Astronomers discover smallest galaxy ever.

A resident of the constellation Leo, the newly discovered galaxy called Leo T is only about 600 light-years across--about one-sixteenth the diameter of the Milky Way--and 50,000 times brighter than the sun. Some 1.4 million light-years from Earth, the galaxy lies far enough away that it's not bound to the Milky Way but is still a member of the Local Group. That family of galaxies includes the Milky Way and Andromeda.

Daniel Zucker of the University of Cambridge in England and his colleagues found the tiny galaxy by analyzing data taken with the Sloan Digital Sky Survey, a map of the nearby universe that covers one-quarter of the sky.

Galaxy-formation models based on the clumping of dark matter, the invisible material that pulls stars and gas into galaxies, indicate that there should be many more small galaxies in the Local Group than have been detected. The new find may help narrow the gap between dark-matter theory and observations, Zucker says. However, he adds that it could also pose a challenge to theorists trying to determine just how small a galaxy that dark matter clumping can produce.

"This is certainly an exciting object, but ... I would not go so far as to say that we theorists would have trouble," says James Bullock of the University of California, Irvine. "Leo T may not be as tiny as seems from visible light, but instead may contain many times its visible mass in dark matter," he notes. "Measuring the velocities of some of its stars would help test this idea, and perhaps even shed light on the nature of dark matter itself."