This **glossary of astronomy** is a list of definitions of terms and concepts relevant to Astronomy and Cosmology. with their related fields, Astronomy is involved with the study of celestial objects and phenomena originate outside the atmosphere of Earth.

A
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A-type star

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*.
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.

The Simple English Wiktionary has a definition for:

 ***scale**, **friction**, **spiral**, **inward**, and **luminosity***.

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

The Simple English Wiktionary has a definition for:

 ***contrast**, **magnitude**, **node**, and **canonical***.

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 called **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.
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 called the **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 called 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.

The Simple English Wiktionary has a definition for:

 ***distinguish**, **arbitrarily**, **diameter**, **composition**, and **beyond***.

asteroid
A minor planet of the inner Solar System, for example the one that orbits the Sun at a distance is no greater than the orbit of 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.

The Simple English Wiktionary has a definition for:

 ***interdisciplinary** and **extraterrestrial***.

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

astrometric binary
A type of binary system where evidence for an unseen orbiting companion is revealed by its 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.

The Simple English Wiktionary has a definition for:

 ***structure**, **entity**, **natural**, **contiguous**, and **interchangeable***.

The Simple English Wiktionary has a definition for:

 ***distinction**, **examinine**, **induce**, **orientation**, and **equivalent***.

astronomical body
Also called a **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 unit (AU)
A unit of length used primarily for measuring astronomical distances within the Solar System or between the Earth and distant stars.
astronomy
The scientific study of celestial objects and phenomena, the origins of those objects and phenomena, and their evolution.
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).
axial precession
A slow, continuous, gravity-induced change (a precession) in the orientation of an astronomical body's axis of rotation.
axial tilt
Also called **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.

B
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The Simple English Wiktionary has a definition for:

 ***outnumber** and **particle***.

barycenter
The common center of mass about which any two or more bodies of a gravitationally bound system orbit.
baryogenesis
How 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.