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astronomical unit | Space and Astronomy | 1 | A unit of length used primarily for measuring distances within the Solar System or secondarily between the Earth and distant stars. Originally conceived as the semimajor axis of the Earth's orbit around the Sun, the astronomical unit is now more rigidly defined as exactly 149,597,870.7 kilometres (92,956,000 miles; 4.8481×10−6 parsecs; 1.5813×10−5 light-years). |
au | Space and Astronomy | 1 | An astronomical unit – the distance between Earth and the Sun, or about 93 million miles (150 million kilometers). |
astronomy | Space and Astronomy | 1 | The scientific study of celestial objects and phenomena, the origins of those objects and phenomena, and their evolution. |
astrophotography | Space and Astronomy | 1 | The photography of astronomical objects, celestial events, or areas of the night sky. |
astrophysics | Space and Astronomy | 1 | The study of physical aspects of stars, galaxies and the universe, such as temperature, gravity and light. |
atmosphere | Space and Astronomy | 1 | The various layers of gases surrounding the Earth and held in place by gravity. The Earth's atmosphere is the origin of the weather phenomena studied in meteorology. Atmospheric composition, temperature, and pressure vary across a series of distinct sublayers including the troposphere and stratosphere. |
axial precession | Space and Astronomy | 1 | A slow, continuous, gravity-induced change (a precession) in the orientation of an astronomical body's axis of rotation. The term often refers in particular to the gradual shift in the orientation of Earth's rotational axis with respect to its orbital plane over a cycle of approximately 25,772 years, which is caused predominantly by the gravitational influence of the Moon and the Sun on the Earth's equatorial bulge. The phenomenon is similar to but much larger in magnitude than other changes in the alignment of Earth's axis such as nutation and polar motion, and is the cause of the apparent precession of the equinoxes in the night sky. |
axial tilt | Space and Astronomy | 1 | The angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane. Axial tilt usually does not change considerably during a single orbital period; Earth's axial tilt is the cause of the seasons. Axial tilt is distinct from orbital inclination. |
azimuth | Space and Astronomy | 1 | An angular measurement of an object's orientation along the horizon of the observer, relative to the direction of true north. When combined with the altitude above the horizon, it defines an object's current position in the spherical coordinate system. |
babcock model | Space and Astronomy | 1 | A model that attempts to explain magnetic and sunspot patterns observed on the Sun. |
barycenter | Space and Astronomy | 1 | The common center of mass about which any two or more bodies of a gravitationally bound system orbit. The barycenter is one of the foci of the elliptical orbit of each body participating in the system; its location is strongly influenced by the mass of each body and the distances between them. For example, in a planetary system where the mass of the central star is significantly larger than the mass of an orbiting planet, the barycenter may actually be located within the radius of the star, such that the planet appears to orbit the star itself, though both bodies actually orbit the shared barycenter. |
baryogenesis | Space and Astronomy | 1 | The process by which the class of subatomic particles known as baryons were generated in the early Universe, including the means by which baryons outnumber antibaryons. |
big bang | Space and Astronomy | 1 | The widely accepted theory describing the origin of the universe as an initial singularity followed by rapid expansion. |
binary star | Space and Astronomy | 1 | A star system consisting of exactly two stars orbiting around their common barycenter. The term is often used interchangeably with double star, though the latter can also refer to an optical double star, a type of optical illusion which is entirely distinct from true binary star systems. |
black hole | Space and Astronomy | 1 | A concentration of mass so compact that it creates a region of space from which not even light can escape. The outer boundary of this region is called the event horizon. |
break-up velocity | Space and Astronomy | 1 | The surface velocity at which the centrifugal force generated by a rapidly spinning star matches the force of Newtonian gravity. At rotational velocities beyond this point, the star begins to eject matter from its surface. |
brown dwarf | Space and Astronomy | 1 | A substellar object that is too low in mass to sustain the nuclear fusion of hydrogen-1 in its core, with the latter being a characteristic of stars on the main sequence. Brown dwarfs can still generate energy from gravitational contraction and by the fusion of deuterium. |
