Old stars gleaned neighbors’ gas, Hubble data show

Stealing keeps some stars looking young. The thieves, called blue stragglers, swipe material from a neighbor, leaving behind a dead stellar companion as a calling card, data from the Hubble Space Telescope show.

Natalie Gosnell, an astronomer at the University of Wisconsin–Madison, and her colleagues discovered three blue stragglers that share orbits with white dwarfs, the remnants of dead stars. The findings, posted January 29 on arXiv.org, match astronomers’ ideas about what would be left if a blue straggler took gas from a now-defunct companion star. The stolen goods allow the blue stragglers to burn hotter and look bluer, as a much younger star would.

“These blue stragglers absolutely did form from taking matter from another star. It is the first time we have been able to say that for a specific population of blue stragglers,” Gosnell says.

Determining how blue stragglers pilfer matter and what happens to them may help astronomers better understand the physics of the dozens of other cosmic phenomena that involve mass transfer, such as exchanges between two stars that ultimately cause explosions.

Astronomers first identified blue stragglers in 1953. The stars were puzzling because they were as old as the cooler, redder stars around them. Over the years, astronomers have come up with three explanations for how blue stragglers cover their age: Each straggler could have collided with another star and absorbed matter from it; it could have existed in a star trio and merged with a companion; or it could have stolen mass from another star. These hypotheses have been difficult to confirm.

Gosnell and colleagues used the Hubble telescope to study ultraviolet light coming from blue stragglers in NGC 188, a 7-billion-year-old cluster of stars 5,800 light-years away in the constellation Cepheus. The researchers knew how much UV light should come independently from a blue straggler and from a white dwarf. Finding blue stragglers that have extra UV light, equivalent to the amount a white dwarf gives off, is a signal that a straggler and a white dwarf orbit each other and once exchanged material. Three blue stragglers in NGC 188 fit the profile, the team reports.

The results do not rule out that collisions and mergers also form blue stragglers, but the findings do provide “waterproof evidence” that mass stealing occurs, says astronomer Michał Różyczka of the Nicolaus Copernicus Astronomical Center in Warsaw. In 2013, his team reported what appeared to be a blue straggler in the act of stealing from its companion.

The new data make an even stronger case for the transfer of mass among these stars because they show that it happened multiple times in a population of blue stragglers. And since astronomers know what the stars looked like before they exchanged mass, they can map out the history of these objects. “This is the first time that has been achieved for any blue stragglers,” Gosnell says.