Galaxies come in all sorts of shapes and sizes. Sometimes, they're egg-shaped behemoths capable of crashing into one another like a slow-motion car wreck. Other times, they spin with unfathomable power, whipping their enormous arms through space like a pinwheel.
Recently, a few Earthlings armed with physics degrees spotted an odd collection of stars loosely packed together in the night sky that had all the characteristics of a galaxy, including a dark matter halo.
But it was incredibly tiny. With about 1,000 stars total, it was just a small fraction of the Milky Way, which boasts 100 billion stars.
This itty-bitty galaxy was dubbed Segue 2, and is described in the latest issue of the Astrophysical Journal. Researchers think the miniature galaxy may hold "the key to a long-standing mystery about the evolution of the universe," reports the Los Angeles Times:
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. [LA Times]
According to study coauthor James Bullock, an astronomer at UC Irvine, "finding a galaxy as tiny as Segue 2" is akin to "discovering an elephant smaller than a mouse."
Bullock and his team aren't sure whether Segue 2 is what remains of a larger galaxy that's been ripped apart by the Milky Way's gravitational pull, or if it's a fossilized mini that never had a chance to grow up. Yet Segue 2's discovery, says Bullock, means it's likely more small galaxies are out there just beyond the Milky Way. The hard part, now, will be picking them out of obscurity.