Glossary of astronomy

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 jargon.

A
A-type star: In the system, a class of star having spectra dominated by Balmer absorption lines of hydrogen. Stars of spectral class A are typically blue-white or white in color, measure between 1.4 and 2.1 times, and have surface temperatures of 7,600–10,000 kelvin.

absolute magnitude: A measure of a 's absolute brightness. It is defined as the the star would show if it were located at a distance of 10, or 32.6.

accretion disk: A roughly circular mass of diffuse material in around a central object, such as a or. 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 displaying a much higher than normal over some part of the electromagnetic spectrum with characteristics indicating that the luminosity is not produced by. A galaxy hosting an AGN is called an active galaxy.

albedo: A measure of the proportion of the total solar radiation received by an, such as a, 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 ) 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 compared to adjacent areas.

Am star: A chemically peculiar star belonging to the more general class of. The spectrum of the Am stars shows abnormal enhancements and deficiencies of certain metals. See.

aphelion: The point at which a body orbiting the Earth's is furthest from the Sun. Contrast.

apoapsis: The point at which an orbiting body is furthest from its. Contrast.

apogee: The point at which a body orbiting the Earth (such as the or an ) is furthest from the Earth. Contrast.

apparent magnitude:

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 of a, one of the two extreme points of distance between the body and its – either the point of minimal distance, called the, or the point of maximal distance, called the. The term may also be used to refer to the value of the distance rather than the point itself. All have exactly two apsides.

argument of periapsis:

The angle from an body's to its, measured in the direction of motion. It is one of six canonical used to characterize an orbit.

artificial satellite: An object that has been intentionally placed into by humans, often around the Earth but also around other bodies within the. Contrast.

ascending node:

The at which an orbiting object moves north through the (in geocentric and heliocentric orbits) or at which the orbiting object moves away from the observer (in orbits outside of the ). The position of the ascending node with respect to a reference direction, called the, is used along with other to describe an orbit. Contrast.

aspect: The position of a or Earth's with respect to the, as viewed from Earth.

asterism: Any pattern of recognizable in Earth's. An asterism may form part of an official or it may be composed of stars from more than one constellation.

asteroid: A of the inner, i.e. one that orbits the at a distance no greater than the orbit of Jupiter. Asteroids are somewhat arbitrarily distinguished from many different types of similar objects: primarily composed of dust and ice instead of mineral and rock are known as ; bodies less than one meter in diameter are known as ; very large asteroids are sometimes called  or ; and bodies similar to asteroids in size and composition but which lie beyond Jupiter are known as.

asteroid belt: The in the located roughly between the orbits of Mars and Jupiter that is occupied by numerous irregularly shaped  ranging in size from dust particles to  and. 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:

An interdisciplinary field that studies the origins, evolution, distribution, and future of living systems in the universe, encompassing research on organic compounds in, abiogenesis and extreme-environment adaptation on Earth, the habitability of, the possible existence of extraterrestrial life, and how humans might be able to detect extraterrestrial biosignatures, among other topics.

astrodynamics: See.

astrogeology:

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 where evidence for an unseen orbiting companion is revealed by its periodic of the visible component. See also.

astrometry: The branch of astronomy that involves precise measurements of the positions and movements of and other.

astronomical body:

A type of naturally occurring physical entity, association, or structure within the that is a single, tightly bound, contiguous structure, such as a,, , or. Though the terms astronomical "body" and are often used interchangeably, there are technical distinctions.

astronomical catalogue:

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:

A type of naturally occurring physical entity, association, or structure that exists within the but is a more complex, less cohesively bound structure than an, consisting perhaps of multiple bodies or even other objects with substructures, such as a, , , or. 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, events, or theoretical constructs, e.g. those of the of the, the , the , and the  and. 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 and in other forms of shorthand.

astronomical unit (AU): A unit of length used primarily for measuring distances within the 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 149597870.7 km.

astronomy: The scientific study of and, the origins of those objects and phenomena, and their evolution.

astrophotography:

astrophysics: The branch of astronomy that employs principles of physics and chemistry to determine the nature of 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 ).

atmosphere: A gaseous envelope held in place by the gravity of a planet. 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 ) in the orientation of an 's. The term often refers in particular to the gradual shift in the orientation of Earth's rotational axis with respect to over a cycle of approximately 25,772 years, which is caused predominantly by the gravitational influence of the and the  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 and polar motion, and is the cause of the apparent  in the.

axial tilt:

The angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between its plane and. Axial tilt usually does not change considerably during a single ; Earth's axial tilt is the cause of the seasons. Axial tilt is distinct from.

axis of rotation:

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
barycenter: The common center of mass about which any two or more bodies of a gravitationally bound system. The barycenter is one of the foci of the 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 where the mass of the central  is significantly larger than the mass of an orbiting, 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 for the origin of the. 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 consisting of exactly two orbiting around their common. The term is often used interchangeably with, 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.

blazar:

break-up velocity:

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: A 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. Brown dwarfs can still generate energy from and by the fusion of deuterium.

bulge:

C
Calibrator star: A star used for calibration of high-powered teleescopes.

coudé spectrograph: 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: The imaginary great circle of a body's that is coplanar with the body's terrestrial. On Earth, the plane of the celestial equator is the basis of the. Due to Earth's, this plane is currently inclined at an angle of 23.44 degrees with respect to the.

celestial mechanics: The branch of that studies the motions of all types of, including, , and and , among others.

celestial meridian: See.

celestial pole: One of two coordinates in the Earth's at which a hypothetical indefinite extension of the Earth's "intersects" the, i.e. the two points in the sky that are directly overhead the terrestrial North and South Poles, around which all  appear to revolve during the course of a day. The celestial poles form the north and south poles of the.

celestial sphere: An imaginary sphere that encompasses the Earth's entire and is stationary with respect to the. It is the basis for.

centaur: A with either a or a  between those of the, i.e. generally inward of the  but beyond the. Centaurs are that typically exhibit characteristics of both  and, and generally also have unstable orbits because they cross the orbits of one or more of the.

central massive object (CMO): Any very large concentration of mass at the center of a, typically either a or a, but sometimes both.

chromosphere:

chromospheric activity index: A parameter indicating the magnetic activity in a star's. 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:

cis-Neptunian object (CNO):

clearing the neighbourhood:

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. 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 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 and 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:

Any 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,, and , or to with very small radii.

compact stellar nucleus: See.

conjunction: A phenomenon during which two or spacecraft have either the same or the same  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 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 in 1930. Compare.

corona: An aura of plasma that surrounds cooler stars such as the. It can be observed during a 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:

coronal mass ejection (CME): A significant release of plasma and the accompanying magnetic field from the 's, often following a or present during a eruption.

cosmic distance ladder:

cosmic dust:

Dust which exists in 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):

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 or from outside the. 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 or the cosmos.

cosmology: The scientific study of the origin, evolution, and eventual fate of the.

critical rotation:

critical 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. See also.

culmination:

The apparent movement of an (e.g. the, the, a , a , a , etc.) across the observer's local. During each day, the Earth's rotation causes every astronomical object to along a circular path on the, creating two points at which it crosses the meridian: an upper culmination, at which the object reaches its highest point above the , 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 of dust and debris orbiting its host star. It is created by collisions between. 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, the celestial equivalent of terrestrial latitude. Coordinates north of the are measured in positive degrees from 0° to 90°, while coordinates to the south are measured in negative degrees. See also.

decretion disk: A formed from gas ejected from a central star that now follows a nearly around it. This type of disk can be found around many Be stars.

deep-sky object (DSO): Any that is not an individual or an object within the Earth's. The classification is used mostly in amateur to distinguish faint objects in the  such as,, and.

degenerate star: A star composed of degenerate matter, e.g. a or a. These stars are in an advanced state of and have suffered extreme, such that normal atoms cannot exist in them.

descending node:

The at which an orbiting object moves south through the (in geocentric and heliocentric orbits) or at which the orbiting object moves toward the observer (in orbits outside of the ). Contrast.

detached object:

A class of in the outer reaches of the  whose  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, such as and  objects.

direct motion: See.

diurnal motion: The apparent motion of an (e.g. the, a, or a distant ) around the two in the Earth's  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 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 system.

dwarf planet:

dwarf star: Any belonging to a category of ordinary stars like the, in contrast to  giant stars like Betelgeuse and Antares. Confusingly, the term has also come to include known as  as well as low-mass  known as.

