If you look at a map of just about any state, you’ll see a few major cities, a few more mid-sized ones, and a whole bunch of small towns. And the same thing happens if you look at a map of our galactic neighborhood. There are a couple of giant galaxies, a few mid-sized ones, and a whole bunch of little ones. They’re all bound to each other by their mutual gravitational pull.
This collection of galaxies is known as the Local Group. It spans about 10 million light-years, and contains a hundred or so known members, with more added every year.
The “big cities” of the Local Group are our home galaxy, the Milky Way, and the Andromeda Galaxy, M31. Each is a disk that’s at least a hundred thousand light-years wide, and contains hundreds of billions of stars.
After these two behemoths, the list includes a few mid-size members. The largest is the Triangulum Galaxy. It’s about half the diameter of M31 and the Milky Way, but only a few percent as massive.
A couple of mid-size galaxies are satellites of M31. One is a spiral, while the other is a small elliptical galaxy, which looks like a fuzzy rugby ball. And two more members of the mid-size brigade orbit the Milky Way — the Large and Small Magellanic Clouds.
Most of the other members of the Local Group are tiny — far smaller, and with far fewer stars, than the larger galaxies. Many are satellites of the Milky Way, and we’ll have more about them tomorrow.
Script by Damond Benningfield