calibrator star | Space and Astronomy | 1 | A star used for calibration of high-powered telescopes. |
coudé spectrograph | Space and Astronomy | 1 | This is a spectrograph placed at the Coudé focus of a reflecting telescope. The focus remains stationary as the telescope is re-oriented, which is advantageous for the stable mounting of heavy spectroscopic instruments. |
celestial equator | Space and Astronomy | 1 | The imaginary great circle of a body's celestial sphere that is coplanar with the body's terrestrial equator. On Earth, the plane of the celestial equator is the basis of the equatorial coordinate system. Due to Earth's axial tilt, this plane is currently inclined at an angle of 23.44 degrees with respect to the ecliptic. |
celestial mechanics | Space and Astronomy | 1 | The branch of astronomy that studies the motions of all types of astronomical objects, including stars, planets, and natural and artificial satellites, among others. |
celestial meridian | Space and Astronomy | 1 | See meridian. |
celestial pole | Space and Astronomy | 1 | One of two coordinates in the Earth's sky at which a hypothetical indefinite extension of the Earth's axis of rotation "intersects" the celestial sphere, i.e. the two points in the sky that are directly overhead the terrestrial North and South Poles, around which all fixed stars appear to revolve during the course of a day. The celestial poles form the north and south poles of the equatorial coordinate system. |
celestial sphere | Space and Astronomy | 1 | An imaginary sphere that encompasses the Earth's entire sky and is stationary with respect to the background stars. It is the basis for spherical astronomy. |
centaur | Space and Astronomy | 1 | A small Solar System body with either a perihelion or a semi-major axis between those of the outer planets, i.e. generally inward of the Kuiper belt but beyond the Jupiter trojans. Centaurs are cis-Neptunian objects that typically exhibit characteristics of both asteroids and comets, and generally also have unstable orbits because they cross the orbits of one or more of the giant planets. |
central massive object | Space and Astronomy | 1 | Any very large concentration of mass at the center of a galaxy, typically either a supermassive black hole or a compact stellar nucleus, but sometimes both. |
cmo | Space and Astronomy | 1 | Any very large concentration of mass at the center of a galaxy, typically either a supermassive black hole or a compact stellar nucleus, but sometimes both. |
chromospheric activity index | Space and Astronomy | 1 | A parameter indicating the magnetic activity in a star's chromosphere. One measure of this activity is log R′HK, where R′HK is the ratio of the equivalent width of a star's singly ionized calcium H and K lines, after correction for photospheric light, to the bolometric flux. Schröder et al. (2009) divide solar-type stars into four groups depending on their activity index: very active (log R′HK above −4.2), active (−4.2 to −4.75), inactive (−4.75 to −5.1), and very inactive (below −5.1). |
color index | Space and Astronomy | 1 | A numeric value that is used to compare the brightness of a star measured from different frequency bands of the electromagnetic spectrum. Because the energy output of a star varies by frequency as a function of temperature, the color index can be used to indicate the star's temperature. |
comet | Space and Astronomy | 1 | A relatively small, icy body that displays extended features when it approaches the Sun. The energy from the Sun vaporizes volatiles on a comet's surface, producing a visible coma around the cometary body. Sometimes a comet can produce a long tail radiating away from the Sun. |
commensurability | Space and Astronomy | 1 | A property of two objects orbiting the same body whose orbital periods are in a rational proportion. For example, the orbital period of Saturn around the Sun is very nearly 5/2 the orbital period of Jupiter. |
common proper motion | Space and Astronomy | 1 | A term used to indicate that two or more stars share the same motion through space, within the margin of observational error. That is, either they have nearly the same proper motion and radial velocity parameters, which may suggest that they are gravitationally bound or share a common origin, or they are known to be gravitationally bound (in which case their proper motions may be rather different but average to be the same over time). |
compact star | Space and Astronomy | 1 | Any astronomical body with a very high mass relative to its radius, compared to most ordinary atomic matter. The term typically refers to very high-density objects such as white dwarfs, neutron stars, and black holes, or to stellar remnants with very small radii. |
