Galaxies

Galaxies are large systems of stars and interstellar matter, typically containing several million to some trillion stars, of masses between several million and several trillion times that of our Sun, of an extension of a few thousands to several 100,000s light years, typically separated by millions of light years distance. They come in a variety of flavors: Spiral, lenticular, elliptical and irregular.

Spiral galaxies usually consist of two major components: A flat, large disk which often contains a lot of interstellar matter (visible sometimes as reddish diffuse emission nebulae, or as dark dust clouds) and young (open) star clusters and associations, which have emerged from them, often arranged in striking spiral patterns and/or bar structures, and an ellipsoidally formed bulge component, consisting of an old stellar population without interstellar matter, and often associated with globular clusters.

Lenticular Galaxies are, in short, "spiral galaxies without spiral structure", i.e. smooth disk galaxies, where stellar formation has stopped long ago, because the interstellar matter was used up. They consist of old stars mainly. From their appearance and stellar contents, they can often hardly be distinguished from ellipticals observationally.

Elliptical galaxies are actually of ellipsoidal shape, and it is now quite safe from observation that they are usually football shaped. They have little or no global angular momentum, i.e. do not rotate as a whole. Normally, they contain very little or no interstellar matter, and consist of old population II stars only: They appear like luminous bulges of spirals, without a disk component.

Irregular Galaxies do not fit well into the scheme of disks and ellipsoids, but exhibit peculiar shapes. A subclass of distorted disks is however frequently occuring.

We live in a giant spiral galaxy, the Milky Way Galaxy, of 100,000 light years diameter and a mass of roughly a trillion solar masses. The nearest dwarf galaxies, satellites of the Milky Way, are only a few 100,000 light years distant, while the nearest giant neighbor, the Andromeda Galaxy, also a spiral, is about 2-3 million light years distant.

For more detailed information about astronomy post your queries on our message board.