E
early-type star: A hotter and more massive star, in contrast to 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.

ecliptic:

The plane defined by the Earth's orbit around the. Hence, the position of the Sun as viewed from the Earth defines the intersection of this plane with the. The ecliptic is widely used as a for describing the position of other bodies within various. It differs from the because of the  of the Earth.

ecliptic coordinate system: An commonly used to specify the apparent positions, orbits, and axial orientations of objects within the, with an origin at the geometric center of either the or the Earth, a fundamental plane defined by the plane of Earth's orbit around the Sun (i.e. the ), a  towards the, and a right-handed convention. This system is convenient because most of the and many  orbit the Sun with only slight  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 amount of electromagnetic radiation.

elliptical galaxy: A type of with an approximately ellipsoidal shape and a smooth, nearly featureless appearance. They are one of three main morphological classes of galaxy, along with and.

elliptical orbit:

A type of with an of less than 1 (often inclusive of, which have eccentricity equal to 0), or one with negative. Elliptical orbits take the shape of an ellipse, and are very common in two-body astronomical systems.

elongation: The angular separation between the and an orbiting body, such as a, as it appears from Earth.

ephemeris: A list or table of the expected positions of or in the  at various dates and times. Modern ephemerides are often provided by computer software.

epoch: A moment in time used as a reference point for some time-varying astronomical quantity, such as the or of an, because such quantities are subject to  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 that represents the intersection of the spheroid's surface with a plane perpendicular to its and equidistant from its geographical poles. The plane of the Earth's terrestrial equator is the basis for the.

equatorial coordinate system: An defined by an origin at the geometric center of the Earth, a fundamental plane created by projecting the Earth's terrestrial onto the  (forming the ), a  towards the, 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: Either of the two precise times of year when the imaginary plane of the Earth's,, passes through the center of the (i.e. the two points at which this plane intersects the ); or, equivalently, when the Sun's apparent geocentric is either 0 degrees or 180 degrees. The two equinoxes, known as the and the, occur on or near  and  each year. On the day of an equinox, the center of the visible Sun appears to be the equator, and the durations of day and night are approximately equal all over the planet. Compare.

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 of any radius.

evolutionary track: A curve on the that a solitary of a particular mass and composition is expected to follow during the course of its. This curve predicts the combination of temperature and 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 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, i.e. all objects not covered by.

extrasolar object: Any that exists outside the. The term is generally not applied to or any objects larger than a star or the Solar System itself, such as.

extrasolar planet:

Any outside the Earth's.

exobiology: See.

F
facula: A bright spot on a 's formed by concentrations of magnetic field lines. For the in particular, faculae (see ) are most readily observed near the. An increase in faculae as a result of a stellar cycle increases the star's total irradiance.

field galaxy: Any that does not belong to a larger cluster of galaxies and is gravitationally isolated.

field star: A randomly situated that lies along the line of sight to a group of physically associated stars under study, such as a. These field stars are important to identify in order to prevent them from contaminating the results of a study.

first light: The first use of a newly constructed or other instrument to take an.

first magnitude star: 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 (♈︎):

The location of the upon the, used as a reference point in. Located in the Pisces, the First Point of Aries defines the  of (0°, 0°) and represents the point at which the  meets the  while traveling from south to north each year. It is directly opposite the.

First Point of Libra: The location of the upon the, used as a reference point in. Located in the Virgo, the First Point of Libra represents the point at which the  meets the  while traveling from north to south each year. It is directly opposite the.

fixed stars:

The "background" of in the 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  which do. The fixed stars are typically taken to include all other than the, as well as all other  and.

flare star: A class of 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.

Fulton gap: The apparent uncommonness of having a size between 1.5 and 2 times that of the Earth.

G
galactic astronomy: The branch of astronomy that studies objects and phenomena within the, as opposed to everything outside of the Milky Way, which is the domain of.

galactic anticenter: The direction in space that is directly opposite the of the, as viewed from Earth; considered as a point on the, the Milky Way's anticenter is in the Auriga.

Galactic Center: The rotational of the, consisting of a of 4.100 ± 0.034 million. It is approximately 8,200 pc away from Earth in the direction of the Sagittarius, Ophiuchus, and Scorpius, where the Milky Way appears brightest.

galactic coordinate system:

galactic corona:

galactic nucleus:

The region at the center of a, usually home to a very dense concentration of stars and gas. It almost always includes a which, when active, can generate a much higher in a compact region than its surroundings. This excess luminosity is known as an, and the brightest such active galaxies are known as.

galactic period:

The time a given within a takes to complete one  around the. Estimates of the duration of one revolution of the about the center of the  range from 225 to 250 million terrestrial years.

galactic tide: The experienced by objects subject to the gravitational field of a such as the.

galactocentric distance: A star or cluster's distance from the of a particular. For example, the is about 27,000  (approximately 8 kiloparsecs) away from the  of the. Galactocentric distance may also refer to a galaxy's distance from another galaxy.

galaxy: A large, gravitationally bound system of,, , , and , each of which orbits a. Galaxies may contain hundreds of billions of stars and are categorized according to their visual morphology as, , or. Most of the galaxies in the are between 1000 and 3000 pc in diameter though some, including the, are much larger.

galaxy cluster: A large-scale structure consisting of hundreds or thousands of bound together by gravity. Galaxy clusters are distinct from similarly named and other types of  and from smaller aggregates of galaxies known as. Galaxy groups and galaxy clusters can themselves cluster together to form.

galaxy group:

A gravitationally bound aggregation of up to 50, each at least as luminous as the. Larger aggregations may be called, and galaxy groups and clusters can themselves cluster together to form.

Galilean moons: A collective name for the four of Jupiter discovered by Galileo Galilei in 1610: Io, Europa, Ganymede, and Callisto.

gamma-ray astronomy: The subfield of that studies 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 away. The source of most GRBs is theorized to be or  explosions of high-mass stars. Short GRBs may also result from the collision of.

gas giant: A composed mainly of hydrogen and helium gases rather than heavier elements, e.g. Jupiter and Saturn in the.

geocenter: The geometric center of the Earth, i.e. the arithmetic mean position of all points within the oblate spheroid that is the precise shape of the Earth.

geocentric: With reference to, or pertaining to, the of the Earth; centered upon the Earth, e.g. a geocentric orbit.

geocentric zenith: The point projected upon the by a straight line that passes through the and an observer; i.e. the observer's  as defined with respect to the center of the Earth.

geometric albedo: The ratio of the brightness of an astronomical body at a of zero to an idealized flat, fully reflecting, diffusively scattering (Lambertian) disk with the same cross-section. It is a measure of how much of the incoming illumination is being scattered back toward an observer and has a value between zero and one.

geometric position: The position of an object ( or otherwise) with respect to the or to the position of an observer, i.e. as defined by a straight line between the center of the Earth (or the observer) and the object at a given time, without any corrections for,, etc.

geostationary orbit:

A, which maintains a constant altitude of 35786 km directly above Earth's in the  as Earth's rotation such that, to an observer on Earth's surface, the orbiting object appears motionless, in a fixed position in the sky. are often placed in geostationary orbit so that antennas on Earth do not have to rotate to track them.

geosynchronous orbit (GSO): A about the Earth, i.e. with an equal to Earth's, such that the orbiting object appears to return to exactly the same position in the sky after a period of one. All geosynchronous orbits have a equal to 35786 km;  are a special case of geosynchronous orbits.

giant planet: Any very large or massive, including and.

globular cluster: A tight, spherical conglomeration of many thousands of which are gravitationally bound to each other and which a  as a. They differ from in having a much higher combined mass, with a typical lifespan extending for billions of years.

gravitational collapse:

gravitational lens: Any very large distribution of mass, such as a, which can bend passing light from a distant source by a noticeable degree. The effect, known as gravitational lensing, can make background objects appear to an observer to take on a ring or arc shape.

gravitational-wave astronomy: A branch of which analyzes minute distortions in the curvature of spacetime known as gravitational waves to collect observational data about astronomical objects and events such as,, , and the.

