Galaxy

Elliptical galaxies have a three-dimensional ellipsoidal shape, so they appear in their two dimensional projections on the sky as ellipses. In his scheme, Hubble denoted elliptical galaxies with the letter E. He further subdivided ellipticals according to the amount of elongation of the ellipse, using numbers from 0 to 7. An E0 galaxy appears spherical. The most elongated elliptical galaxies are E7. The E1 through E6 galaxies are intermediate.

Note that this classification is based on the appearance of a galaxy, which may be different from its true shape owing to projection effects. Since Hubble's time, astronomers have learned that some ellipticals are relatively small and others are large. We now have the additional classification of either dwarf ellipticals or giant ellipticals. For finer divisions astronomers use the luminosity classes I, for the supergiant ellipticals, down to V for the smallest dwarf ellipticals.

Dwarf elliptical galaxies tend to be fairly small. They average about 30,000 light years in diameter, but can be as small as about 10,000 light years. The diameters of galaxies are a little uncertain because galaxies do not end sharply. Instead, they tend to gradually fade out with increasing distance from the center. By contrast, Giant elliptical galaxies average about 150,000 light years in diameter. The largest supergiant ellipticals are a few million light years in diameter.

The dwarf ellipticals have masses ranging from 100,000 to 10 million times the mass of the sun, suggesting that they have about that many stars. Giant ellipticals on the other hand will typically have 10 trillion times the mass of the sun and therefore roughly that many stars. Both giant and dwarf elliptical galaxies have only old stars and very small amounts of the interstellar gas and dust that is the raw material for forming new stars, probably due to the loss of gas clouds to star formation during the collisions that formed the elliptical shape.