galaxies unit review

galaxy

(astronomy) a collection of star systems

globular cluster

A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite.

Cepheid variable

A variable star's brightness changes over time. Cepheids are extremely important to astronomers because their periodic changes in brightness can be used to figure out how far away they are.

irregular galaxy

Irregular galaxies have no particular shape. They are among the smallest galaxies and are full of gas and dust.

inflationary universe

It is like the Big Bang except that the rate of expansion fluctuates during the first stages of the expansion.

halo

Spherical region that surrounds the Milky Way's nuclear bulge and disk.

spiral galaxy

The most common type of galaxy is called a "spiral galaxy." Not surprisingly, spiral galaxies look like spirals, with long arms winding toward a bright bulge at the center.

Milky Way

The name of our galaxy, a spiral galaxy that contains about 400 billion stars

Hubble's Law

The observation that the farther away a galaxy is, the faster it is moving away.

radio galaxy

a galaxy that emits an unusually large amount of radio waves

dark matter

a hypothetical form of matter that is believed to make up 90 percent of the matter in the universe

Barred spiral galaxy

a spiral galaxy with a central bar-shaped structure composed of stars.

variable star

a star whose brightness as seen from Earth

quasars

a starlike object that may send out radio waves and other forms of energy

steady state theory

a theory in which the universe is assumed to have average properties that are constant in space and time so that new matter must be continuously and spontaneously created to maintain average densities as the universe expands.

elliptical galaxy

a type of galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile.

Supercluster

large groups of smaller galaxy clusters or galaxy groups and are among the largest known structures of the cosmos. The Milky Way is in the Local Group of galaxies

RR Lyrae variable

periodic variable stars, commonly found in globular clusters, and often used as standard candles to measure galactic distances.

spiral arm

sites of strong star formation: we see dust, HII regions, blue stars, lots of gas.

Doppler effect

the change in frequency of a wave (or other periodic event) for an observer moving relative to its source.

big bang theory

the leading explanation about how the universe began

disk

the outer portion of a galaxy containing more dust and gas and fewer clusters

cosmology

the science of the origin and development of the universe