http://www.nytimes.com/2015/03/06/s...ova-and-find-theyre-watching-reruns.html?_r=2 It’s “Groundhog Day” in the cosmos. In the 1993 Bill Murray movie, a weatherman finds himself reliving the same day over and over again. Now astronomers using the Hubble Space Telescope say they have been watching the same star blow itself to smithereens in a supernova explosion over and over again, thanks to a trick of Einsteinian optics. The star exploded more than nine billion years ago on the other side of the universe, too far for even the Hubble to see without special help from the cosmos. In this case, however, light rays from the star have been bent and magnified by the gravity of an intervening cluster of galaxies so that multiple images of it appear. Four of them are arranged in a tight formation known as an Einstein Cross surrounding one of the galaxies in the cluster. Since each light ray follows a different path from the star to here, each image in the cross represents a slightly different moment in the supernova explosion. This is the first time astronomers have been able to see the same explosion over and over again, and its unique properties may help them better understand not only the nature of these spectacular phenomena but also cosmological mysteries like dark matter and how fast the universe is expanding.
