People used to think that the heavens were a vast clockwork, with planets and moons moving in circular orbits like a vast timepiece. Recent advances have shown us that this stellar machinery is far vaster than they ever suspected - even our galaxy has satellites, mini-galaxies orbiting the Milky Way and some of them could be very interesting indeed.
We now know about two dozen of these satellite galaxies. One of the most recent is "Segue 1", uncovered by the Sloan Digital Sky Survey (SDSS), whose extremely low light-to-mass ratio makes it a particularly significant cosmic find. Despite having a mass of a million suns it is nowhere near as luminous as astronomers would expect, with only a couple of hundred stars visible. They think "How can so much matter be so dark?", then they go "Dark matter!" and at this point we like to believe their monocle flies out and they dash down the street shouting Eureka.
Of course, the actual physics of arriving at this conclusion is a tiny bit more complicated, but the result is the same: Marla Geha (Yale professor of astronomy) and colleagues believe that it's a galaxy composed mainly of dark matter. A handy thing to have around when you're trying to study the stuff or even prove that it exists.
The SDSS has made many such observations possible, picking out objects in the sky which have the bad manners not to twinkle twinkle bright enough, despite being little stars. These mini-galaxies and other previously unobservable objects offer a wealth of data on galaxy formation, evolution, and even the composition of the universe itself.
Posted by Luke McKinney