Glossary items by topic: Galaxies

Accretion Disk

A relatively flat, rapidly rotating disk of gas surrounding a black hole, a newborn star, or any massive object that attracts and swallows matter. Accretion disks around stars are expected to contain dust particles and may show evidence of active planet formation. Beta Pictoris is an example of a star known to have an accretion disk.

Active Galactic Nucleus (AGN)

A very bright, compact region found at the center of certain galaxies. The brightness of an active galactic nucleus is thought to come from an accretion disk around a supermassive black hole. The black hole devours matter from the accretion disk, and this infall of matter provides the firepower for quasars, the most luminous type of active galactic nucleus.

Active Galaxy

A galaxy possessing an active galactic nucleus at its center.


The fading fireball of a gamma-ray burst — a sudden burst of gamma rays from deep space — that is observable in less energetic wavelengths, such as X-ray, optical, and radio. After an initial explosion, an expanding gamma-ray burst slows and sweeps up surrounding material, generating the afterglow, which is visible for several weeks or months. The afterglow is usually extremely faint, making it difficult to locate and study.

Barred Spiral Galaxy

A galaxy with a “bar” of stars and interstellar matter, such as dust and gas, slicing across its center. The Milky Way is thought to be a barred spiral galaxy.

Black Hole

A region of space containing a huge amount of mass compacted into an extremely small volume. A black hole’s gravitational influence is so strong that nothing, not even light, can escape its grasp. Swirling disks of material — called accretion disks — may surround black holes, and jets of matter may arise from their vicinity.


The spherical structure at the center of a spiral galaxy that is made up primarily of old stars, gas, and dust. The Milky Way’s bulge is roughly 15,000 light-years across.

Colliding Galaxies

A galactic “car wreck” in which two galaxies pass close enough to gravitationally disrupt each other’s shape. The collision rips streamers of stars from the galaxies, fuels an explosion of star birth, and can ultimately result in both galaxies merging into one.

Dark Matter

Matter that is too dim to be detected by telescopes. Astronomers infer its existence by measuring its gravitational influence. Dark matter makes up most of the total mass of the universe.

Dwarf Galaxy

A relatively small galaxy. The Large and Small Magellanic Clouds, visible in the Southern Hemisphere, are two dwarf irregular galaxies that are neighbors of the Milky Way.

Elliptical Galaxy

A galaxy that appears spherical or football-shaped. Elliptical galaxies are comprised mostly of old stars and contain very little dust and “cool” gas that can form stars.

Galactic Center

The central hub or nucleus of a galaxy. The Milky Way’s galactic center is about 28,000 light-years from Earth.

Galactic Disk

A flattened disk of gas and young stars in a galaxy. Some galactic disks have material concentrated in spiral arms (as in a spiral galaxy) or bars (as in barred spirals).

Galactic Halo

Spherical regions around spiral galaxies that contain dim stars and globular clusters. The radius of the halo surrounding the Milky Way extends some 50,000 light-years from the galactic center.

Galactic Nucleus

The central concentration of matter (stars, gas, dust, and perhaps a black hole) in a galaxy, typically spanning no more than a few light-years in diameter.

Galactic Plane

The imaginary projection of the Milky Way’s disk on the sky. Most of the galaxy’s stars and interstellar matter reside in this disk. Objects in the galaxy are often referred to as being above, below, or in the galactic plane.


A collection of stars, gas, and dust bound together by gravity. The smallest galaxies may contain only a few hundred thousand stars, while the largest galaxies have thousands of billions of stars. The Milky Way galaxy contains our solar system. Galaxies are classified or grouped by their shape. Round or oval galaxies are elliptical galaxies and those showing a pinwheel structure are spiral galaxies. All others are called irregular because they do not resemble elliptical or spiral galaxies.

Galaxy Cluster

A collection of dozens to thousands of galaxies bound together by gravity.

Galaxy Evolution

The study of the birth of galaxies and how they change and develop over time.

Galaxy Supercluster

A vast collection of galaxy clusters that may contain tens of thousands of galaxies spanning over a hundred million light-years of space. Galaxy superclusters are the largest structures in the universe.

Globular Cluster

A collection of hundreds of thousands of old stars held together by gravity. Globular clusters are usually spherically shaped and are often found in the halos of galaxies. Each star belonging to a cluster revolves around the cluster’s common center of mass.

Gravitational Clustering

The process by which a large-scale structure grows as its gravity attracts smaller building blocks. Astronomers believe that all the large-scale structures (such as galaxies, galaxy clusters, and galaxy superclusters) that we see in the universe today formed through gravitational clustering.

