July 10, 2012: Astronomers have puzzled over why some puny, extremely faint dwarf galaxies spotted in our Milky Way galaxy's back yard contain so few stars. These ghost-like galaxies are thought to be some of the tiniest, oldest, and most pristine galaxies in the universe. They have been discovered over the past decade by astronomers using automated computer techniques to search through the images of the Sloan Sky Survey. But astronomers needed NASA's Hubble Space Telescope to help solve the mystery of these star-starved galaxies.
Hubble views of Leo IV and two other small-fry galaxies in this study reveal that their stars share the same birth date. The galaxies all started forming stars more than 13 billion years ago — and then abruptly stopped — all in the first billion years after the universe was born in the big bang. Because the stars in these galaxies are so ancient and share the same age, astronomers suggest that a global event, such as reionization, shut down star formation in them. Reionization is a transitional phase in the early universe when the first stars burned off a fog of cold hydrogen.See the rest: