Hubble image of M51 Whirlpool Galaxy dated November 7 2002 NASA Goddard Space Flight Center

was formed). Most of the stars in elliptical galaxies appear to be old, relatively cold, reddish low-mass stars, similar to the stars in the halo of the Milky Way and other spiral galaxies. Ellipticals are therefore old systems in which the gas and dust was all swept up by the star systems long ago, and the stars are moving about in irregular paths within the elliptical mass. As with most generalized statements, there are exceptions. Recent observations have shown that some giant elliptical galaxies have smaller areas that contain disks of gas and dust, but some astronomers speculate that these may be other spiral galaxies that collided with the giant ellipticals.

Irregular galaxies include the whole range of other galaxies that do not fit into Hubble's spiral, barred spiral, or elliptical galaxy classes. These galaxies tend to lack systematic structure such as spiral arms or bulges, but they do contain large amounts of interstellar material such as dust and gas. Irr I type irregular galaxies slightly resemble distorted spiral galaxies, with famous examples including the Magellanic Clouds that orbit the Milky Way. The less common Irr II galaxies exhibit explosive or filamentous characteristics. Different models have been advanced to explain these characteristics of Irr II galaxies, including massive explosions inside the galaxies, or effects of close encounters with other galaxies. Most irregular galaxies contain between 1,000,000,000

to 100,000,000,000 stars, with the smaller "dwarf" irregular being more common than the larger elliptical galaxies.

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