compact stellar nucleus | Space and Astronomy | 1 | See nuclear star cluster. |
conjunction | Space and Astronomy | 1 | A phenomenon during which two astronomical objects or spacecraft have either the same right ascension or the same ecliptic longitude as observed from a third body (usually the Earth), such that, from the observer's perspective, the objects appear to closely approach each other in the sky. |
constellation | Space and Astronomy | 1 | An area of the night sky that contains a group of stars that seems to form a certain shape or picture. |
corona | Space and Astronomy | 1 | The outer layer of the Sun’s atmosphere. It is visible to the naked eye as a white halo during a solar eclipse when the photosphere and chromosphere have been completely occluded. |
coronal mass ejection | Space and Astronomy | 1 | An explosive release from the Sun’s outer atmosphere of a massive cloud of billions of tonnes of plasma material. |
cme | Space and Astronomy | 1 | An explosive release from the Sun’s outer atmosphere of a massive cloud of billions of tonnes of plasma material. |
cosmic dust | Space and Astronomy | 1 | Clouds of fine solid particles of matter in interstellar space. |
cosmic ray | Space and Astronomy | 1 | High-energy rays of charged particles coming from Space. |
cosmogony | Space and Astronomy | 1 | Any model concerning the origin of either the universe or the cosmos. |
cosmology | Space and Astronomy | 1 | The study of the origin, development and structure of the universe. |
critical velocity | Space and Astronomy | 1 | The surface velocity at the equator of a rotating body where the centrifugal force balances the Newtonian gravity. At this rotation rate, mass can be readily lost from the equator, forming a circumstellar disc. See also break-up velocity. |
culmination | Space and Astronomy | 1 | The apparent movement of an astronomical object (e.g. the Sun, the Moon, a planet, a star, a constellation, etc.) across the observer's local meridian. During each day, the Earth's rotation causes every astronomical object to appear to move along a circular path on the celestial sphere, creating two points at which it crosses the meridian: an upper culmination, at which the object reaches its highest point above the horizon, and a lower culmination, at which it reaches its lowest point, nearly 12 hours later. When not otherwise qualified, the time of culmination typically refers to the time at which the upper culmination occurs. |
debris disk | Space and Astronomy | 1 | A ring-shaped circumstellar disc of dust and debris orbiting its host star. It is created by collisions between planetesimals. A debris disk can be discerned from an infrared excess being emitted from the star system, as the orbiting debris re-radiates the star's energy into space as heat. |
declination | Space and Astronomy | 1 | In the equatorial coordinate system, the celestial equivalent of terrestrial latitude. Coordinates north of the celestial equator are measured in positive degrees from 0° to 90°, while coordinates to the south are measured in negative degrees. See also right ascension. |
decretion disk | Space and Astronomy | 1 | A circumstellar disc formed from gas ejected from a central star that now follows a nearly Keplerian orbit around it. This type of disk can be found around many Be stars. |
deep-sky object | Space and Astronomy | 1 | Any astronomical object that is not an individual star or an object within the Earth's Solar System. The classification is used mostly in amateur observational astronomy to distinguish faint objects in the night sky such as star clusters, nebulae, and galaxies. |
dso | Space and Astronomy | 1 | Any astronomical object that is not an individual star or an object within the Earth's Solar System. The classification is used mostly in amateur observational astronomy to distinguish faint objects in the night sky such as star clusters, nebulae, and galaxies. |
degenerate star | Space and Astronomy | 1 | A star composed of degenerate matter, e.g. a white dwarf or a neutron star. These stars are in an advanced state of evolution and have suffered extreme gravitational collapse, such that normal atoms cannot exist in them. |
descending node | Space and Astronomy | 1 | The orbital node at which an orbiting object moves south through the plane of reference (in geocentric and heliocentric orbits) or at which the orbiting object moves toward the observer (in orbits outside of the Solar System). Contrast ascending node. |
detached object | Space and Astronomy | 1 | A dynamical class of minor planet in the outer reaches of the Solar System whose point of closest approach to the Sun is so distant that the object is only moderately or weakly affected by the gravitational influence of Neptune and the other known planets, such that it appears to be "detached" from the rest of the Solar System. Detached objects are thus distinct from other populations of trans-Neptunian objects, such as cubewanos and scattered disc objects. |
