This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth. The field of astronomy features an extensive vocabulary and a significant amount of sophisticated terminology.
A
A-type main-sequence star In the Harvard spectral classification system, a class of main-sequence star having spectra dominated by Balmer absorption lines of hydrogen. Main-sequence stars of spectral class A are typically blue-white or white in color, measure between 1.4 and 2.1 times the mass of the Sun, and have surface temperatures of 7,600–10,000 kelvin.
absolute magnitude A measure of a star's absolute brightness. It is defined as the apparent magnitude the star would show if it were located at a distance of 10 parsecs, or 32.6 light-years.
accretion disk A roughly circular mass of diffuse material in orbit around a central object, such as a star or black hole. The material is acquired from a source external to the central object, and friction causes it to spiral inward towards the object.
active galactic nucleus (AGN) A compact region in the center of a galaxy displaying a much higher than normal luminosity over some part of the electromagnetic spectrum with characteristics indicating that the luminosity is not produced by stars. A galaxy hosting an AGN is called an active galaxy.
airborne observatory An airplane with an astronomical telescope, which relies upon altitude to reduce atmospheric absorption and improve seeing conditions. Drawbacks include the instability of the lifting platform and higher costs.
albedo A measure of the proportion of the total solar radiation received by an astronomical body, such as a planet, that is diffusely reflected away from the body. It is a dimensionless quantity typically measured on a scale from 0 (indicating total absorption of all incident radiation, as by a black body) to 1 (indicating total reflection). The albedo reported for an astronomical body may vary widely by the spectral and angular distribution of the incident radiation, by the "layer" of the body being measured (e.g. upper atmosphere versus surface), and by local variation within these layers (e.g. cloud cover and geological or environmental surface features).
albedo feature A large area on the surface of a reflecting object that shows a significant contrast in brightness or darkness (albedo) compared to adjacent areas.
Alfvén surface The boundary separating a star's corona from the stellar wind, defined as the point at which the coronal plasma's Alfvén speed equals the large-scale stellar wind speed.
Am star A chemically peculiar star belonging to the more general class of A-type stars. The spectrum of the Am stars shows abnormal enhancements and deficiencies of certain metals. They are generally slow rotators, and are often found in binary systems where tidal braking has slowed the rate of spin. See metallicity.
aphelion The point at which a body orbiting the Earth's Sun is furthest from the Sun. Contrast perihelion.
apoapsis The point at which an orbiting body is furthest from its primary. Contrast periapsis.
apogee The point at which a body orbiting the Earth (such as the Moon or an artificial satellite) is furthest from the Earth. Contrast perigee.
apparent magnitude Also visual brightness (V). A measure of the brightness of a celestial body as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere. The brighter the object appears, the lower its magnitude.
appulse The closest approach of one celestial object to another, as viewed from a third body.
apsis In the orbit of a planetary body, one of the two extreme points of distance between the body and its primary – either the point of minimal distance, called the periapsis, or the point of maximal distance, called the apoapsis. The term may also be used to refer to the value of the distance rather than the point itself. All elliptical orbits have exactly two apsides.
argument of periapsis Also argument of perifocus or argument of pericenter. The angle from an orbiting body's ascending node to its periapsis, measured in the direction of motion. It is one of six canonical orbital elements used to characterize an orbit.
artificial satellite An object that has been intentionally placed into orbit by humans, often around the Earth but also around other bodies within the Solar System. Contrast natural satellite.
ascending node Also the north node. The orbital node at which an orbiting object moves north through the plane of reference (in geocentric and heliocentric orbits) or at which the orbiting object moves away from the observer (in orbits outside of the Solar System). The position of the ascending node with respect to a reference direction, called the longitude of the ascending node, is used along with other parameters to describe an orbit. Contrast descending node.
aspect The position of a planet or Earth's Moon with respect to the Sun, as viewed from Earth.
