Galaxies

By definition galaxies are a large aggregation of stars, gas, and dust, typically containing billions of stars.

Recognition that galaxies are independent star systems outside the Milky Way came from a study of the Andromeda Galaxy (1926—29) by Edwin P. Hubble that indicated the great distances at which this and other galaxies are located. Previously, the galaxies had been classified with the luminous gas clouds, or bright nebulae, within the Milky Way.

The sun and its solar system, as well as the visible stars, are all in our galaxy, the Milky Way galaxy.

Billions of galaxies are within the optical range of the largest telescopes; in 1996 analysis of photographs taken from the Hubble Space Telescope increased the estimated number of galaxies from 10 billion to 50 billion.

A galaxy is held together by the gravitational attraction between its constituent parts, while its rotational motion prevents it from collapsing on itself.

Many large galaxies have smaller galaxies, called satellite galaxies, in close proximity.

A typical spiral galaxy is shaped like a flat disk, about 100,000 light-years in diameter, with a central bulge, or nucleus, containing old stars; winding through the disk are the characteristic spiral arms of dust, gas, and young stars. This type of galaxy is further classified as being either a normal or a barred spiral.

The elliptical galaxies, lacking spiral arms entirely and containing little or no gas and dust, resemble the nuclei of spiral galaxies. Their shapes vary from nearly spherical to highly flattened ellipsoids.

Spiral galaxies contain a larger number of bluer, younger stars, while elliptical galaxies contain a larger number of redder, older stars.

Many galaxies radiate a large fraction of their energy in forms other than visible light.

The Milky Way's nearest large galactic neighbor is the Andromeda Galaxy. It is wider and possibly brighter than our Milky Way.

A galaxy might contain a million solar systems like ours.

The milky way got its name from the way it makes a white path through the night sky (billions of distant stars packed close together make it look uniformly white).

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