Secret of Blue Stragglers’ Youth Revealed

The reason why “blue stragglers” are hotter, larger, and age more slowly than stars their own age appears to be because they are accompanied by invisible white dwarf stars, according to new findings to be published in Nature on Oct. 20.

The origin of blue stragglers has been a mystery since their discovery in 1953. However, a mechanism called mass transfer could explain how they form. The straggler appears to be fueled by the outer envelope of its giant stellar companion, while the companion is stripped down to a white dwarf core.

“It’s really the companion star that helped us determine where the blue straggler comes from,” said study co-author Aaron M. Geller at Northwestern University in a press release.

“The companion stars orbit at periods of about 1,000 days, and we have evidence that the companions are white dwarfs. Both point directly to an origin from mass transfer.”

The astronomers studied the NGC 188 open cluster in the constellation Cepheus, which is one of the most ancient open star clusters with about 3,000 stars around the same age and 21 blue stragglers.

Using this observational data, they looked at the three main theories behind blue straggler formation, namely star collisions, star mergers, and mass transfer between stars.

They found that only mass transfer fitted the observations, although the companion stars are not visible. However, their gravitational pull on the blue stragglers can be seen, causing a “wobble” in their orbits that allows the mass of the companion stars to be measured.

According to the data, each companion star is about half a solar mass, typical of a white dwarf. For the other two theories to work, the companion stars would have to be more massive, with some expected to be visible.

“As so often happens in astronomy, it is the objects that you don’t see that provide the critical clues,” said co-author Robert Mathieu at University of Washington-Madison.

“Now we will use the Hubble Space Telescope to search for the ultraviolet light in which white dwarf secondary stars shine.”

With this observational data, the team will be able to confirm whether the companions are actually white dwarf in about one year.

The paper has been accepted for publication in The Astrophysical Journal.