Elliptical Galaxies

Ellipticals galaxies have a round or elliptical shape (like an American football), with a smooth appearance, contain no gas to fuel new star formation, and show little or no signatures of dust or any distinct features. The stars which make up elliptical galaxies are old and red, not terribly bright, and are found crowded together in a giant centralized sphere (the galaxy bulge). Most ellipticals stopped forming stars more than 10 billion years ago, having used up their supply of gas in one great star-forming epoch. They are supported against self-gravity, which would cause them to shrink in size, by the random velocities of the stars, like molecules in a hot gas. The distribution of stellar motion determines the final shape of the galaxy (spherical, oblate, or very flattened). In the Hubble sequence E0, E1, E2, ... E7, the number is a function of how flattened the ellipse appears to be, with E0 corresponding to no flattening and E7 to a very elongated ellipse. This depends on the apparent ellipticity, so it refers to the projection of the galaxy's shape on the celestial sphere, not its actual shape (whether it is facing us head-on or side-on).

The masses of elliptical galaxies cover a large range: from about 10 up to 10 solar masses. The corresponding range of diameters is about 1/10 kiloparsecs to about 100 kiloparsecs, and the absolute blue magnitude varies over a correspondingly large range from -8 to -23 (the brightest elliptical galaxy is a million times brighter than the faintest). The smallest of the elliptical galaxies, called dwarf ellipticals, may thus be only a little larger than globular clusters, while the giant elliptical galaxies like M87 are among the largest galaxies in the Universe.

Many ellipticals are believed to be the result of collisions between two equally massive spiral galaxies. The collision process serves to disorder the disk shape of the spirals, and the end result is a swirl of stars rotating in all directions. Large ellipticals act as cannibals, consuming any smaller companion galaxies near to them. The accumulated material sinks slowly into the galaxy's core, sometimes creating new generations of massive, luminous stars. Over the course of millions of years the gas reaches the center of the galaxy, where supermassive black holes may lie in wait for a new supply of fuel.

NGC 4406 NGC 4486 (M87) NGC 4486 NGC 5078 (plus IC 879)
Leo I Dwarf Galaxy NGC 4472 NGC 5128 (Centaurus A) NGC 4636
Many elliptical galaxies are as similar as peas in a pod. [NASA/HST, AAO]