Today the productivity of the astronomy department dropped markedly, as the coffee shop in our building closed up for the winter break. No more precious java will flow forth from the hallowed espresso machines until mid-January. So it is no wonder that most of the astronomers have left for the year.
Yours truly, however, is still plugging away, albeit in a caffeine-starved state. This evening, I was baking cookies while watching that holiday classic, "It's a Wonderful Life." And the opening of the movie contains some astronomy!
In the scene where the angels are speaking (Joseph, Franklin, and later Clarence), the angels are portrayed as galaxies. Not just any galaxies, but Stephan's Quintet, a tight grouping of five galaxies in the constellation Pegasus. Since, in reality, these five galaxies do not represent a meeting between two angels, what are we seeing?
A color picture of Stephan's Quintet was taken at Kitt Peak can be seen here. Note that four of the galaxies are a yellowish-white, and the fifth is almost pure white. The very white galaxy is much closer to us than the other galaxies; it just appears along the same line of sight. The yellow color is caused by the expansion of the Universe "redshifting" the galaxy light.
The four galaxies at the same distance are part of a "compact group." Galaxies, like people, tend to like to travel together in groups ranging in size from a few galaxies to many thousands of galaxies. Our own galaxy is in a group with the Andromeda Galaxy, the Triangulum Galaxy, and several small dwarf galaxies. The Local Group, as it is called, is pretty scattered. Andromeda is two million light-years away, and Triangulum is about three million light-years away. The Milky Way and Andromeda are each about 100 thousand light-years across. So there is a lot of room in our Local Group!
In Stephan's Quintet, on the other hand, the galaxies are very close together. Studies of pictures and spectra of the galaxies in Stephan's Quintet shows that there are two pairs of galaxies, and the galaxies in each pair are so close that gravity is pulling large streams of stars and gas out of each galaxy! We see interacting galaxies in many places, but having four bright interacting galaxies in such a small area of the sky makes this system interesting to study. Why are some groups of galaxies, like the Local Group, so spread apart, while others with similar numbers of galaxies, like Stephan's Quintet, so close together? In several billion years, the Local Group may look like Stephan's Quintet. The Milky Way will eventually collide with the Andromeda Galaxy, and the Triangulum Galaxy will probably collide with Andromeda, too. So, by studying this distant group, we can learn what might happen to our own galaxy in the distant future!