Andromeda galaxy is our neighbor, the closest major galaxy to our Milky Way, and can be seen as a fuzzy blob with a naked eye on a clear, dark sky. According to Messier's catalog, it's classified as M31. In our night sky it occupies a region equivalent to 7 full moons. It is located 2.5 million light-years away in the constallation Andromeda.

The disk of the galaxy is 260,000 light-years across, 2.6 times larger than that of Milky Way.The galaxy contains about 1 trillion stars, while the Milky Way has about 200 billion. The stars are crammed into the galaxies core so much that its center blazes with bright starlight.  In the pink and blue images, galaxy's star dust is represented in pink, and swirls around blue stars.

The mass of the stars in the galaxy was calculated to be approximately 10 billion times that of the Sun.

Andromeda has a smaller companion galaxy close by, labeled as M32.

There are a lot of speculations on the different ages of the stars within the galaxy, which are just what they are, speculations.

Sources:

http://nasaimages.org/luna/servlet/detail/nasaNAS~12~12~64342~168847?qvq=q:andromeda+galaxy