Introductory Astronomy: Elliptical Galaxies

Most of the galaxies we know are elliptical in shape. The largest of these elliptical galaxies may contain as many as 10 trillion stars (10 trillion solar masses) and may be as large as 100,000 parsecs in diameter. (This is comparable to the size of our galaxy - including the entire disk of our galaxy - but with about 100 times more stars. No wonder they are so bright!) Such huge galaxies are called Giant Ellipticals(an example is shown above). They are rare but spectacular. Most of the ellipticals are Dwarf Ellipticals , which have approximately a few million solar masses and diameters of about 2000 parsecs. They are low surface brightness objects. Dwarf Ellipticals generally are found in galaxy clusters or near large galaxies.

Ellipticals are different from spirals in that they have very little (or no) gas and dust. They have only stars which are concentrated near their centers (the nucleus of the galaxy). This means that very little (if any) star formation is taking place in elliptical galaxies and therefore that their stars are basically old, low mass stars (which have long lifetimes). This also means that elliptical galaxies contain very few (if any) hot, bright stars; these massive stars are short lived and would have died long ago.

Ellipticals are given their name because of their shapes. They are not flattened, but instead range from nearly spherical to highly elongated (like a football). Hubble classified elliptical galaxies according to their elongation. The most spherical galaxies he called E0 and the most elongated he called E7.