Free Astronomy Magazine July-August 2018

22 JULY-AUGUST 2018 SPACE CHRONICLES Stellar thief is the surviving companion to a supernova by NASA/ESA S eventeen years ago, astron- omers witnessed a supernova go off 40 million light-years away in the galaxy called NGC 7424, located in the southern constella- tion Grus, the Crane. Now, in the fading afterglow of that explosion, NASA’s Hubble has captured the first image of a surviving compan- ion to a supernova. This picture is the most compelling evidence that some supernovas originate in dou- ble-star systems. “We know that the majority of mas- sive stars are in binary pairs,” said Stuart Ryder from the Australian As- tronomical Observatory (AAO) in Sydney, Australia and lead author of the study. “Many of these binary pairs will interact and transfer gas from one star to the other when their orbits bring them close to- gether.” The companion to the su- pernova’s progenitor star was no innocent bystander to the explo- sion. It siphoned off almost all of the hydrogen from the doomed star’s stellar envelope, the region that transports energy from the star’s core to its atmosphere. Mil- lions of years before the primary star went supernova, the compan- ion’s thievery created an instability in the primary star, causing it to episodically blow off a cocoon and shells of hydrogen gas before the catastrophe. The supernova, called SN 2001ig, is categorized as a Type IIb stripped-envelope supernova. This type of supernova is unusual because most, but not all, of the hy- drogen is gone prior to the explo-