity's strength and direction vary, and those differences are called tidal forces. The Earth and the Moon, for instance, orbit a common center of mass due to their mutual gravitational attraction, but objects on the side of Earth nearer the Moon will be accelerated a bit more strongly towards the Moon, objects on the opposite side of Earth somewhat less. Earth's ocean water follows those tidal accelerations, forming \"tidal bulges\" directly under the Moon and on the opposite side, which produces Earth's tides.",
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"term_name": "Time",
"term_definition": "All of us have a basic idea of what time is: A progression of events, one after the other, from the past through the present to the future. Yet time is not something that can be seen, heard, smelled, touched, or tasted. It can be measured, though, and that is the aspect of time that matters in astronomy and in physics.\r\n\r\nWe measure time by comparing the duration of whatever is happening with the duration measured on a clock or, for longer periods of time, on a calendar. Traditionally, measurements of time of this kind have been based on the Earth's rotation. The time it takes the Earth to complete one full rotation (as judged by the position of the Sun) defines the length of one day. The usual subdivisions – one day divided into 24 hours, each hour divided into 60 minutes, each minute into 60 seconds – provide additional units of time.\r\n\r\nSince 1967, the definition of time has instead been based on the duration of a second, as measured by a Caesium-133 atomic clock (\"SI second\"). Several systems of time are defined on this basis, notably Universal Time (UTC) which is used in official time-keeping world-wide, and what is known as the Julian Date and its variations, a continuous counting of days used in astronomy.\r\n\r\nEinstein's special theory of relativity and his general theory of relativity have shown that the time that passes on a clock depends both on the motion and on the influence of gravity of that clock. These relativistic effects need to be accounted for in highly accurate measurements of time, such as the ones using the satellites of the Global Positioning System (GPS).",
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"term_name": "Time Zone",
"term_definition": "A time zone is a region where a uniform standard time is used. Our measures of time are linked to Earth's rotation. In particular, the moment of noon is linked to the Sun's highest position in the sky. Local noon strongly depends on longitude: An observer located eastward of another observer will see the Sun rise, and reach noon, earlier. All locations at the same longitude have the same moment of local noon, so in principle, we could define a common measure of time for all those locations. In practice, a common time is defined for a segment of longitude or for a country or a region of a country. For most locations, local noon will be close to, but slightly before or after the time zone's official noon. In this scheme, differences in official time almost always amount to an integer number of hours (although some defined time zones deviate from this scheme by, say, half an hour).",
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"term_name": "Titan",
"term_definition": "Titan is the largest moon of Saturn. It is larger than Earth's Moon and Mercury, and is second largest amongst the moons in the Solar System. It is the only body other than Earth known to have liquid oceans, seas, and rivers on the surface. These are, however, made up of chemicals called hydrocarbons. Titan is believed to have a rocky core and an ice-crust made of water. It is thought that the liquid water beneath the ice-crust could possibly harbor life. It has a thick atmosphere, made up mostly of nitrogen, that makes it look very smooth in pictures.",
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"term_name": "Total Lunar Eclipse",
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"term_name": "Total Solar Eclipse",
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"term_name": "Trans-Neptunian Object",
"term_definition": "Trans-Neptunian objects (TNOs) are a class of small objects and dwarf planets that orbit the Sun beyond the orbit of Neptune. These are defined as having a typical distance from the Sun (the semi-major axis of their orbit) that is greater than Neptune's typical distance from the Sun (30.1 astronomical units or Earth–Sun distances). There are over 2000 known TNOs. Most are members of the Kuiper Belt although more distant objects are members of a population called the scattered disk.",
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"term_name": "Transit",
"term_definition": "A transit occurs when a celestial body passes between another celestial body of larger angular size and the observer. From the point of view of the observer, the obscuring celestial body moves in front of the obscured body, blocking the view of a fraction of the surface of the obscured object. If the obscuring body that passes between the observer and the obscured object has a larger angular size than the latter, the event is referred to as an occultation rather than a transit. A transit viewed by one observer may not be seen by another observer looking at the same system from a different angle.\r\n\r\nIn the Solar System both Mercury and Venus can sometimes transit across the face of the Sun when viewed from Earth. Moons orbiting Solar System planets are commonly seen to transit across the face of their host planet when viewed from Earth.\r\n\r\nPlanets orbiting other stars (exoplanets) are often discovered when they transit their host star, blocking out a little of the star's light and making it appear to dim slightly when viewed from the observer's location. A planet orbiting a star will only transit when viewed from Earth if the plane of its orbit intersects with the line of sight from the Earth. As such searching for planets using the transit method can only ever detect a fraction of the planets in the Galaxy. Transits can also be used to estimate the size of an exoplanet.",
