“Fossil” galaxy, Segue 1, is a satellite of the Milky Way and, with just a few hundred stars, it’s so tiny that it’s not clear whether it’s actually a galaxy or just a globular cluster (a tightly-packed grouping of old stars that orbits in the halo of our galaxy). What makes Segue 1 interesting is that, unlike other galaxies, it appears to have stopped changing with time almost immediately after it formed. Galaxies go through periods of intense activity throughout their lifetimes — e.g. bursts of star formation — but not so with Segue 1.
Elements heavier than helium are almost entirely produced inside the cores of stars or during supernovae, and are expelled back into space via red giant winds or supernova explosions, where these elements mix with what’s called the interstellar medium (mostly hydrogen and helium). The next generation of stars forms out of this mixed gas, therefore we expect to find heavier elements in the atmospheres of younger stars, and that’s what astronomers indeed observe. The stars in Segue 1, however, are pristine and comprised almost entirely of hydrogen and helium — its stars contain 300 times less heavy elements than our Sun. It appears that star formation in this tiny galaxy shut off almost as soon as it began. Astronomers refer to it as a “fossil” galaxy, because it’s been preserved just as it was nearly 13 billion years ago, and as such it should give some important insights into the conditions of the early universe.