Gravitational Lens

A massive object that magnifies or distorts the light of objects lying behind it. For example, the powerful gravitational field of a massive cluster of galaxies can bend the light rays from more distant galaxies, just as a camera lens bends light to form a picture.

Group of Galaxies

A small collection of galaxies bound together by gravity. The number of galaxies in a group can range from a few to dozens. The Milky Way is a member of the Local Group, a collection of more than 30 galaxies.


Hubble Deep Field North (HDF-N) is a tiny region of the northern sky near the Big Dipper toward which the Hubble Space Telescope was pointed for ten straight days in 1995. Because this observation was designed to detect very faint light from the most distant galaxies Hubble can observe, the field contains few bright celestial objects. Seemingly devoid of light, this small area provided a “keyhole” view of the universe’s past, reaching across space and time to see infant galaxies. By probing these remote regions of space, astronomers are gaining more information on galaxy development.


Hubble Deep Field South (HDF-S) is a tiny region of the southern sky near the Southern Cross toward which the Hubble Space Telescope was pointed for ten straight days in 1998. Because this observation was designed to detect very faint light from the most distant galaxies Hubble can observe, the field contains few bright celestial objects. Seemingly devoid of light, this small area provided a “keyhole” view of the universe’s past, reaching across space and time to see infant galaxies. By probing these remote regions of space, astronomers are gaining more information on galaxy development.

Host Galaxy

A galaxy in which a cosmic phenomenon, such as a supernova explosion or a gamma-ray burst, has occurred.

Interstellar Dust

Small particles of solid matter, similar to smoke, in the space between stars.

Interstellar Medium (ISM)

The sparse gas and dust located between the stars of a galaxy.

Interstellar Space

The dark regions of space located between the stars.

Irregular Galaxy

A galaxy that appears disorganized and disordered, without a distinct spiral or elliptical shape. Irregular galaxies are usually rich in interstellar matter, such as dust and gas. The Large and Small Magellanic Clouds are examples of nearby irregular galaxies.

Local Group

A small cluster of more than 30 galaxies, including the Andromeda galaxy, the Magellanic Clouds, and the Milky Way galaxy.

Magellanic Clouds

The Magellanic Clouds are two dwarf irregular galaxies. Known as the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), the galaxies are in the Local Group. The closer LMC is 168,000 light-years from Earth. Both galaxies can be observed with the naked eye in the southern night sky.

Milky Way Galaxy

The Milky Way, a spiral galaxy, is the home of Earth. The Milky Way contains more than 100 billion stars and has a diameter of 100,000 light-years.

Open Cluster

Also known as a galactic cluster, an open cluster consists of numerous young stars that formed at the same time within a large cloud of interstellar dust and gas. Open clusters are located in the spiral arms or the disks of galaxies. The Pleiades is an example of an open cluster.


Matter that is beginning to come together to form a galaxy. It is the precursor of a present-day galaxy and is sometimes called a “baby galaxy.”


The brightest type of active galactic nucleus, believed to be powered by a supermassive black hole. The word “quasar” is derived from quasi-stellar radio source, because this type of object was first identified as a kind of radio source. Quasars also are called quasi-stellar objects (QSOs). Thousands of quasars have been observed, all at extreme distances from our galaxy.

Seyfert Galaxy

A galaxy characterized by a moderately bright, compact active galactic nucleus, presumably powered by a black hole.


A black hole’s center, where the matter is thought to be infinitely dense, the volume is infinitely small, and the force of gravity is infinitely large.

Spiral Arms

A pinwheel structure, composed of dust, gas, and young stars, that winds its way out from the core of a normal spiral galaxy and from the ends of the bar in a barred spiral galaxy.

Spiral Galaxy

A spiral-shaped system of stars, dust, and gas clouds. A typical spiral galaxy has a spherical central bulge of older stars surrounded by a flattened galactic disk that contains a spiral pattern of young, hot stars, as well as interstellar matter.

Starburst Galaxy

A galaxy undergoing an extremely high rate of star formation. Starburst galaxies contain massive, deeply embedded stars that are among the youngest stars observed.

Stellar Black Hole

A black hole formed from the death of a massive star during a supernova explosion. A stellar black hole, much like a supermassive black hole, feeds off of nearby material — in this case, the dead star. As it gains mass, its gravitational field increases.

Supermassive Black Hole

A black hole possessing as much mass as a million or a billion stars. Supermassive black holes reside in the centers of galaxies and are the engines that power active galactic nuclei and quasars.

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