H
H II region: An ionized powered by young, massive O-type stars. Ultraviolet photons from these hot stars ionize gas in the surrounding environment, and the nebular gas shines brightly in spectral lines of hydrogen and other elements. Because O-type stars have relatively short lifetimes (typically a few million years), the presence of an H II region indicates that massive star formation has taken place recently at that location. H II regions are often found in the arms of and in star-forming.

heliocenter: The precise geometric center of the Earth's, i.e. the arithmetic mean position of all points within the approximate spheroid that is the shape of the Sun.

heliocentric: With reference to, or pertaining to, the of the Earth's ; centered upon the Sun, e.g. a heliocentric orbit.

heliopause:

heliosphere: The vast, bubble-like cavity in the which surrounds and is created by the emanating from the Earth's. The heliosphere encompasses the entirety of the and a vast region of space beyond it. Its outer limit is often considered the boundary between matter originating from the Sun and matter originating from the rest of the galaxy.

Hertzsprung–Russell diagram: A plot of versus for a population of ; depending on the usage, the star's  may be substituted for luminosity, and its  or  for temperature. Single stars of known mass and composition follow predictable across this chart over the course of their. Hence, knowing a star's mass and allows its age to be estimated. Stars of similar types are also found grouped together in specific regions of the chart, including,, and stars.

Hill sphere:

The approximate region around an within which its gravitational attraction dominates the motions of. It is computed with respect to the next most gravitationally attractive object, such as the nearest star or the. Satellites moving outside this radius tend to be perturbed away from the main body.

horizon: The apparent boundary between the surface of a celestial body and its sky when viewed from the perspective of an observer on or near that body's surface; more specifically, the plane perpendicular to a line from an observer to the that passes through the point of observation.

hour angle: For a given, the angular distance on the measured westward along the from the observer's local  to the  that passes through the celestial object; or, equivalently, the angle between the plane containing Earth's  and the, and the plane containing Earth's rotational axis and the object of interest. Analogous to, the hour angle is one of many ways commonly used to specify the longitudinal position of an object upon the celestial sphere.

hour circle: Any imaginary great circle drawn upon the celestial sphere that passes through both of the and is therefore perpendicular to the. Similar to a but additionally taking into account the terrain and the depth to the  at a ground observer's particular location, the concept of the hour circle is employed to describe the longitudinal position of a celestial object relative to the observer's local meridian.

hybrid pulsator: This is a class of pulsating stars that display pulsation frequencies of two different classes of variables. An example are variables displaying characteristic frequencies of both Delta Scuti and Gamma Doradus variables. On the, these stars are positioned where the instability strips of both variable classes overlap.

hydrogen burning limit: A critical mass below which an astronomical object cannot sustain its surface through nuclear fusion. This mass limit, equal to about 7% of the, forms the dividing line between and hydrogen-fusing stars.

hypergalaxy: A system consisting of a large accompanied by multiple smaller (often elliptical) as well as its. The and Andromeda systems are examples of hypergalaxies.

I
ice giant: A composed mainly of elements heavier than hydrogen or helium (such as oxygen, carbon, nitrogen, and sulfur), especially chemical with freezing points above 100 K, e.g. Uranus and Neptune in the.

inclination: See.

inferior planet: An archaic term that is sometimes used to refer to the planets Mercury and Venus. The name originated from the fact that these planets orbit closer to the than the Earth and hence, in the geocentric of Ptolemy, both appear to travel with the Sun across the sky. This is in contrast to the so-called, such as Mars, which appear to move independently of the Sun.

infrared astronomy: The subfield of that studies detectable at infrared wavelengths.

International Astronomical Union (IAU):

interstellar medium (ISM): The matter that exists in the space between the in a. This medium mainly consists of hydrogen and helium, but is enhanced by traces of other elements contributed by matter expelled from stars.

interstellar reddening: An effect produced by the incremental absorption and scattering of electromagnetic energy from interstellar matter, known as. This effect causes more distant objects such as stars to appear redder and dimmer than expected. It is not to be confused with the separate phenomenon of.

invariable plane:

The imaginary plane passing through the of a and perpendicular to its angular momentum vector, and which may be regarded as the weighted average of all planetary  and rotational planes comprising the system.

ionosphere:

irregular galaxy:

irregular moon: A following a distant,, and often and  orbit about its. Irregular moons are thought to be captured from other orbits, as opposed to, which are thought to form in situ.

isochrone: A curve on the that represents the positions of stars having the same age but differing masses. This is in contrast to an, which is a plot of stars having the same mass but differing ages. In fact, multiple evolutionary tracks can be used to build isochrones by putting curves through equal-age points along the tracks. When the mass of a star can be determined, an isochrone can be used to estimate the star's age.

J
Jeans instability: A physical state in which an of gas will begin to undergo collapse and form stars. A cloud can become unstable against collapse when it cools sufficiently or has perturbations of density, allowing gravity to overcome the gas pressure.

Julian year (a): A unit of time defined as exactly 365.25 days of 86,400 SI seconds each. Because these are units of constant duration, the Julian year is also constant and does not vary with a specific calendar or with any of the other means of determining the length of a year, such as the. It is therefore widely used as the basis for defining the standard astronomical and the.

K
Kelvin–Helmholtz mechanism:

Kepler orbit:

The motion of one body relative to another, as an ellipse, parabola, or hyperbola, which forms a two-dimensional (or sometimes a straight line) in three-dimensional space. Kepler orbits are idealized mathematical constructions which consider only the point-like gravitational attraction of two bodies, neglecting more complex orbital that may exist in reality.

Kuiper belt:

A of such as,, and  in the outer , extending between 30 and 50  from the. It is similar to the but far larger, and is home to several, including Pluto.

L
Lagrangian point:

Any of a set of points near two large in at which a smaller object will maintain a constant position relative to the larger bodies. At other locations, a small object would eventually be pulled into its own orbit around one of the large bodies, but at the Lagrangian points the gravitational forces of the large bodies, the centripetal force of orbital motion, and (in certain scenarios) the Coriolis acceleration all align in a way that causes the small object to become "locked" in a stable or nearly stable relative position. For each combination of two orbital bodies, there are five such Lagrangian points, typically identified with the labels L1 to L5. The phenomenon is the basis for the stable orbits of and is commonly exploited by.

Laniakea Supercluster:

late-type star:

libration: A slight oscillating motion of the as seen from the Earth, a result of the Moon's elliptical orbit. It can allow normally hidden parts of the Moon's far side to be visible along the limbs of the lunar disk.

light-year (ly): A unit of length used to express astronomical distances that is equivalent to the distance that an object moving at the in vacuum would travel in one : approximately 9.46 trillion kilometres ($9.46 km$) or 5.88 trillion miles ($5.88 mi$). Though the light-year is often used to measure -scale distances in non-specialist publications, the unit of length most commonly used in professional is the.

limb darkening: An optical effect seen in stars (including the ), where the center part of the disk appears brighter than the edge or limb of the image.

line of apsides: The imaginary line connecting the two (the and the ) of an, and which therefore represents the distance of the orbit's longest axis.