asterism Any pattern of stars recognizable in Earth's night sky. An asterism may form part of an official constellation or it may be composed of stars from more than one constellation.
asteroid A minor planet of the inner Solar System. They are primarily found orbiting the Sun between Jupiter and Mars, but can approach the Earth or occupy trojan orbits with Jupiter. Asteroids are somewhat arbitrarily distinguished from many different types of similar objects: small Solar System bodies primarily composed of dust and ice instead of mineral and rock are known as comets; bodies less than one meter in diameter are known as meteoroids; very large asteroids are sometimes called planetoids or planetesimals; and bodies similar to asteroids in size and composition but which lie beyond Jupiter are known as distant minor planets.
asteroid belt The circumstellar disc in the Solar System located roughly between the orbits of Mars and Jupiter that is occupied by numerous irregularly shaped small Solar System bodies ranging in size from dust particles to asteroids and minor planets. The asteroid belt is often called the main asteroid belt or main belt to distinguish it from other asteroid populations in other parts of the Solar System.
astrobiology Also exobiology. An interdisciplinary field that studies the origins, evolution, distribution, and future of living systems in the universe, encompassing research on organic compounds in space, abiogenesis and extreme-environment adaptation on Earth, the habitability of extrasolar planets, the possible existence of extraterrestrial life, and how humans might be able to detect extraterrestrial biosignatures, among other topics.
astrodynamics See orbital mechanics.
astrogeology Also planetary geology. A field that studies the geology of solidified bodies such as the planets and their moons, asteroids, comets, and meteorites. Investigations are centered around the composition, structure, processes, and history of these objects.
astrometric binary A type of binary system where evidence for an unseen orbiting companion is revealed by periodic gravitational perturbation of the visible component. See also spectroscopic binary.
astrometry The branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies.
astronomical body Also celestial body. A type of naturally occurring physical entity, association, or structure within the observable universe that is a single, tightly bound, contiguous structure, such as a star, planet, moon, or asteroid. Though the terms astronomical "body" and astronomical "object" are often used interchangeably, there are technical distinctions.
astronomical catalogue Also spelled astronomical catalog. A list of astronomical objects, typically grouped together because they share a common type, morphology, origin, means of detection, or method of discovery.
astronomical object Also celestial object. A type of naturally occurring physical entity, association, or structure that exists within the observable universe but is a more complex, less cohesively bound structure than an astronomical body, consisting perhaps of multiple bodies or even other objects with substructures, such as a planetary system, star cluster, nebula, or galaxy. Though the terms astronomical "object" and astronomical "body" are often used interchangeably, there are technical distinctions.
astronomical symbol Any abstract pictorial symbol used to represent one or more astronomical objects, events, or theoretical constructs, e.g. those of the planets of the Solar System, the phases of the Moon, the zodiacal constellations, and the solstices and equinoxes. Many of these symbols were commonly used historically, though in the modern era they are usually limited to almanacs and astrology, and their appearance in scientific literature has become increasingly infrequent. Exceptions include the symbols for the Sun (☉), the Earth (🜨), and the Moon (☾), which are sometimes used for astronomical constants and in other forms of shorthand.
astronomical unit (AU) 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 semi-major 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).
astronomy The scientific study of celestial objects and phenomena, the origins of those objects and phenomena, and their evolution.
astrophotography The photography or imaging of astronomical objects, celestial events, or areas of the night sky.
astrophysics The branch of astronomy that employs principles of physics and chemistry to determine the nature of astronomical objects and phenomena, examining properties such as luminosity, density, temperature, and chemical composition (rather than the positions or motions of objects in space, which is more specifically the emphasis of celestial mechanics).
atmosphere An envelope of gases surrounding an astronomical body such as a planet and held in place by its gravity. This shell of gas has no clearly defined exterior boundary, but instead grows increasingly tenuous with altitude. The term can also be applied to a stellar atmosphere, referring to the visible outer layers of a star.
axial precession 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 25,771.5 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 Also obliquity. 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.
axis of rotation The imaginary central line around which a compact body such as a star or planet undergoes circular rotation. On Earth, the points where this axis intersect the surface define the geographical poles. A rotational axis can "wobble" due to precession and nutation.
azimuth 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.