direct motion | Space and Astronomy | 1 | See prograde motion. |
diurnal motion | Space and Astronomy | 1 | The apparent motion of an astronomical object (e.g. the Sun, a planet, or a distant star) around the two celestial poles in the Earth's night sky over the course of one day. Diurnal motion is caused by Earth's rotation about its own axis, such that every object appears to follow a circular path called the diurnal circle. |
double star | Space and Astronomy | 1 | Any pair of stars which appear near each other on the celestial sphere, either because the two stars coincidentally lie along nearly the same line of sight from the Earth, though they are in fact physically distant from each other, or because the two stars are actually located in physical proximity to each other, by which they may form a co-moving pair or a binary star system. |
dwarf star | Space and Astronomy | 1 | Any star belonging to a category of ordinary main-sequence stars like the Sun, in contrast to evolved giant stars like Betelgeuse and Antares. Confusingly, the term has also come to include stellar remnants known as white dwarfs as well as low-mass substellar objects known as brown dwarfs. |
early-type star | Space and Astronomy | 1 | A hotter and more massive star, in contrast to late-type stars that are cooler and less massive. The term originated from historical stellar models that assumed stars began their early life at a high temperature then gradually cooled off as they aged. It may be used to refer to the higher-temperature members of any particular population or category of stars, rather than of all stars in general. |
eccentricity | Space and Astronomy | 1 | See orbital eccentricity. |
ecliptic | Space and Astronomy | 1 | The plane defined by the Earth's orbit around the Sun. Hence, the position of the Sun as viewed from the Earth defines the intersection of this plane with the celestial sphere. The ecliptic is widely used as a reference plane for describing the position of other Solar System bodies within various celestial coordinate systems. It differs from the celestial equator because of the axial tilt of the Earth. |
ecliptic coordinate system | Space and Astronomy | 1 | An astronomical coordinate system commonly used to specify the apparent positions, orbits, and axial orientations of objects within the Solar System, with an origin at the geometric center of either the Sun or the Earth, a fundamental plane defined by the plane of Earth's orbit around the Sun (i.e. the plane of the ecliptic), a primary direction towards the vernal equinox, and a right-handed convention. This system is convenient because most of the planets and many small Solar System bodies orbit the Sun with only slight inclinations to the ecliptic. It may be implemented in either spherical or rectangular coordinates. |
effective temperature | Space and Astronomy | 1 | (of a star or planet) The temperature of an ideal black body that would emit the same total amount of electromagnetic radiation. |
elliptical galaxy | Space and Astronomy | 1 | A type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless appearance. They are one of three main morphological classes of galaxy, along with spiral and lenticular galaxies. |
elliptical orbit | Space and Astronomy | 1 | A type of Kepler orbit with an orbital eccentricity of less than 1 (often inclusive of circular orbits, which have eccentricity equal to 0), or one with negative energy. Elliptical orbits take the shape of an ellipse, and are very common in two-body astronomical systems. |
elongation | Space and Astronomy | 1 | The angular separation between the Sun and an orbiting body, such as a planet, as it appears from Earth. |
ephemeris | Space and Astronomy | 1 | A list or table of the expected positions of astronomical objects or artificial satellites in the sky at various dates and times. Modern ephemerides are often provided by computer software. |
epoch | Space and Astronomy | 1 | A moment in time used as a reference point for some time-varying astronomical quantity, such as the celestial coordinates or orbital elements of an astronomical object, because such quantities are subject to perturbations and change over time. The primary use of astronomical quantities specified by epochs is to calculate other relevant parameters of motion in order to predict future positions and velocities. In modern usage, astronomical quantities are often specified as a polynomial function of a particular time interval, with a given epoch as the temporal point of origin. |
