The Andromeda galaxy is the nearest major galaxy to our own and it's actually the largest in our local group of galaxies, which contains the also famous Triangulum galaxy (M33) and at least 44 other, smaller members. Recent studies show quite different results but we can assume that the Andromeda galaxy is easily more than 50% larger than the Milky Way and that it contains a lot more stars too. It's 220.000 lightyears across, twice as much as our own galaxy, and that still isn't big at all if you compare it to some of the giant galaxies out there.
If you look up at the autumn sky you can't actually miss it since it's certainly visible to the naked eye, unless you're observing from a city centre. It's easy to find, just above Mirach (Bèta Andromedae) and if you've got a pair of binoculars you may even see two of its companions: M32 (to the left on my sketch) and M110 (a bit further out to the right). Both are mini-galaxies that accompany their master, much like the Magellanic Clouds, which can be seen on the southern hemisphere, are dwarf-galaxies that accompany ours. The distance of this galaxy system is estimated at 2,5 million lightyears and this brings me to an interesting thought. 2,5 million lightyears means that light, with its formidable speed, needs 2,5 million years to travel from the Andromeda galaxy to us. As a consequence we don't see the galaxy as it is now, but as it was 2,5 million years ago. Yes! The image of it that has reached us today actually left the galaxy when the first humanoids appeared on the African plains! We haven't got a clue what the Andromeda galaxy, or by extension any other star or object in the night's sky, looks like today. For as far as we know, it may already have blown up! Of course, a galaxy the size that it is doesn't disappear overnight and we can rest assured that it's probably still there. However, some other stars or objects may have disappeared, or appeared without us knowing it. We can only wait until the light from those events has eventually reached us.
In summary, looking at the night's sky actually means looking into the past, because everything that you see doesn't exist anymore as it is today. The closest star, Proxima Centauri, is only 4,3 lightyears away so we see it as it was 4 years ago. But the further we look into the universe, the further we look back into time. Isn't that exciting?
Another interesting thing is that the Andromeda galaxy's heading towards us. It's expected to collide with ours within 3,75 billion years. That's still a lot of time and that doesn't necessarily mean that some star's going to crash into us. Most of the space in a galaxy is absolutely void (think of the distances between us and the closest stars) and therefore collisions are highly unlikely. Most probably, as is the case with other colliding galaxies, the two will simply merge through gravity into a single, giant galaxy.
I know that my observation seems a bit out of season. But the Andromeda galaxy happened to be at a very comfortable altitude above the horizon for my binoculars so I couldn't resist it. There was a fair bit of wind which gave me some trouble but the sky's transparency was excellent. I immediately noticed the dark dustlane on the right and the two companions just leapt out at me. Mind you, the stars you see, also the ones "within" the galaxy, are much closer to us as they all belong to our own. A galaxy's so far away that it's impossible to distinguish any individual stars in it, at least not with an amateur telescope.