Astronomers might have solved the long-held mystery of the blue stragglers. These are stars that appear hotter and bluer than they should be – the old stellar populations they are found in should be devoid of high-mass, short-lifespan stars like these, but somehow they defy the expected evolutionary sequence. One-quarter of evolved older stars are thought to be blue stragglers.
Using the Hubble Space Telescope, researchers from the University of Texas observed 21 blue stragglers in the open cluster NGC 188 and discovered that two-thirds of them were able to acquire material that made them able to “rejuvenate” themselves.
Although they have been known for more than 60 years, the formation mechanism behind blue stragglers has remained mysterious. Most scientists agree that these objects form through mass accretion, but observational evidence has so far been elusive.
“This was really great. Until now there was no concrete observational proof, only suggestive results,” lead author Dr Natalie Gosnell said in a statement. “It’s the first time we can place limits on the fraction of blue stragglers formed through mass transfer.”
The open star cluster NGC 188 observed by Gosnell and her team. Digitized Sky Survey 2 (STScI/AURA, Palomar/Caltech, and UKSTU/AAO)
Binary stars, although they form together, tend to have a different evolution. The bigger star will become a red giant more quickly, turning into a large, bloated star with an outer layer of material that is only tenuously held by the star's gravity. While the bigger star turns into a red giant, the smaller companion seizes the opportunity to change things. It starts stealing the material from the red giant, becoming a hotter and bluer while only the hot core of its companion remains: the red giant has turned into a white dwarf.
The study, published in the Astrophysical Journal, shows that seven out the 21 blue stragglers have a white dwarf companion, which were so hot (between 11,000 and 12,000 Kelvins) they were identified by their ultraviolet glow. Another seven objects were shown to have formed by a different mass accretion mechanism by comparing their properties to theoretical predictions. Some formed by merging into a triple star system.
The method is limited because if the blue straggler had an old white dwarf (older than 250 million years) it would now be too cool to be detected using UV light. So, the result is a lower limit on the amount of blue stragglers formed in this way.
Gosnell is planning to continue study blue stragglers at the McDonald Observatory, hoping to constrain the number of objects that form through mergers into a three-star system. Models of how stars form and evolve are very important, both in studying stars in the Milky Way and producing estimations on distant galaxies. “You don’t want to be ignoring 25 percent of the evolved stars” in those galaxies, Gosnell concluded.