equator | Space and Astronomy | 1 | The imaginary line on a gravitationally rounded spheroid such as a planet that represents the intersection of the spheroid's surface with a plane perpendicular to its axis of rotation and equidistant from its geographical poles. The plane of the Earth's terrestrial equator is the basis for the celestial equator. |
equatorial coordinate system | Space and Astronomy | 1 | An astronomical coordinate system defined by an origin at the geometric center of the Earth, a fundamental plane created by projecting the Earth's terrestrial equator onto the celestial sphere (forming the celestial equator), a primary direction towards the vernal equinox, and a right-handed convention. This system is widely used to specify the positions of celestial objects as viewed from Earth. It may be implemented in either spherical or rectangular coordinates. |
equinoctial | Space and Astronomy | 1 | Of, relating to, or occurring at an equinox. |
equinox | Space and Astronomy | 1 | Either of the two precise times of year when the imaginary plane of the Earth's equator, extended indefinitely in all directions, passes through the center of the Sun (i.e. the two points at which this plane intersects the plane of the ecliptic); or, equivalently, when the Sun's apparent geocentric longitude is either 0 degrees or 180 degrees. The two equinoxes, known as the vernal equinox and the autumnal equinox, occur on or near March 20 and September 22 each year. On the day of an equinox, the center of the visible Sun appears to be directly above the equator, and the durations of day and night are approximately equal all over the planet. Compare solstice. |
escape velocity | Space and Astronomy | 1 | The minimum speed that must be achieved for a free, non-propelled object to escape from the gravitational influence of a massive body, i.e. to achieve an infinite distance from it; more generally, escape velocity is the speed at which the sum of an object's kinetic energy and gravitational potential energy is equal to zero. It is a function of the mass of the body and of the distance between the object and the body's center of mass. An object which has achieved escape velocity is neither on the surface nor in a closed orbit of any radius. |
evolutionary track | Space and Astronomy | 1 | A curve on the Hertzsprung–Russell diagram that a solitary star of a particular mass and composition is expected to follow during the course of its evolution. This curve predicts the combination of temperature and luminosity that a star will have during part or all of its lifetime. |
extinction | Space and Astronomy | 1 | The absorption and scattering of electromagnetic radiation by matter (dust and gas) between an emitting astronomical object and the observer. Atmospheric extinction varies by the wavelength of the radiation, with the attenuation being greater for blue light than for red. |
extragalactic astronomy | Space and Astronomy | 1 | The branch of astronomy that studies objects and phenomena outside of the Milky Way galaxy, i.e. all objects not covered by galactic astronomy. |
extrasolar object | Space and Astronomy | 1 | Any astronomical object that exists outside the Solar System. The term is generally not applied to stars or any objects larger than a star or the Solar System itself, such as galaxies. |
extrasolar planet | Space and Astronomy | 1 | Planets that orbit stars outside our own Solar System. Also called exoplanets. |
exobiology | Space and Astronomy | 1 | See astrobiology. |
facula | Space and Astronomy | 1 | A bright spot on a star's photosphere formed by concentrations of magnetic field lines. For the Sun in particular, faculae (see solar facula) are most readily observed near the solar limb. An increase in faculae as a result of a stellar cycle increases the star's total irradiance. |
field galaxy | Space and Astronomy | 1 | Any galaxy that does not belong to a larger cluster of galaxies and is gravitationally isolated. |
field star | Space and Astronomy | 1 | A randomly situated star that lies along the line of sight to a group of physically associated stars under study, such as a star cluster. These field stars are important to identify in order to prevent them from contaminating the results of a study. |
field of view | Space and Astronomy | 1 | The angular extent of the observable world that is seen at any given moment. In astronomy, the field of view is usually expressed as an angular area viewed by the instrument, in square degrees, or for higher magnification instruments, in square arc-minutes. |
first light | Space and Astronomy | 1 | The first use of a newly constructed telescope or other instrument to take an astronomical image. |
first magnitude star | Space and Astronomy | 1 | A term used to classify the brightest stars in the night sky, with apparent magnitudes lower (i.e. brighter) than 1.50. There are 22 stars that are classified as first magnitude stars. |
