Where the Stars Live

If you've ever spent any time under clear dark skies, you've seen the cloudy band of light running across the sky that we call the Milky Way.  Its name comes from the resemblance the ancient Greeks saw to  milk, which gives us our other name for it, the Galaxy.  Galileo was the first to penetrate the nature of this cloudy band of light when he turned his telescope upon it and saw a myriad of stars.

As telescopes and astronomical techniques improved it became clear that the Milky Way, our galaxy, was a sort of island universe in which our star, the Sun, was embedded as one of enormously many stars.  There are a few other cloudy patches of light in the sky, three of which are visible to the keen eyed - Andromeda and, very prominent in the Southern Hemisphere, the Magellanic Clouds.  There are also more temporary cloudy glows, the comets, which were long found more interesting.  Telescopes revealed ever more of these cloudy patches.

It eventually became clear that at least some of these were part of our local island universe, the Milky Way.  Others were more ambiguous.  Immanuel Kant suggested that some of these were other island universes, like our own Milky Way.  This question was decisively answered in Kant's favor in the early decades of the Twentieth Century, not quite 100 years ago.

We have now discovered billions of such galaxies, some very like our own and others quite different.  What they have in common is that they are the places where stars are born, stars live, and stars die.  To be sure, there are some wandering stars outside galaxies, but they are like fish out of water - they got there through accident or catastrophe.  So far as we know, star birth takes place only in galaxies.

As we know, our solar system is almost all empty space, but the galaxy away from stars is mostly far more empty, and the space between galaxies is about a million times emptier than a galaxy too.

Galaxies vary a lot in size and other properties, but there does seem to be a definite range.  The very biggest galaxies have about as much mass as a few trillion Suns, and the smallest ones, about as much as ten billion Suns.

The Magellanic Clouds, mentioned above, are satellite galaxies of the Milky Way, and Andromeda, that tiny fuzzy patch of light, is, like the Milky Way, one of the two big dogs in the local group of a few dozen galaxies, with Andromeda being somewhat larger.  Not all galaxies live in sparse groups like ours.  Some are loners and others may live in massive cluster which may contains thousands or even tens of thousands of galaxies.

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