Andromeda Galaxy 
One of the highlights of the autumn sky is M31, the Andromeda galaxy. It’s the most distant object that’s readily visible to the unaided eye — about two-and-a-half million light-years away.
M31 and our own galaxy, the Milky Way, are near twins.
Both are spiral galaxies. They’re shaped like wide, thin disks. Their hottest, brightest stars outline spiral arms, so from afar, the galaxies look like pinwheels. The Milky Way spans a hundred thousand light-years, while Andromeda is a bit bigger.
Both galaxies are also at least a trillion times as massive as the Sun. Astronomers used to think that Andromeda was the heavier of the two, but more recent research seems to give the nod to the Milky Way.
Each galaxy has several hundred billion stars, but most of their mass is in the form of dark matter — matter that produces no detectable energy, but that exerts a gravitational pull on the stars and gas clouds around it.
The dark matter is concentrated in vast “halos” that surround the galaxies — ball-shaped volumes of space that contain only a few stars. Andromeda’s halo may extend a half-million light-years from the galaxy’s heart — a long way toward our own galactic home. Over the eons, though, the two galaxies will get even closer; more about that tomorrow.
M31 is well up in the east-northeast by the time it gets good and dark. It looks like a small, fuzzy patch of light, so you need to get away from city lights to see it.
Script by Damond Benningfield, Copyright 2007, 2012