M31 - The Andromeda Galaxy
Introduction
Probably the most famous galaxy other than the Milky Way, the Andromeda galaxy is our nearest neighbour in space as far as large galaxies are concerned. Andromeda provides a great opportunity to astronomers as it is thought to be very simmilar to our own galaxy, allowing us to look at galaxies with a different perspective. It is also slightly odd becasue Andromeda is one of the very few blue shifted galaxies, this blue shifting indicates the galaxy is moving towards us. Astronomers have measured the speed of the galaxy to be about 300km/s and is expected to collide with our own galaxy in approximately 2 billion years.
Position
M31 lies in the constealtion of andromeda and is very easy to find for the amature astronomer with almost any telescope. Even thought the visual magnitude of andromeda is very bright +4.4mag it is quite hard to see anything more than a smudge as it has a very low surface brightness this is due to the physical size of the galaxy. To view detail visually it is important to have very dark skies and as much contrast as you can get, I reckon that andromeda is probably the most photographed galaxy out there as it is easy to catch on camera with short exposures. M31 lies approximately 0.8Mpc(~2 million light years) away from us and is part of the local group of which m31 and the milky way are the two largest galaxies.
Structure
Andromeda is a SAb type galaxy, this means it is a barred spiral with pronounced spiral arms(~70 degree inclination), we can easily see the dust lanes on images and can measure a redshift to get a rotation velocity of around 200km/s at the edges with a diameter of ~200,000 light years. (see here for calculation for the size of m31.)There is some evidence that M31 may have a double centre as shown in the image to the bottom left taken by the Hubble Space Telescope(HST). M31 shows clear spiral arms these are the birthplace of stars and are very active regions of the galaxy. There are many O-B type stars in these regions and as you would expect from such stars they are easily seen in the UV image below(centre). Another distinct feature we can easily see is the dustclouds orbiting the galaxy these are very clear in visible wavelength images but show up even more so in the infra-red as the dust radiates heat. We can see from the image on the right that the dust a ring around the centre of the galaxy.
Left: Double Nucleus of M31 Credit: T. R. Lauer (KPNO/NOAO) et al. HST || Centre: M31 in UV Credit: NASA Caltech || Right: M31 in infra-red Credit: NASA,IPAC,IRAS