first point of aries | Space and Astronomy | 1 | The location of the March equinox upon the celestial sphere, used as a reference point in celestial coordinate systems. Located in the constellation Pisces, the First Point of Aries defines the ecliptic coordinate of (0°, 0°) and represents the point at which the Sun meets the celestial equator while traveling from south to north each year. It is directly opposite the First Point of Libra. |
first point of libra | Space and Astronomy | 1 | The location of the September equinox upon the celestial sphere, used as a reference point in celestial coordinate systems. Located in the constellation Virgo, the First Point of Libra represents the point at which the Sun meets the celestial equator while traveling from north to south each year. It is directly opposite the First Point of Aries. |
fixed stars | Space and Astronomy | 1 | Stars that appear to rise and set but keep the same relative arrangement over time. It applies to all stars except the Sun. The other group – wandering stars – are actually planets. (People used to think they were stars that changed position.) |
flare star | Space and Astronomy | 1 | A class of variable star that undergoes sudden, dramatic increases in brightness due to magnetic activity on its surface. This change in brightness occurs across the electromagnetic spectrum from radio waves to X-rays. Most flare stars are faint red dwarfs. |
fulton gap | Space and Astronomy | 1 | The apparent uncommonness of planets having a size between 1.5 and 2 times that of the Earth. |
galactic astronomy | Space and Astronomy | 1 | The branch of astronomy that studies objects and phenomena within the Milky Way galaxy, as opposed to everything outside of the Milky Way, which is the domain of extragalactic astronomy. |
galactic anticenter | Space and Astronomy | 1 | The direction in space that is directly opposite the center of the Milky Way Galaxy, as viewed from Earth; considered as a point on the celestial sphere, the Milky Way's anticenter is in the constellation Auriga. |
galactic center | Space and Astronomy | 1 | The rotational center of the Milky Way galaxy, consisting of a supermassive black hole of 4.100 ± 0.034 million solar masses. It is approximately 8,200 parsecs (27,000 ly) away from Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius, where the Milky Way appears brightest. |
galactic nucleus | Space and Astronomy | 1 | The region at the center of a galaxy, usually home to a very dense concentration of stars and gas. It almost always includes a supermassive black hole which, when active, can generate a much higher luminosity in a compact region than its surroundings. This excess luminosity is known as an active galactic nucleus, and the brightest such active galaxies are known as quasars. |
galactic period | Space and Astronomy | 1 | The time a given astronomical object within a galaxy takes to complete one orbit around the galactic center. Estimates of the duration of one revolution of the Solar System about the center of the Milky Way range from 225 to 250 million terrestrial years. |
galactic tide | Space and Astronomy | 1 | The tidal force experienced by objects subject to the gravitational field of a galaxy such as the Milky Way. |
galactocentric distance | Space and Astronomy | 1 | A star or cluster's distance from the gravitational center of a particular galaxy. For example, the Sun is about 27,000 light-years (approximately 8 kiloparsecs) away from the Galactic Center of the Milky Way. Galactocentric distance may also refer to a galaxy's distance from another galaxy. |
galaxy | Space and Astronomy | 1 | A large-scale collection of stars, gas and dust. Galaxies are held together by gravitational attraction. The Solar System is situated in the Milky Way galaxy. |
galaxy cluster | Space and Astronomy | 1 | A large-scale structure consisting of hundreds or thousands of galaxies bound together by gravity. Galaxy clusters are distinct from similarly named galactic clusters and other types of star clusters and from smaller aggregates of galaxies known as galaxy groups. Galaxy groups and galaxy clusters can themselves cluster together to form superclusters. |
galaxy group | Space and Astronomy | 1 | A gravitationally bound aggregation of up to 50 galaxies, each at least as luminous as the Milky Way Galaxy. Larger aggregations may be called galaxy clusters, and galaxy groups and clusters can themselves cluster together to form superclusters. |
galilean moons | Space and Astronomy | 1 | A collective name for the four moons of Jupiter discovered by Galileo Galilei in 1610: Io, Europa, Ganymede, and Callisto. |
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