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, which means they're shaped like wide, thin disks. The youngest, brightest stars in the galaxies outline spiral arms, so from afar, the galaxies look like cosmic pinwheels. The Milky Way's disk spans a hundred thousand light-years, while Andromeda is a bit bigger.
Both galaxies are also quite massive -- 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."
The dark matter is concentrated in "halos" that surround the galaxies -- big ball-shaped volumes of space that contain sprinklings of stars. The stars are among a galaxy's oldest, and they extend far into space. Andromeda's halo, in fact, may extend a half-million light-years from the galaxy's heart -- a long way toward our own galactic home.
M31 is well up in the northeast by early evening. It looks like a small, fuzzy patch of light, so you need to get away from city lights to see it. Binoculars will enhance the view of this beautiful spiral galaxy.
More about M31 tomorrow.
Script by Damond Benningfield, Copyright 2007
For more skywatching tips, astronomy news, and much more, read StarDate magazine.
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