An unusual group of binary stars that look deceptively young and bright for their age are able to maintain their appearance by cannibalizing their older partners, astronomers from the University of Wisconsin-Madison have discovered.

Writing earlier this month in The Astrophysical Journal, astronomy professor Robert Mathieu and former UW student Natalie Gosnell found that one of these stars, which are also known as blue straggler stars, siphoned hydrogen gas from a companion red giant until it became a white dwarf—an old, small, dense, bright burnt-out shell of its former self.

For decades, scientists have understood that stars age, evolve, and ultimately die off, and as they grow older, they change their patterns of color, their light output, and their size. Nearly half of all stars are in binary systems, meaning they have a nearby partner star whose gravity can drastically alter the evolution of that star. The one notable exception is the blue straggler star.

Blue stragglers, the authors explained, seem to defy stellar evolution by appearing brighter and younger than they should for their age. Now, though, Mathieu, Gosnell and their co-authors have discovered why these unusual stars appear to have found the cosmic fountain of youth.

Binary blue stragglers are parasites to their companion stars

Using the Hubble Space Telescope, they studied the colors of far ultraviolet light coming from blue stragglers and their companion stars in an open cluster known as NGC 188, located nearly 5,500 light years from Earth. By analyzing the UV rays combing from the companion, the team found solid evidence that it was a white dwarf, helping them to solve the puzzle.

Having identified the stars they were studying as being a binary pair, they also found indications of the presence of a blue straggler. Furthermore, the brightness of the white dwarf indicated that it was only 300 million years old, meaning that the pair had formed fairly recently. White dwarf stars lose their atmosphere when they form, and that mass had to end up going somewhere.

In this case, the lost mass was going to the companion normal star, which was able to draw it in because of its close proximity. Therefore, the normal star’s added mass made it a blue straggler, and the white dwarf formed from the red giant that had been surrendering its atmosphere.

While Gosnell, who is currently a postdoctoral fellow at the University of Texas, explained that these findings will help shed new light on a poorly understood area of stellar evolution, there are still questions that remain, as this only solves the mystery for about two-thirds of blue stragglers (those in binary pairs with white dwarfs)—leaving another 33 percent still unaccounted for.

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Feature Image: NASA/ESA, A. Feild (STScI)

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