M51, the Whirlpool Galaxy

Photo credit: Randy and Betty Ivins and granddaughter Miranda. Image has been color-enhanced to show faint structure.

This is M51, the Whirlpool Galaxy. It is a classic "grand design" spiral galaxy, conveniently tilted face-on toward us so that we can get a great look at its well-defined spiral arms. Below and to the right on this image, we see its companion galaxy NGC 5195. Though just a small blob by comparison, NGC 5195 is probably responsible for the spiralling structure of the larger M51 galaxy, as the two of them undergo a long, slow, mid-space collision.

Follow up:

The pair are about 20-30 million light-years from here, in the direction of the constellation Canes Venatici. As with many galaxies, they're part of a galaxy group. In such a group, galaxies are fairly close together compared with the space between galaxies, so collisions between galaxies can be fairly common. Such a collision, however, isn't like two solid objects smacking into each other. Stars, unlike galaxies, are very, very tiny compared to the distances between them, so it's highly unlikely that any stars will bump into each other - even though the galaxies containing those stars are in mid-collision!

However, the great clouds of dust and gas between stars do get pulled around and collide, causing the formation of multiple new stars, including some bright blue short-lived stars that tell us that this happened fairly recently on astronomical timescales.

The Whirlpool galaxy can actually be found with binoculars in a very dark sky, but it will take at least a 4-inch telescope before you start to see hints of a spiral.

To find M51, locate the Big Dipper in your northern sky - it's high up during the summer evenings. Follow the curve of the Dipper's handle, which is made up of what look like three bright stars. The star in the middle is the bright star Mizar (with its binary companion Alcor), and at the end of the handle is Alkaid (Eta Ursae Majoris). Imagine, then, that Mizar and Alkaid form a line, which is the long side of a letter "L". Draw another imaginary line from Alkaid out to form the short side of the "L", in the direction the Dipper's handle curves; M51 will be at the end of that shorter line. (Wikipedia has a nice illustration.)