Elliptical Galaxies
Dwarf Elliptical Galaxy M32
Credits: 1.1 Meter Hall Telescope, Lowell Observatory, Bill Keel
(U. Alabama).
Elliptical galaxies, the second major galaxy type, are far less flattened than spirals and have apparently turned all of their original gas and dust into stars. Typically, a large fraction of their stars is old, and there isn't much matter converted into new stars.
Spiral galaxies still have a few percent of the mass in the disk in interstellar gas, while irregular galaxies, with no recognizable structure, have 20 to 50 percent of their mass in interstellar matter. Thus, these galaxies have active star formation regions. We now know that spiral galaxies, and quite likely elliptical galaxies as well, have enormous halos of "dark matter" surrounding their visible distributions of stars.
In spiral and elliptical galaxies, and in irregular galaxies as well, a small fraction of the stars belong to compact "star clusters." A spiral galaxy contains two distinctly different types of star clusters. Globular star clusters represent the first parts of the galaxy to form as the protogalaxy contracted; their immense, elongated orbits can carry them much farther from a galaxy's center than most stars ever go. In contrast, open clusters are found only in spirals and irregulars, not in elliptical galaxies, and always appear in spiral galaxies near the plane of symmetry. Open clusters usually consist of young stars, such as those in the Pleiades, and have only a few hundred members rather than the million or so stars in a globular cluster.
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