Local Group:

longitude of the ascending node (☊ or Ω): The angle between a specified reference direction, called the, and the direction of an 's, as measured on a specified. The angle is typically measured eastwards from the reference direction to the ascending node (i.e. counterclockwise as seen from the north). It is one of six canonical used to characterize an orbit.

luminosity: The total amount of energy emitted per unit time by a,, or other. In SI units, luminosity is measured in joules per second or watts, and is often given in terms of. Luminosity is related to but distinct from.

lunar: Of or relating to the Earth's.

lunar phase:

The shape of the portion of the that is illuminated by direct sunlight as viewed from Earth. This shape is referred to as a phase because it gradually changes in a regular cycle over the course of a : as the orbital positions of the Moon around Earth and Earth around the change, the visibility of the side of the Moon that  Earth alternates between completely illuminated (known as a full moon) and completely darkened by the Moon's own shadow (known as a new moon). There are also intermediate phases, during which the visible side may be only partially sunlit, e.g. when the Moon appears as a crescent. During the part of the lunar cycle in which the illuminated portion is growing larger, the Moon is said to be ; when the illuminated portion is becoming smaller, it is said to be. The phase of the Moon at any particular time appears the same from every point on Earth.

M
Massive compact halo object (MACHO): A kind of  that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a body that emits little or no radiation and drifts through interstellar space unassociated with any . Examples of MACHOs include  or  as well as  and .

magnetosphere: A mostly convex region formed when a plasma, such as the, interacts with the magnetic field of a body, such as a or.

magnitude: A numerical logarithmic scale indicating the brightness of an, where the lower the value, the brighter the object. By convention, a first magnitude star is 100 times as bright as a sixth magnitude star. Magnitude 6 is considered the lower limit of objects that can be seen with the naked eye, although this can vary depending on sky conditions and eyesight.

main sequence: A category of which form a continuous and distinctive band on plots of stellar temperature versus, in particular the. These stars are characterized by being in hydrostatic equilibrium and undergoing nuclear fusion of hydrogen-1 in their core region. The is a main-sequence star.

major axis: See.

March equinox:

The precise time of year on Earth when the Sun appears to cross the, while generally trending northward at each passage. It represents the moment at which the North Pole of the Earth begins to tilt toward the Sun, and typically occurs on or near March 20 each year. It is the vernal equinox in the Northern Hemisphere and the autumnal equinox in the Southern Hemisphere. Contrast.

mean anomaly: The fraction of an 's that has elapsed since the orbiting body passed, expressed as the angular distance from the which a fictitious body would have if it moved in a perfectly  in the same  as the actual body in its elliptical orbit. Unlike the, the mean anomaly does not correspond to a real geometric angle but is instead a contrived parameter used to make calculating the position of the orbiting body in the mathematically convenient.

mean-motion resonance (MMR): See.

meridian: A line running north–south across the and passing through the point directly overhead known as the.

meridian astronomy: The measurement of positions of celestial objects based on observation of the times of their across the and of their  distance at those times, with the intention of obtaining accurate  positions which are self-consistent over large areas of sky.

Messier object: One of a set of 110 "nebulous", 103 of which were catalogued as non- by French comet hunter Charles Messier between 1771 and 1781. The Messier catalogue includes most of the easily visible from the Northern Hemisphere.

meteor:

The visible passage of a glowing,, , or through the Earth's , usually as a long streak of light produced when such an object is heated to incandescence by collisions with air molecules in the upper atmosphere, leaving an ionization trail as a result of its rapid motion and sometimes also shedding material in its wake.

meteorite: A solid piece of debris from a that originated in outer space and survived its passage through the atmosphere to reach the surface of a planet or moon.

meteoroid: A small rock or boulder that has entered a planetary. If it survives to reach the surface, it is then termed a.

meteor shower: A series of that seemingly radiate from a single area in the. These are produced by debris left over from a larger body, such as a, and hence they follow roughly the same. This makes many meteor showers predictable events, as they recur every year.

metallicity: A measure of the abundance of elements other than hydrogen and helium within an astronomical object. Note that this definition includes elements that are not traditionally considered metallic by chemical convention.

micrometeorite: A very small that has survived its passage through the atmosphere to reach the surface of a planet or moon, usually ranging in size from 50 μm to 2 mm. Micrometeorites are a major component of.

micrometeoroid: A very small, usually weighing less than one gram. If it survives to reach a planetary surface, it is then termed a.

microvariable: A stellar object such as a that undergoes very small variations in, in which the amplitude of the fluctuations amounts to just a few thousandths of a. Detecting microvariability typically requires a sufficient number of observations to rule out random error as a source.

Milky Way: The that includes the Earth's. The name describes the galaxy's appearance from the Earth: a hazy band of light visible in the, formed from billions of that cannot be individually distinguished by the naked eye. The Milky Way Galaxy has a diameter of 100,000–200,000 and is estimated to contain 100–400 billion stars and at least that number of planets. The Solar System is located on the inner edge of one of the Milky Way's spiral arms, about 27,000 light-years from the, which the orbits with a period of 240 million years. The hazy band of light itself, which from Earth appears as a band because the galaxy's disk-shaped structure is viewed side-on from within.

minor axis: See.

minor planet: An object in direct around the that is neither a dominant  nor originally classified as a. A is not a minor planet because it orbits another body instead of the Sun.

minor-planet moon: A that orbits a. See also ' and '.

molecular cloud: An in which the prevailing physical conditions allow molecules to form, including molecular hydrogen.

moment of inertia factor:

A dimensionless quantity that characterizes the radial distribution of mass inside a planet or moon.

moon: See.

Moon: The solid, rocky body that orbits the Earth as its only, completing a full orbit every 27.3 days. The Moon's gravitational influence is responsible for on Earth; because of, only one side of the Moon is ever visible from the Earth. Sunlight reflected from its surface makes the Moon appear very bright in the, though its orbital position with respect to the Earth and the causes its visibility to change in a regular cycle of  when viewed from the Earth. The adjectival lunar is often used specifically to describe the orbit, gravity, and other properties of the Earth's Moon.

moonlet:

An especially small orbiting a,, or other. See also ' and '.

moonmoon: See.

Morgan–Keenan stellar classification system:

morning width:

The horizontal angular distance between the rise of a and the east direction.

moving group:

A loose grouping of which travel together through space. Although the members were formed together in the same, they have since moved too far apart to be gravitationally bound as a.

multi-messenger astronomy: A type of based on the acquisition of information about through the coordinated observation and interpretation of four disparate classes of "messenger" signals with  origins: electromagnetic radiation, gravitational waves,, and. Because these four extrasolar messengers are created by different astrophysical processes, their presence or absence during a celestial event can reveal useful information about their sources.

multiverse:

N
N galaxy: An early classification for active galaxies that had the visual appearance of a galaxy with a particularly bright, star-like nucleus. As a group, they are intermediate between Seyfert galaxies and Quasar. Most are giant ellipticals that are radio sources and display prominent emission lines.

naked eye:

The human eye as used without any magnifying or light-collecting optical aid, such as a telescope, nor any eye protection. Many astronomical objects emit or reflect visible light that is sufficiently bright to fall within the limits of normal human visual perception, allowing observers to see them from the Earth's surface without any special equipment. Vision corrected to normal acuity using eyeglasses or contact lenses is still considered unaided.

natural satellite:

Any that a,, or sometimes another.

near-Earth object (NEO): Any, such as an or, whose orbit brings it into proximity with Earth, generally by being less than 1.3 from the  at its.

nebula: Any of indistinct nebulosity. In modern usage, the term typically refers to an of, hydrogen, helium, and other ionized gases. Historically, it was also used to refer to extended sources of that could not be resolved into their individual components, such as  and.

neutrino: A type of elementary particle, electrically neutral and with an extremely small rest mass, that interacts with other particles only via the weak interaction and the gravitational interaction. Neutrinos therefore typically pass through normal matter unimpeded and undetected.

neutron star: A type of that is composed almost entirely of neutrons, which are a type of subatomic particle with no electrical charge. Typically, neutron stars have a mass between about 1.35 and 2.0 times the mass of the, but with a radius of only 12 km, making them among the densest known objects in the universe.

