Astronomers have discovered a strangely tiny galaxy in the Milky Way's neighborhood - one with less than 1,000 stars held together by the smallest dark-matter halo ever observed.
The galaxy known as Segue 2, described in the Astrophysical Journal, might hold the key to a long-standing mystery about the evolution of the universe.
"These little clumps are almost certainly the first things to form in the universe," said study co-author James Bullock, an astronomer at the University of California-Irvine.
Segue 2, discovered by an extension of the Sloan Digital Sky Survey in 2009, puts out about as much light as 900 suns, Bullock said. The Milky Way galaxy, in contrast, contains about 100 billion stars. Segue 2 is so small that it's dwarfed by many star clusters, which are collections of stars inside a galaxy that can contain 100,000 solar masses.
Astronomers have come to realize that size isn't the key difference between a star cluster and a tiny galaxy. Unlike a star cluster, all galaxies great and small are filled with and surrounded by a halo of dark matter - the invisible, mysterious stuff that fills the universe and acts as a sort of glue within and between galaxies. Thin tendrils of dark matter connect nodes of galaxy clusters, creating a cosmic web that has given the universe its structure.
Theorists are trying to understand how this structure evolved, figuring that there must have been a kind of early intermediate stage when tiny dark matter clumps formed, grew, pulled in stars and other visible matter and finally became the giant masses we see today.