Glossary items by topic: Stars & Nebulae

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.

Binary Star System

A system of two stars orbiting around a common center of mass that are bound together by their mutual gravitational attraction.

Blue Star

A massive, hot star that appears blue in color. Spica in the constellation Virgo is an example of a blue star.

Brown Dwarf

An object too small to be an ordinary star because it cannot produce enough energy by fusion in its core to compensate for the radiative energy it loses from its surface. A brown dwarf has a mass less than 0.08 times that of the Sun.

Cepheid Variable

A type of pulsating star whose light and energy output vary noticeably over a set period of time. The time period over which the star varies is directly related to its light output or luminosity, making these stars useful standard candles for measuring intergalactic distances.

Dark Dust Cloud

A region of interstellar space that contains a rich concentration of gas and dust. Such a cloud is often irregular in shape but sometimes has a well-defined edge. Visible light cannot pass through these clouds, so they obscure the light from stars beyond them.

Gaseous Nebula

A glowing cloud of gas in interstellar space. The cloud of gas may be either an emission nebula, which absorbs ultraviolet light from nearby stars and re-radiates visible light, or a reflection nebula, which reflects light off of its dust particles.

Giant Star

A dying star that has used up the hydrogen fuel in its core and has begun to expand. Giant stars are generally larger than our Sun.

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.

Hertzsprung-Russell Diagram

A plot showing the relationship between the brightness (luminosity) and the surface temperatures of many stars. Often the spectral class, which is based on the temperature of the star, is used as a label.

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.

Light Curve

A plot showing how the light output of a star (or other variable astronomical object) changes with time.

Molecular Cloud

A relatively dense, cold region of interstellar matter where hydrogen gas is primarily in molecular form. Stars generally form in molecular clouds. Molecular clouds appear as dark blotches in the sky because they block all the light behind them.

Neutron Star

An extremely compact ball of neutrons created from the central core of a star that collapsed under gravity during a supernova explosion. Neutron stars are extremely dense: they are only 10 kilometers or so in size, but have the mass of an average star (usually about 1.5 times more massive than our Sun). A neutron star that regularly emits pulses of radiation is known as a pulsar.


A binary star system (consisting of a white dwarf and a companion star) that rapidly brightens, then slowly fades back to normal.

Period-Luminosity Law

A relationship that describes how the luminosity or absolute brightness of a Cepheid variable star depends on the period of time over which that brightness varies.

Planetary Nebula

An expanding shell of glowing gas expelled by a star late in its life. Our Sun will create a planetary nebula at the end of its life.


A collection of interstellar gas and dust whose gravitational pull is causing it to collapse on itself and form a star.


A neutron star that emits rapid and periodic pulses of radiation.

Red Giant Star

An old, bright star, much larger and cooler than the Sun. Betelgeuse (alpha Orionis) is an example of a red giant.

Spectral Class (Spectral Type)

A classification scheme that groups stars according to their surface temperatures and spectral features.

Spectral Line

In a spectrum, an emission (bright) or absorption (dark) at a specific frequency or wavelength.


A huge ball of gas held together by gravity. The central core of a star is extremely hot and produces energy. Some of this energy is released as visible light, which makes the star glow. Stars come in different sizes, colors, and temperatures. Our Sun, the center of our solar system, is a yellow star of average temperature and size.

Star Cluster

A group of stars born at almost the same time and place, capable of remaining together for billions of years because of their mutual gravitational attraction.

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 Evolution

The process of change that occurs during a star’s lifetime from its birth to its death.


The explosive death of a massive star whose energy output causes its expanding gases to glow brightly for weeks or months. A supernova remnant is the glowing, expanding gaseous remains of a supernova explosion.

Supernova Remnant

The glowing, expanding gaseous remains of a supernova explosion.

T-Tauri Star

A class of very young, flaring stars on the verge of becoming normal stars fueled by nuclear fusion.

Variable Star

A star whose luminosity (brightness) changes with time.

White Dwarf Star

The hot, compact remains of a low-mass star like our Sun that has exhausted its sources of fuel for thermonuclear fusion. White dwarf stars are generally about the size of the Earth.

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