New General Catalogue (NGC):

night sky: The appearance of the Earth's at nighttime, when the is below the horizon, and more specifically when clear weather and low levels of ambient light permit visibility of  such as,, and the. The night sky remains a fundamental setting for both amateur and professional.

non-inclined orbit: Any that is coplanar with a specified, such that the is 0 degrees for  orbits and 180 degrees for  ones.

nuclear star cluster (NSC): A compact and dense concentration of stars located at the center of a galaxy.

number density: The quantity of some specified particle or object class per unit volume. For atoms, molecules, or subatomic particles, the volume is typically in cm3 or m3. With stars, cubic parsecs (pc3) are often used.

nutation: A continuous, gravity-induced change in the orientation of an 's which results from the combined effects of small, short-term variations. Nutation is distinguished from, which is a similar but longer-term change in axial orientation.

O
O–C diagram: A diagram of observed minus calculated values over time, showing how observed data differ from theoretical values which have been calculated according to a particular scientific model. It is often used as a diagnostic tool to determine the accuracy of the model. With a variable star, it is typically used to compare phase differences over time.

OB association: A group of massive which are not gravitationally bound to each other, but move together through space in a loose association. The OB in the name is a reference to stars of O and B.

obliquity: See.

observation arc:

The duration of time between the earliest and latest made by astronomers of an object within the, which defines the length of the path traced by the object between these same observations. The term is primarily used in the discovery and tracking of and, which can be difficult to continuously track because of their size and great distance from Earth. Very short observation arcs, e.g. where the time between the initial observation and the most recent observation is less than 30 days, are of limited descriptive power because they represent only a very small fraction of the total path traced by the object in its around the Sun (or other ), and therefore result in a high degree of  when estimating the shape and characteristics of the object's orbit.

observable universe:

observational astronomy: The practice and study of directly observing with the use of and other astronomical instruments. It is concerned with recording data about the, as opposed to, which is concerned with calculating the measurable implications of astronomical models.

occultation: A celestial event that occurs when a distant or is hidden by another, nearer body or object that passes between it and the observer, thereby blocking the first object from view. and eclipses are specific types of occultations.

Oort cloud:

A vast theoretical cloud of predominantly icy hypothesized to surround the at distances ranging from 2,000 to 200,000. It is thought to be divided into two regions: a disc-shaped inner Oort cloud and a spherical outer Oort cloud. The outer limit of the Oort cloud is often considered the cosmographical boundary of the.

opacity: A measure of the resistance of a medium to the radiative transmission of energy. Within a star, it is an important factor in determining whether convection occurs.

open cluster: A gravitationally bound group of up to one thousand stars that formed together in the same.

opposition: The positioning of two celestial objects on opposite sides of the sky, from the perspective of an observer. This occurs, for example, when a planet makes its closest approach to the Earth, placing it in opposition to the Sun.

orbit: The gravitationally curved trajectory of an, such as the trajectory of a around a or a  around a planet. Though the smaller body is often said to orbit the larger body itself, both bodies actually follow approximately around a common  positioned at a focal point of each ellipse. The word "orbit" can variously refer to the elliptical trajectory itself or the act of following this trajectory, and can refer to a stable, regularly repeating trajectory as well as a non-repeating trajectory.

orbit plot:

A schematic diagram of a complete. For a binary system, it is typically presented from the 's frame of reference.

orbital eccentricity: A parameter that determines how much an deviates from a perfect circle. For an, the eccentricity ranges from greater than zero to less than one.

orbital elements: The set of parameters that uniquely define an.

orbital inclination: The tilt of an object's around an astronomical body, expressed as the angle between the or axis of direction of the orbiting object and a.

orbital mechanics:

orbital node: One of two points at which the of an intersects a specified  to which it is ; in some contexts, the two nodes may be distinguished as the  and the. A, which is coplanar with the reference plane, has no nodes.

orbital period:

The time a given takes to complete one around another object. For objects in the, the orbital period is often referred to as the.

orbital plane: The imaginary geometric plane defined by the of an around its. The Earth's orbital plane, which defines the, is commonly used as a for the orbits of other objects in the.

orbital resonance: The situation that occurs when two or more bodies exert regular, periodic gravitational influences on each other such that one or more of their (e.g.,, , etc., or any combination thereof) exist in some definite mathematical relationship with each other. Most commonly, the term refers to mean-motion orbital resonance, in which the bodies' are related by a ratio of small integers. For example, the Pluto exists in a stable 2:3 resonance with Neptune, such that Pluto completes two orbits around the  in the same time it takes Neptune to complete three. Resonance may act on any time scale, from short-term to, and often leads to either long-term stabilization of the orbits or their eventual destabilization.

orbital speed: The speed at which an astronomical body or object around a, or its speed relative to the center of mass of the most massive body in the system. The term may be used to refer to either the mean orbital speed, i.e. the average speed over the entire, or the instantaneous speed at a particular point in the orbit. Maximum instantaneous orbital speed typically occurs at.

origin of longitude:

osculating orbit: The hypothetical, idealized that an orbiting object would follow around its if all  were absent, i.e. the orbit that coincides with the instantaneous orbital state vectors at a given moment in time.

outer space:

The vast, nearly empty expanse that exists beyond the Earth and between all, characterized generally by extremely low densities of particles, extremely low temperatures, and minimal gravity. Most of the volume of the is intergalactic space, and even galaxies and star systems consist almost entirely of empty space.

P
parsec (pc): A unit of length defined as the distance at which a star would show a shift of exactly one arcsecond as observed from Earth's orbit. It is equal to 3.2616 or 206,265. The word "parsec" is a portmanteau of the words parallax and second.

partial solar eclipse:

peak magnitude: For a variable star, this is the highest amplitude achieved during a rise in luminosity, followed by a decline. This data point can provide useful distance information for a cataclysmic variable. It can be determined from a light curve of the stellar variability.

periapsis:

The point at which an orbiting body is closest to its. Contrast.

perigee: The point at which a body orbiting the Earth (such as the or an ) is closest to the Earth. Contrast.

perihelion: The point at which a body orbiting the Earth's is closest to the Sun. Contrast.

perturbation: The complex motion of an astronomical body that is subject to forces other than the gravitational attraction of its alone, or any force which complicates the characteristics of the body such that the idealized  of the  is not an accurate representation of the body's actual orbit. Perturbing forces may include the gravitational forces exerted by any number of additional bodies, the off-center gravitational forces which are consequences of bodies not being perfectly spherical, and/or atmospheric resistance.

phase angle: The or angle between an orbiting body and the Sun as viewed from a particular perspective, such as the Earth. It determines the amount of a planet or moon's visible surface that lies in shadow. such as Venus generally have low phase angles as seen from Earth, so they often appear as a slim crescent; such as Mars and Jupiter usually have high phase angles, so that little of the shadowed side is visible.

photometric system:

photosphere:

plane of reference:

An arbitrarily chosen, imaginary plane from which to measure and define such as and. The,, and are all commonly used as reference planes in various contexts.

plane of the sky: An imaginary plane that is perpendicular to the line of sight. Typically this is used as a reference plane for the of an orbital plane of a distant star system.

