Elliptical galaxies are spheroidal concentrations of billions of stars that resemble Globular Clusters on a grand scale. They have very little internal structure; the density of stars declines smoothly from the concentrated center to the diffuse edge, and they can have a broad range of ellipticities (or aspect ratios). They typically contain very little interstellar gas and dust, and no young stellar populations (although there are exceptions to these rules). Edwin Hubble referred to Elliptical galaxies as “early-type” galaxies, because he thought that they evolved to become Spiral Galaxies (which he called “late-type” galaxies). Astronomers actually now believe the opposite is the case (i.e., that Spiral galaxies can turn into Elliptical galaxies), but Hubble's early- and late-type labels are still used.
Once thought to be a simple galaxy type, ellipticals are now known to be quite complex objects. Part of this complexity is due to their amazing history: ellipticals are thought to be the end product of the merger of two Spiral galaxies. You can view a computer simulation MPEG movie of such a merger at this NASA HST webpage (warning: the file is 3.4 MB).
Elliptical galaxies span a very wide range of sizes and luminosities, from giant Ellipticals hundreds of thousands of light years across and nearly a trillion times brighter than the sun, to dwarf Ellipticals just a bit brighter than the average globular cluster. They are divided to several morphological classes:
- cD galaxies:
Immense and bright objects that can measure nearly 1 Megaparsec (3 million light years) across. These titans are only found near the centers of large, dense clusters of galaxies, and are likely the result of many galaxy mergers.
- Normal Elliptical galaxies
Condensed Object with relatively high central surface brightness. They include the giant ellipticals (gE'e), intermediate-luminosity ellipticals (E's), and compact ellipticals.
- Dwarf elliptical galaxies (dE's)
This class of galaxies is fundamentally different from normal ellipticals. Their diameters on the order of 1 to 10 kiloparsec with surface brightness that is much lower than normal ellipticals, giving them a much more diffuse appearance. They display the same characteristic gradual decline of star density from a relatively dense core out to a diffuse periphery.
- Dwarf spheroidal galaxies (dSph's)
Extreme low-luminosity, low surface-brightness and have only been observed in the vicinity of the Milky Way, and possibly other very nearby galaxy groups, such as the Leo group. Their absolute magnitudes are only -8 to -15 mag. The Draco dwarf spheroidal galaxy has an absolute magnitude of -8.6, making it fainter than the average globular cluster in the Milky Way!
- Blue compact dwarf galaxies (BCD's)
Small galaxies that are unusually blue. Thehave photometric colors of B-V = 0.0 to 0.30 mag, which is typical for relatively young stars of spectral type A. This suggests that BCDs are currently actively forming stars. These systems also have abundant interstellar gas (unlike other Elliptical galaxies).
You can see examples of Elliptical galaxies in KStars, using the Find Object window (Ctrl+F). Search for NGC 4881, which is the Giant cD galaxy in the Coma cluster of galaxies. M 86 is a normal Elliptical galaxy in the Virgo cluster of galaxies. M 32 is a dwarf Elliptical that is a satellite of our neighbor, the Andromeda galaxy (M 31). M 110 is another satellite of M 31 that is a borderline dwarf spheroidal galaxy (“borderline” because it is somewhat brighter than most other dwarf spheroidals).