Blue Stragglers Explained Thanks to Mass Transfer and White Dwarfs

December 12, 2015

A new study offers concrete proof that blue straggler stars are rejuvenated by the gas of their companion stars, which continue to shine on as white dwarfs.

Illustration of a mass transfer between blue straggler star and its companion. Credit: NASA/ESA, A. Feild (STScI)

For stars like our Sun and many others, we know exactly how they come to be, how they evolve and how they die. Then there are stars like so-called blue stragglers that evolve differently and thanks to a study by Natalie Gosnell, at the University of Texas, Austin, USA and her former professor Robert Mathieu, at the University of Wisconsin-Madison, Madison, USA, we now have strong evidence to support one of the existing theories.

Stars like Sun grow in size as they get older, becoming red and eventually their outer layers escape the gravity, leaving only the core behind, which is then known as white dwarf. White dwarfs continue to shine for another 250 million years, then they become undetectable.

Blue stragglers don’t evolve the same way. They are old stars and should be red, yet they are blue, which is normally an indication a star is young. They must have been rejuvenated by acquiring additional gas from somewhere. This has been puzzling astronomers since 1953 when blue stragglers were first discovered.

One of the theories is that a blue straggler star is a part of a binary system - two stars orbiting each other - and that one star, if it’s close enough, takes the gas from its larger, Sun-like partner star and uses it as new fuel, rejuvenating itself. This process is called mass transfer and it transforms the bigger star into a white dwarf.

Mathieu and Gosnell used the Hubble Space Telescope and observed 21 blue stragglers in the open star cluster NGC 188. Open cluster is a group of stars made of the same gas cloud at the same time, so if the majority of the stars in it are red, blue stragglers stick out.

The result of their observation - out of 21 stars, seven had a white dwarf companion and another seven showed evidence of mass transfer in other ways. Hubble is able to detect ultraviolet light coming from white dwarfs, but as mentioned above, they need to be younger than 250 million years to be detectable, which also implies that blue stragglers were ordinary stars not so long ago, astronomically speaking.

"These blue stragglers were formed 'yesterday,'" says Mathieu.

The Impact This Study Will Have

“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 stragglers formed through mass transfer.”

Blue stragglers’ numbers in the Universe isn’t insignificant either. If half of all the stars are in a binary system and half of those systems have stars close enough to each other to allow for mass transfer, then understanding the evolution of those stars means understanding a significant portion of star population.

"For the evolution of single stars like our sun, by and large, we got it right, from birth to death,” added Mathieu. ”Now we're starting to do the same thing for the one-quarter of stars that are close-orbiting binaries. This work allows us to talk not about points of light, but about the evolution of galaxies, including our own Milky Way. That's a big deal, and getting it right is an even bigger deal."

Source 1, Source 2

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