B
B-type main-sequence star A type of main-sequence star which is typically blue in color, temperatures ranges from 10,700 to 31,400 K and have 2.75 to 18 solar masses.
Babcock model A model that attempts to explain magnetic and sunspot patterns observed on the Sun.
barycenter 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 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 The prevailing cosmological model for the origin of the observable universe. It depicts a starting condition of extremely high density and temperature, followed by an ongoing expansion that led to the current conditions.
binary star 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 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.
blazar An active galactic nucleus (AGN) with a relativistic jet directed very nearly towards the observer.
break-up velocity Also critical velocity or critical rotation. The surface velocity at which the centrifugal force generated by a rapidly spinning star matches the force of Newtonian gravity. At higher rotational velocities, the star begins to eject matter from its surface.
brown dwarf 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.
bulge A tightly packed group or cluster of stars within a larger star formation.
C
calibrator star A fixed star used for calibration of high-powered telescopes.
celestial equator 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 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 See meridian.
celestial pole 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 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 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 (CMO) 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.
central star In the case of a planetary nebula, the central star is the collapsing remnant that ejected the material forming the nebula. In other cases it may refer to the host star of a system.
chromosphere A thin transition region of a star's outer atmosphere, positioned above the cooler photosphere and below the hot corona. For the Sun, the chromosphere is only visible during a total eclipse, when it gives off a red glow due to Balmer Hydrogen-alpha emission. Rising solar spicules occur in this region of magnetic activity.
chromospheric activity index 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).
circumstellar disc Also circumstellar disk. A loose disk of material in orbit around a star. It can be composed of gas, dust, and/or various smaller bodies. An example is the asteroid belt.
cis-Neptunian object (CNO) Any small Solar System body that orbits the Sun closer than Neptune — that is, with a semi-major axis less than about 30 astronomical units (AU). These include centaurs and Neptunian trojans.
clearing the neighbourhood In celestial mechanics, clearing the neighborhood around a celestial body's orbit describes the body becoming gravitationally dominant such that there are no other bodies of comparable size other than its natural satellites or those otherwise under its gravitational influence.
color index 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 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 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 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 Also compact object. 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 See nuclear star cluster.
conjunction 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 A region on the celestial sphere surrounding a specific and identifiable grouping of stars. The names of constellations are assigned by tradition and often have an associated folklore based in mythology, while the modern demarcation of their borders was established by the International Astronomical Union in 1930. Compare asterism.
corona An aura of plasma that surrounds cooler stars such as the Sun. It can be observed during a solar eclipse as a bright glow surrounding the lunar disk. The temperature of the corona is much higher than that of the stellar surface, and the mechanism that creates this heat remains subject to debate among astronomers.
coronal loop An arch-like structure in the Sun's atmosphere made up of relatively dense plasma confined and isolated from the surrounding medium by magnetic flux tubes.
coronal mass ejection (CME) A significant release of plasma and the accompanying magnetic field from the Sun's corona, often following a solar flare or during a solar prominence eruption.
cosmic distance ladder
cosmic dust Also space dust. Dust which exists in outer space or has fallen on Earth, generally composed of fine particles of solid matter far smaller than those found in terrestrial dust.
cosmic microwave background (CMB) Also cosmic microwave background radiation (CMBR).
cosmic ray A type of radiation consisting of high-energy protons and atomic nuclei which move through space at nearly the speed of light, and which may originate from the Sun or from outside the Solar System. Collisions of cosmic rays with the Earth's atmosphere can produce dramatic effects both in the air and on the surface.
cosmogony Any model concerning the origin of either the universe or the cosmos.
cosmology The scientific study of the origin, evolution, and eventual fate of the Universe.
coudé spectrograph A type of optical spectrometer 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.
critical rotation
critical velocity Also break-up velocity. 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 Also meridian transit. 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.
