What is a Galaxy?

It's pretty hard not to have sometime seen pictures of the beautiful island universes, or galaxies, in which all stars are apparently born and most live out their lives. So what makes a galaxy, and what is their history?

Unlike stars, there don't seem to be any young galaxies. They all seem to have been born shortly after the big bang. There are, however, galaxies with lots of young stars and galaxies consisting almost entirely of old stars, but so far as I know, there aren't any galaxies with *no* old stars.

I mentioned that some stars no longer live in galaxies, but they are thought to have been expelled by various cosmic accidents, the most important of which are collisions between galaxies. Many or perhaps most of all existing large galaxies are the product of multiple mergers of smaller galaxies. Our own Milky Way galaxy, for example, is currently in the process of gobbling up several smaller neighbors, and almost certainly has dined on many more in the past. It in turn is likely to merge with our larger neighbor galaxy, Andromeda, some 4 billion years from now.

Such collisions and mergers are unlikely to produce collisions between stars because there is simply too much space between them, but they do produce collisions between gas clouds, triggering violent bursts of star formation. Although direct collisions are unlikely, close encounters can slingshot some stars out into intergalactic space.

Our Milky Way is a fairly good sized barred spiral galaxy, with maybe a couple of hundred billion stars, and weighing in at maybe 700 billion solar masses, counting dark matter.

The dark matter component is vital. If current theories are correct, dark matter played a crucial role in galaxy formation. In the early millenia after the big bang, protons and electrons existed in a fully ionized soup too hot and electrified to condense under gravity, but dark matter, not coupled to electromagnetism, could, and condensed under gravity into gravitational wells, or so-called dark matter halos, of mutual attraction. After recombination of electrons and nuclei at about the 300,000 year mark, ordinary matter was able to fall into these dark matter created overdensities, becoming the nuclei of the galaxies to come.

The very smallest galaxies that we see today may have only about one million stars, or only about 1/200,000 th of the Milky Way complement, but they do seem to have a dark matter halo. It's at least possible that they lost most of their stars in encounters with other galaxies. The very largest galaxies, the so called CD ellipticals, have several hundred times as many stars as the Milky Way, and have masses of perhaps fifty times as great or more. They are likely products of many, many mergers.


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