planet: A type of the Sun, which is massive enough to be rounded by its own gravity (but not massive enough to achieve thermonuclear fusion) and has  of all. The term exoplanet is used in reference to a planet-like object that is not orbiting the Sun.

planetary: Of or relating to a or planets.

planetary body:

Any that is geologically differentiated or in hydrostatic equilibrium and therefore has a -like geology, such as a,, or other , but excluding smaller objects such as.

planetary differentiation: The process of separating out different constituents of a planetary body, causing it to develop compositionally distinct layers (such as a metallic core).

planetary nebula: A type of emission nebula formed from a glowing shell of expanding plasma that has been ejected from a star late in its life. The name derives from their resemblance to a. An example is the Ring Nebula.

planetary science:

The scientific study of,, and , with the aim of understanding their formation, composition, topography, dynamics, and interactions with other bodies.

planetary system: Any set of gravitationally bound non- objects in or out of around a or. In general, planetary systems include one or more, though such systems may also consist of, , , , , and , among other objects.

planetary-mass object (PMO):

planetesimal: Any solid object (generally larger than 1 km in diameter) that arises during the formation of a whose internal strength is dominated by self-gravity and whose orbital dynamics are not significantly affected by gas drag. The term is most commonly applied to small bodies thought to exist in and  during the process of planet formation, but is also sometimes used to refer to various types of  which are left over from the formation process. There is no precise distinction between a planetesimal and a.

planetoid: Another name for a or.

polar orbit: An in which the orbiting object passes directly over or nearly over both poles of the body being orbited during each revolution. It therefore has an equal or nearly equal to 90 degrees to the body's.

precession: Any slow change in the orientation of an object's. For the Earth in particular, this phenomenon is referred to as the. Apsidal precession refers to a steady change in the orientation of an orbit, such as the precession in the orbit of Mercury that was explained by the theory of general relativity.

precession of the equinoxes:

primary:

The main physical body of a gravitationally bound, multi-object system. The primary constitutes most of the system's mass and is generally located near the system's.

prograde motion:

Orbital or rotational motion of an object in the same direction as the rotation of the object's. The direction of rotation is determined by an inertial frame of reference such as the. Contrast.

projected separation: The observed physical separation between two astronomical objects, as determined from their angular separation and estimated distance. For planets and, this distance is usually given in. The actual separation of the two objects depends on the angle of the line between the two objects to the line-of-sight of the observer.

proper motion: The rate of angular motion of an object over an interval of time, usually years. For stars, this is typically given in milliarcseconds per year.

protoplanet: A large embryo that originated within a and has since undergone internal melting to produce an interior of non-uniform composition. Protoplanets represent an intermediate step in the formation of a full-sized ; they are thought to form out of smaller as they collide with each other and gradually coalesce into larger bodies.

protoplanetary disk:

protostar: A concentration of mass formed out of the contraction of a collapsing interstellar cloud. Once sufficient mass has fallen onto this central core, it becomes a pre-main-sequence star.

pseudo-synchronous rotation: For an eccentric orbit, this is a near synchronization of revolution and rotation at periastron.

pulsar: A highly magnetized rotating or that emits a beam of electromagnetic radiation. This beam is observed only when it is pointing toward Earth, making the object appear to pulse.

Q
quadratic field strength: A method of computing the mean strength of a varying stellar magnetic field. It is determined by calculating the root mean square of a series of longitudinal magnetic field strength measurements taken at different times.

quadrature: A configuration in which two celestial bodies have apparent that differ by 90 degrees as viewed from a third body, e.g. when a planet's is perpendicular to the direction of the  as viewed from the Earth. The term is used especially to describe the position of a or the  at its first and last quarter phases.

quasar:

A distant, point-like energy source originating from a powerful. Its is generated by the of gas onto a. Quasars emit radiation across the electromagnetic spectrum from radio waves to X-rays, and their ultraviolet and optical spectra are characterized by strong, broad emission lines.

R
radial velocity: The velocity of an object along the line of sight to the observer, which in astronomy is usually determined via Doppler spectroscopy. Positive values are used to indicate a receding object. An object such as a star can undergo changes in its radial velocity because of the of another body, or because of radial pulsations of its surface. The latter, for example, occurs with a Beta Cephei variable star.

radio astronomy: The subfield of that studies at radio frequencies, using large radio antennas known as radio telescopes.

radio source: Any that emits strong radio waves into space. These objects are the basis for.

red-giant branch: A conspicuous trail of enlarged red stars found on the for a typical. It begins at the main-sequence turnoff point and extends toward the higher luminosity and lower temperature range until reaching the red-giant tip. This branch consists of older stars that have evolved away from the main sequence but have not yet initiated helium fusion in their core region.

redshift:

regular moon: A following a relatively close and orbit with little or no  or. Regular moons are thought to form in situ about their, as opposed to, which are thought to be captured.

relativistic jet:

réseau: A grid of fine lines or crosshatches engraved upon a transparent glass plate, which when placed in front of film during a photographic exposure produces a corresponding grid in the resulting photograph by creating permanent shadows on the film negative. These grids are used in some photographic telescopes to produce reference markers in photographs of distant stars, allowing precise and convenient measurement of astrometric positions.

retrograde motion: Orbital or rotational motion of an object in the direction opposite the rotation of the object's. The direction of rotation is determined by an inertial frame of reference such as the. Contrast.

right ascension: In the, the celestial equivalent of terrestrial longitude. It divides the into 24 hours, each of 60 minutes.

ring system: A disk- or ring-shaped accumulation of various solid material such as and that orbits an  such as a. Ring systems are common components of around, as with the Rings of Saturn. See also.

Roche limit: The distance from an astronomical object at which the matches an orbiting body's gravitational self-attraction. Inside this limit, the tidal forces will cause the orbiting body to disintegrate, usually to disperse and form a. Outside this limit, loose material will tend to coalesce.

rogue planet:

Any that orbits a directly rather than a  or. Such objects have often been ejected from the in which they formed, or otherwise have never been gravitationally bound to any star system.

Rosseland optical depth: An extinction coefficient of an, which describes the net to radiation at a given depth. See.

rotation period: The time that an object takes to complete a single revolution about its own relative to the. It is not necessarily the same as the object's or.

rotational modulation: A phenomenon which causes the of a star to vary as rotation carries or other localized activity across the line of sight. Examples include RS CVn and BY Dra variables.

S
Saber's beads: A broken arc of illuminations seen at the limb of very young or very old lunar crescents. The visual similarity to the moments before and after a was first noted by American astronomer Stephen Saber.

satellite galaxy: A smaller companion that orbits within the gravitational potential of a more massive and luminous host galaxy; e.g. the Large Magellanic Cloud is a satellite galaxy of the.

scattered disc:

scintillation:

Rapid variations in the apparent, color, or position of a (or any other distant luminous object) as viewed through a medium, such as the Earth's atmosphere, caused by the passing of light through layers of turbulence in the medium. Most terrestrial scintillation effects are the result of atmospheric refraction caused by small-scale fluctuations in air density, and are much more pronounced near the, since light rays near the horizon must travel longer paths through the atmosphere before reaching the observer.

secular: Continuing, or changing in a non-periodic way, over a long period of time.

secular motion: Any change in movement that happens over a very long time period. Examples include the perihelion precession of Mercury, the tidal acceleration of the Earth–Moon system, and.

selenocentric: With reference to, or pertaining to, the geometric center of the Earth's.

semi-diameter: The angle at the position of an observer subtended by the equatorial radius of the Sun, the Moon, or a planet.

semi-major axis:

One half of the longest diameter (the major axis) of an ellipse. It is expressed in units of length and often used to give a physical dimension to a two-body, such as for a system or star–planet system. When the distance between the orbiting bodies is unknown, the semi-major axis may be given as an angle.