D
debris disk 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 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 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 (DSO) 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 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 Also the south node. 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 Also distant detached object and extended scattered disc object. 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 See prograde motion.
diurnal motion 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 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.
dust astronomy
dwarf planet
dwarf star 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.
E
early-type star 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 See orbital eccentricity.
ecliptic Also ecliptic plane or plane of the ecliptic. 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 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 (of a star or planet) The temperature of an ideal black body that would emit the same total energy as electromagnetic radiation.
elliptical galaxy 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 Also elliptic orbit. 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 The angular separation between the Sun and an orbiting body, such as a planet, as it appears from Earth.
ephemeris 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 generally provided by computer software.
epoch 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 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 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 Of, relating to, or occurring at an equinox.
equinox 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 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 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 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 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 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 Also exoplanet. Any planet outside the Earth's Solar System.
exobiology See astrobiology.
F
F-type main-sequence star A type of main-sequence star which is typically yellow-white in color, temperatures ranges from 6,000 to 7,200 K and have 1.1 to 1.6 solar masses.
facula A bright spot on a star's photosphere formed by concentrations of magnetic field lines. For the Sun in particular, solar faculae 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 Any galaxy that does not belong to a larger cluster of galaxies and is gravitationally isolated.
field star 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 The angular extent of the observable world that can be seen from a given place at a given moment. In astronomy, the field of view is the angular area viewed by an instrument such as a telescope, usually expressed in square degrees, or for higher magnification instruments, in square arc-minutes.
first light The first use of a newly constructed telescope or other instrument to observe or capture an astronomical image.
first magnitude star A term used to classify the brightest stars in the night sky, specifically those having an apparent magnitude lower (i.e. brighter) than 1.50. There are 22 stars that are classified as first magnitude stars.
First Point of Aries (♈︎) Also the Cusp of Aries. 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 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 Also background stars. The "background" of astronomical objects in the night sky which are so distant from observers on Earth that they do not appear to move relative to each other, as opposed to the "foreground" of objects within the Solar System which do. The fixed stars are typically taken to include all stars other than the Sun, as well as all other extrasolar and deep-sky objects.
flare star 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 The apparent uncommonness of planets having a size between 1.5 and 2 times that of the Earth.
G
G-type main-sequence star A type of main-sequence star which is typically yellow in color, temperatures range from 5,300 and 6,000 K and have 0.9 to 1.1 solar masses, it is also the Sun's spectral type.
galactic astronomy 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 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 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 coordinate system
galactic corona
galactic nucleus Also galactic core or galactic center. 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 Also galactic year or cosmic year. 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 The tidal force experienced by objects subject to the gravitational field of a galaxy such as the Milky Way.
galactocentric distance 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 A large, gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter, each of which orbits a center of mass. Galaxies may contain hundreds of billions of stars and are categorized according to their visual morphology as elliptical, spiral, or irregular. Most of the galaxies in the observable universe are between 1,000 and 3,000 parsecs (3,300 and 9,800 ly) in diameter, though some, including the Milky Way, are much larger.
galaxy cluster 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 Also group of galaxies (GrG). 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 The collective name for the four moons of Jupiter discovered by Galileo Galilei in 1610: Io, Europa, Ganymede, and Callisto.
gamma-ray astronomy The subfield of astronomy that studies astronomical objects detectable at gamma-ray wavelengths.
gamma-ray burst (GRB) A cataclysmic event that generates a brief but intense outburst of gamma ray radiation which can be detected from billions of light-years away. The source of most GRBs is theorized to be supernova or hypernova explosions of high-mass stars. Short GRBs may also result from the collision of neutron stars.
gas giant A giant planet composed mainly of hydrogen and helium gases rather than heavier elements, e.g. Jupiter and Saturn in th