Free Astronomy Magazine November-December 2015

SPACE CHRONICLES NASA's Hubble finds supernovae in 'wrong place at wrong time' by NASA S cientists have been fascinated by a series of unusual exploding stars-outcasts beyond the typi- cal cozy confines of their galaxies. A new analysis of 13 supernovae — in- cluding archived data from NASA's Hubble Space Telescope — is help- ing astronomers explain how some young stars exploded sooner than expected, hurling them to a lonely place far from their host galaxies. It's a complicated mystery of double- star systems, merging galaxies, and twin black holes that began in 2000 when the first such supernova was discovered, according to study lead- er Ryan Foley, University of Illinois at Urbana-Champaign. "This story has taken lots of twists and turns, and I was surprised every step of the way," he said. "We knew these stars had to be far from the source of their ex- plosion as supernovae and wanted to find out how they arrived at their current homes." Foley thought that the doomed stars had somehow migrated to their final resting spots. To prove his idea, he studied data from the Lick Obser- vatory in California and the W. M. Keck Observatory and the Subaru Telescope, both in Hawaii, to de- termine how fast the stars were traveling. To his surprise, he dis- covered that the doom- ed stars were zipping along at about the same speed as stars that have been tossed out of our Milky Way galaxy by its central su- permassive black hole, at more than 5 million miles (7 million kilome- ters) an hour. The astronomer then turned his attention to the aging galaxies in the area of the speed- ing supernovae. Study- ing Hubble archival im- ages, he confirmed that many are massive ellip- tical galaxies that were merging or had recent- ly merged with other T hese Hubble Space Telescope images show elliptical galaxies with dark, wispy dust lanes, the signature of a recent galaxy merger. The dust is the only relic of a smaller galaxy that was consumed by the larger elliptical galaxy. The "X" in the images marks the location of supernova explosions that are associated with the galaxies. Each super- nova may have been gravitationally kicked out of its host galaxy by a pair of central supermassive black holes. When two galaxies merge, so do their supermassive black holes. Astronomers suggest the supernovae were stars that were once part of double- star systems. These systems wandered too close to the binary black holes, which ejec- ted them from their galaxies. Eventually, the stars in each system moved close enough together to trigger a supernova blast. [NASA, ESA, and R. Foley (University of Illinois)]