For the past 60 years, astronomers have been puzzled by an unusual type of star that looks hotter and bluer than it should for its age. It has been dubbed a "blue straggler" because it seems to lag behind the evolution of neighboring stars. Blue stragglers dwell inside ancient star clusters that should have stopped making youthful and short-lived blue stars billions of years ago. The most popular explanation among several competing theories is that an aging star spills material onto a smaller companion 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, astronomers using the Hubble Space Telescope conducted a survey of the open star cluster NGC 188 that has 21 blue stragglers. Of those they found that seven had white dwarf companions, by identifying their ultraviolet glow that is detectable by Hubble. This confirms the binary star theory for their origin.