September equinox:

The precise time of year on Earth when the Sun appears to cross the, while generally trending southward at each passage. It represents the moment at which the North Pole of the Earth begins to tilt away from the Sun, and typically occurs on or near September 22 each year. It is the autumnal equinox in the Northern Hemisphere and the vernal equinox in the Southern Hemisphere. Contrast.

sidereal day: The of an object (e.g. the Earth) with respect to the distant of its own  (rather than to its primary star, e.g. the ), measured as the time it takes for the fixed stars, as viewed from a particular point on the object's surface, to return to the same position in the  on consecutive nights. The Earth's sidereal day is equal to approximately 86,164.09 seconds (23 hours, 56 minutes, 4.09 seconds), about four minutes shorter than the, which instead reckons time based on the Sun's position in the sky.

sidereal period: The of an object within the, e.g. the Earth's orbital period around the Sun. The name "sidereal" implies that the object returns to the same position relative to the of the  as observed from the Earth.

sidereal time: The calculation of the passage of time based on the of the in the Earth's. The fundamental unit of sidereal time is the, i.e. the time interval between two successive returns of the fixed stars to the local, as viewed from a given location on the Earth's surface.

sidereal year:

sky: Everything that lies above the surface of the Earth, including the atmosphere and. In the context of, the term "sky" is also used as another name for the. See also.

small Solar System body (SSSB):

solar day: A on Earth, i.e. the of the Earth with respect to the, measured as the time it takes for the Sun, as viewed from a particular point on the Earth's surface, to return to the same position in the  (e.g. to cross the same ) on consecutive days. Because the Earth's orbit around the Sun affects the angle at which the Sun is seen from the Earth, the Sun appears to take slightly longer to return to the same position than do the, which results in the solar day being on average about four minutes longer than the. The length of the solar day is also not constant, but rather changes over the course of the year because the Earth's orbit is and because its rotational axis is  to its orbital plane. One (averaged over the Earth's orbital period) is currently equal to 86,400 seconds, or exactly 24 hours.

solar eclipse: An of the by the Earth's, in which a portion of the Earth passes through the shadow cast by the Moon, temporarily blocking sunlight, fully or partially, from reaching that portion of the Earth's surface. A solar eclipse occurs when the Moon is between the Sun and the Earth. Because all three bodies are continuously moving, the shadow of the Moon traces out a narrow path across the Earth's surface, and from any given location within or very close to this path, the eclipse is visible only for a short duration. Depending on the observer's location and on the apparent sizes of the solar and lunar disks in the sky, an eclipse may appear to be, , or.

solar facula: Solar faculae are bright spots in the of the Sun that form in the canyons between solar granules. They are produced by concentrations of magnetic field lines. The Sun's faculae are most readily observed near the. Faculae occur on other stars (see facula). solar flare:

solar granule:

solar mass: A standard unit of mass equal to the mass of the Earth's, or approximately $1.988 kg$. It is commonly used to express the masses of other and astronomical objects relative to the Sun.

solar prominence: A large, bright, transient feature, often in the shape of a, consisting of plasma extending outward from the 's into the. Prominences may be hundreds of thousands of kilometers long.

solar radius: A standard unit of distance equal to the radius of the Earth's (typically measured from the Sun's center to the layer in the at which the optical depth equals 2/3), or approximately 695,700 km. It is commonly used to express the radii of other  and astronomical objects relative to the Sun.

solar storm: See.

Solar System: The gravitationally bound of the Earth's and all of the objects that orbit it, either directly or indirectly, including the eight true, five, and numerous  such as , , and.

solar time: The calculation of the passage of time based on the of the in the Earth's. The fundamental unit of solar time is the, i.e. the time interval between two successive returns of the Sun to the local, as viewed from a given location on the Earth's surface. Because the duration of this interval changes during the Earth's orbit around the Sun, is distinguished from. Solar time and were employed by astronomers as time reckoning systems before the introduction of.

solar wind: A stream of charged particles, primarily protons, electrons, and alpha particles, released from the 's and flowing outwards at up to 900 km/s into interplanetary space. Phenomena influenced by the solar wind include,, and the plasma tails of.

solstice: Either of the two precise times of year when the reaches its most northerly or most southerly point in the as seen from Earth; or, equivalently, when the Sun's apparent geocentric  is either 90 degrees or 270 degrees. The solstices occur on or near June 20 and December 21 each year. The, called the in the Northern Hemisphere, is the annual date featuring the longest duration of daylight and the shortest duration of nighttime for any given point in the Northern Hemisphere; the reverse is true in the Southern Hemisphere, where the June date is the.

spectral classification: See.

spectroscopic binary: A type of system where the individual components have not been resolved with a. Instead, the evidence for the binarity comes from shifts observed in the spectrum. This is caused by the Doppler effect as the of the components change over the course of each orbit.

spectroscopy:



speed of light:

spherical astronomy:

A branch of which is used to locate the positions of astronomical objects on the as they would appear from a particular date, time, and location on Earth. It relies on the mathematical methods of spherical geometry and the measurements of.

spiral galaxy:

standard gravity:

The nominal gravitational acceleration of an object in a vacuum near the surface of the Earth, as a result of Earth's gravity and, less importantly, the centrifugal force generated by its rotation. It is by definition equal to $9.807 m/s2$ (approximately $32.174 ft/s2$).

star: A massive, luminous spheroid of plasma held together by its own gravity which, for at least a portion of its life, radiates energy into due to the thermonuclear fusion of hydrogen into helium within its core. Astronomers can determine the mass, age, platemperature, chemical composition, and many other properties of a star by observing its motion through space, its, and its.

star catalogue:

star cluster:

star system:

Any small number of that orbit each other, bound by gravitational attraction, such as a system. In the broadest sense, very large groups of stars bound by gravitation such as and  are also star systems. Star systems are distinct from, which include and other bodies such as.

starburst galaxy: Any that has an anomalously high rate of star formation. The criteria for a starburst is a star formation rate that would normally consume the galaxy's available supply of unbound gas within a time period shorter than the age of the galaxy. Most starbursts occur as a result of galactic interactions, such as a merger.

starfield: Any set of visible in an arbitrarily sized field of view of a, usually in the context of some region of interest within the. For example, the starfield surrounding the stars Betelgeuse and Rigel could be defined as encompassing some or all of the Orion constellation.

stellar: Of or relating to a or.

stellar atmosphere:

The outermost region of a, located above the stellar core, radiation zone, and convection zone. Although it constitutes only a small portion of the star's mass, for some evolved stars the stellar envelope can encompass a significant fraction of the radius.

stellar classification:

The categorization of based upon their. The modern scheme is a two-dimensional classification based on temperature and.

stellar designation:

stellar dynamics:

stellar envelope: The region within the volume of a star that transports energy from the stellar core to the ; or another name for the stellar atmosphere itself. The of gases encompassing a system.

stellar evolution:

stellar evolution model:

An model of a star's over time based upon its mass and chemical composition.

stellar magnetic field: A magnetic field generated by the convective motion of plasma inside a, responsible for phenomena such as and.

stellar parallax:

stellar remnant:

submillimetre astronomy: The subfield of that studies detectable at submillimetre wavelengths (i.e. terahertz radiation).

subsatellite: Any or that orbits another natural satellite, i.e. "a moon of a moon".

substellar object:

An whose mass is smaller than the smallest mass at which the fusion of hydrogen nuclei can be sustained (equivalent to approximately 0.08 ), including and some, as well as certain.

