Free Astronomy Magazine January-February 2016

SPACE CHRONICLES Gosnell said she believes these are older white dwarf-blue straggler bi- naries, and indicate two-thirds of blue stragglers form through mass transfer. “This was really great,” Gosnell says. “Until now there was no concrete observational proof, only suggestive results,” Gosnell said. “It’s the first time we can place limits on the fraction of blue strag- glers formed through mass trans- fer.” This discovery sheds light on the physical processes responsible for changing the appearance of 25 percent of evolved stars. The prob- lem came to light because in recent years, astronomers have been able to make a complete and accurate census of stars in a number of open star clusters, Gosnell said. “Open clusters really are the best laboratory for the study of stel- lar evolution,” Gosnell said. “They have a simple stellar population.” The stars in a cluster form at the same time and from the same ma- terials, she explained. The cluster population studies revealed that up “Born again” stars, a solved mystery by NASA/McDonald Obs. U niversity of Texas astronomer Natalie Gosnell has used Hub- ble Space Telescope to better understand why some stars aren’t evolving as predicted. These so-cal- led “blue stragglers” look hotter and bluer than they should for their advanced age. It’s almost as if they were somehow reinvigorated to look much younger than they really are. Though blue stragglers were first identi- fied 62 years ago, astrono- mers have yet to converge on a solution for their odd appearance. The most pop- ular explanation among sev- eral competing theories is that an aging star spills ma- terial onto a smaller com- panion star. The small star bulks up on mass to become hotter and bluer while the aging companion burns out and collapses to a white dwarf – a burned out cinder. To test this theory Gosnell’s team conducted a survey of the open star cluster NGC 188 that has 21 blue strag- glers. Of those, she found that seven had white dwarf companions, by identifying their ultraviolet glow that is detectable by Hubble. Of the remaining 14 of the 21 blue stragglers, a further seven show evidence of so-called mass transfer be- tween stars in other ways.