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"term_name": "Triton",
"term_definition": "Triton is the largest moon of the planet Neptune. It is larger than Pluto, and in fact, because of the unusual properties of its orbit and from what we know of its composition, Triton may well be a dwarf planet captured by Neptune. Triton has an icy surface, with water ice covered by a layer of frozen nitrogen, surrounding a mostly rocky core. Voyager 2, so far the only space probe to visit Triton, discovered that the moon is geologically active, with geysers blowing out nitrogen gas, and very probably with ice volcanoes (\"cryovolcanoes\") – analogous to Earth's volcanoes, but with water and ammonia instead of liquified rock.",
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"term_name": "Trojans",
"term_definition": "Trojans are asteroids which orbit the Sun with the same orbit as one of the major planets, but are either one sixth of an orbit ahead of the planet or one sixth of an orbit behind the planet. These points, ahead of the planet and behind it, are two special gravitational points known as Lagrange points 4 and 5. Here a small body can sit stably in its orbit without being kicked around by the gravity of the planet. \r\n\r\nThe largest groups of trojans in the Solar System are those which share an orbital path with Jupiter. Those which lie 60 degrees ahead of Jupiter are sometimes referred to as Greeks and are named after mythical Greek characters from the Trojan War. Those which are 60 degrees behind Jupiter are named after Trojan characters.",
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"term_name": "Tropical Year (Solar Year)",
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"term_name": "Ultraviolet",
"term_definition": "Ultraviolet radiation, abbreviated \"UV\", is electromagnetic radiation made up of elementary waves with wavelengths between 10 and 380 nanometers. These wavelengths lie between X-rays and visible light. Most UV radiation from astronomical objects is absorbed by Earth's atmosphere, so UV astronomy necessarily requires space telescopes, such as the Hubble Space Telescope, or more specialized observatories like GALEX. UV radiation can be used to study hot, young stars, determine the chemical composition of the interstellar medium, study solar activity and our Sun's corona, or explore certain properties of active galactic nuclei.",
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"term_name": "Umbra",
"term_definition": "Umbra is Latin for \"shadow\". In the context of eclipses, the umbra is that region of space where an observer sees the one body block the other's light completely. For a solar eclipse, for instance, any observer in the umbra will see the Sun's disk covered completely by the disk of the Moon. An observer located in the penumbra region, on the other hand, will only see a partial eclipse – in the case of a solar eclipse, the Sun's disk covered only partially by the Moon. If an observer sees the covering object as too small to cover the object behind it completely, as in an annular solar eclipse, the observer is said to be in the antumbra. Alternate meaning: the inner, darker region of a sunspot is called its umbra, the surrounding less dark region its penumbra.",
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"term_name": "Universal Time (UT)",
"term_definition": "Universal Time, abbreviated UT, is an umbrella term for several ways of defining the measurement of time. UT1 is the mean solar time at longitude zero (the historical location of the Royal Greenwich Observatory). Mean solar time defines the length of a day as the average duration between one noon (highest position of the Sun in the sky) and the next. In practice, the necessary measurements are made not with the Sun, but with distant astronomical objects such as quasars at night. UTC, Universal Time Coordinated, is a time standard based on the timekeeping of a specific large set of atomic clocks (\"international atomic time\" or TAI), but with occasional extra seconds (\"leap seconds\") added to some days to ensure UTC and UT1 never diverge by more than 0.9 seconds.",
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"term_name": "Universe",
"term_definition": "Space, time, matter, energy, objects, phenomena, and laws that govern the underlying fundamental mechanisms are contained within the Universe. The Universe is often referred to as the cosmos, given that all the various aspects of the Universe come together in an \"ordered\" system, although there are subtle technical differences between the two terms. Universe is derived from the Latin universus. Scientists often refer to the observable Universe, to emphasize the limits of what can be observed and measured.",
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