Sun:

supercluster:

superior planet: An archaic term that is sometimes used to refer to that orbit further from the Sun than the Earth, such as Saturn. The name originated from the geocentric of Ptolemy. Contrast.

supermassive black hole (SMBH): One of a class of very large which possess masses ranging from hundreds of thousands to many billions of times the. These are typically found at a, where they can have a profound effect upon the evolution of the surrounding.

supernova: An extremely, stellar explosion occurring during a massive 's final stages or when a  is triggered into runaway nuclear fusion.

surface gravity (g): The gravitational acceleration experienced at the equatorial surface of an or other, including that produced by the effects of rotation. It is typically expressed in units of acceleration such as meters per second squared (m/s2) or as a multiple of the Earth's, which is equal to $9.807 m/s2$.

synchronous orbit: Any in which an object orbits its with an  equal to the average  of the primary and in the  as the primary's rotation.

synodic day:

The time it takes for an object to rotate once about its own (i.e. its ) relative to the it is orbiting (rather than to the much more distant ). The synodic day may be described as the time between two consecutive sunrises (in the case where the primary is a ), which is not necessarily the same as the. As it does on Earth, an object's synodic day may change slightly in duration over the course of the due to  and ; Earth's synodic day is often called a.

synodic period: The time it takes for a body visible from another body (often the Earth) to complete a cycle with respect to the visible in the second body's. Synodic period is most commonly used to indicate the elapsed time between a given body's consecutive appearances in the in the  as observed from Earth, but can in principle be calculated with respect to the sky as observed from any body. It is related to but distinct from the, a result of the fact that both the body being studied (e.g. Jupiter) and the body from which it is being observed (e.g. Earth) are independently orbiting a third body (the ).

synodic time: The calculation of the passage of time based on successive of an astronomical object, such as a (i.e. successive returns of the object to the same  in the Earth's ).

syzygy: The straight-line configuration of three celestial bodies in a gravitational system.

T
tangential velocity: The component of the velocity of a star or other that is perpendicular to the line of sight of the (i.e. in the tangent plane). This component can be computed from the body's observed and its measured distance from the observer.

telescope:

telluric star: A with nearly featureless continuum spectra that can be used to correct for the effect of telluric contamination of the Earth's atmosphere on the spectra of other stars. For example, water vapor in the atmosphere creates significant telluric absorption bands at wavelengths above 6800 Å. These features need to be corrected for in order to more accurately measure the spectrum.

termination shock: The boundary within the, approximately 75 to 90 from the, beyond which the slows to subsonic speeds (relative to the Sun) as a result of interactions with the local.

terminator: The line that divides the illuminated side of a or from its dark side. The line moves as the object rotates with respect to its parent.

theoretical astronomy: A branch of astronomy that uses analytical and computational models based on principles from physics and chemistry to describe, explain, and model the properties of astronomical objects and phenomena, with the ultimate goal of accurately predicting the observable or testable consequences of those models.

thick disk population:

thin disk population: The layer of the galaxy where the spiral arms are found and where most of the star formation takes place. It is about 300 – deep and centered on the. Stars belonging to this population generally follow orbits that lie close to this plane. This is in contrast to members of the and.

tidal braking:

The transfer of momentum between an astronomical body and an orbiting satellite as the result of. This can cause changes in the for both bodies as well as modification of their mutual orbit. A satellite in a orbit will gradually recede from its primary while slowing the rotation rate of both bodies.

tidal force:

tidal locking: The net result of continued such that, over the course of an orbit, there is no net transfer of angular momentum between an astronomical body and its gravitational partner. When the is low, the result is that the  orbits with the same face always pointed toward its. An example is the, which is tidally locked with the Earth.

tidal stream: A stream of and gases which are stripped from gas clouds and star clusters because of interaction with the gravitational field of a such as the.

tilt erosion: The gradual reduction of the of an orbiting due to tidal interactions.

Tisserand's parameter:

A measure of the orbital motion of a relatively small body (e.g. an or ) with respect to a larger, perturbing body (e.g. a ), used for in which the three bodies all differ greatly in mass. The parameter is calculated from the of each body, including the small body's,, and , and is useful in specifically identifying small bodies observed before and after planetary encounters, as its numerical value remains largely constant throughout the body's lifetime. It is also used to distinguish between different kinds of orbits which are characteristic of different classes of bodies.

topocentric: With reference to, or pertaining to, a point on the surface of the Earth.

total solar eclipse:



trans-Neptunian object (TNO):

transit: The passage of a particular across a particular. An astronomical event during which a celestial body or object passes visibly across the face of a much larger body. An example is the transit of Venus across the face of the, which was visible from Earth in 2004 and 2012. Because a transit results in a decrease in the net from the two objects, the transit method can be used to detect as they pass in front of their host stars. A transit by an object that appears roughly the same size or larger than the body it is transiting is called an or.

trojan:

tropical year:

true anomaly: The angle between the direction of and the current position of an orbiting body as it moves along an, as measured from the nearest focus of the ellipse. The true anomaly is one of three angular parameters that define a position along an orbital path, the other two being the and the, and also one of six canonical  used to characterize an orbit.

Tully–Fisher relation: An empirical relationship between the mass or intrinsic of a and its angular velocity or emission line width. It can be used to estimate the distance of the galaxy, and hence forms a rung on the.

twilight: The time period immediately before sunrise and after sunset during which, despite the Sun being completely below the horizon, the scattering of sunlight by the Earth's atmosphere supplies significant illumination to the ambient environment. Several definitions of twilight are commonly distinguished, including,, and.

two-body problem:

U
UBV photometric system:

universe: The entirety of space and time and their contents, including,, , all other forms of matter and energy, and the physical laws and constants that describe them. When not otherwise qualified, "the Universe" usually refers to the entire Universe, whose spatial extent is unknown because it is not directly measurable; this is distinguished from the, whose size it is possible to measure. One of many hypothetical parallel universes which exist as causally disconnected constituent parts of a larger, which itself comprises all of space and time and their contents.

V
variable star: Any that is observed to vary in brightness. This variation may be periodic, with one or more cycles that last hours, days, months, or even years. Some stars vary in an irregular manner, while others undergo cataclysmic changes in brightness. Other forms of variability are intrinsic changes to the star's or its profile of spectral lines.

velocity dispersion: The statistical dispersion of velocities about the mean velocity for a group of objects, such as stars in a or galaxies in a. This value can be used to derive the combined mass of the group by using the virial theorem.

Virgo Supercluster (Virgo SC):

W
weak-line star: A reference to the faintness of the spectral lines for a star compared to standard stars with the same. Since most absorption lines are caused by elements other than hydrogen and helium—what astronomers refer to as "metals"—these are sometimes called metal weak stars.

white dwarf: A type of composed mostly of electron-degenerate matter. A white dwarf lacks the mass needed to continue the nuclear fusion process with its constituent atoms, so the object's energy output normally comes from radiative cooling. See and Type Ia supernova.

Wilson–Bappu effect: A correlation between the width of the singly ionized calcium K-line (Ca II K) at 3933 Å and the of the emitting. This linear relation makes it useful for determining the distances of G, K, and M-type stars.

X
XBONG:

A seemingly normal that does not appear to have an, yet displays an anomalous level of excess X-ray emission. X-ray source: A source of X-rays. They are usually produced when a high-mass object, usually a neutron star or black hole and a companion star are in a binary system.

Z
zenith: The point in the that is directly overhead from the perspective of a particular location on the Earth.

zero-age main sequence (ZAMS): The sequence of positions along the achieved by newly formed, chemically homogeneous which have finished contracting and have reached hydrostatic equilibrium, with energy being derived solely from nuclear fusion.

zodiac: The area of the that extends approximately 8 degrees north or south (in ) of the, the apparent path of the across the  over the course of the year as observed from Earth. The Sun,, and visible planets appear to travel across a band of twelve Zodiac constellations within this belt as the Earth orbits the Sun.

zodiacal light: A band of light in the night sky, thought to be sunlight reflected from cometary dust concentrated in the plane of the zodiac, or ecliptic.