Blue Stragglers Can Be Either Vampires or Stellar Bad-Boys

Messier 30, from HST’s Advanced Camera for Surveys. Credit: NASA, ESA and Francesco Ferraro (University of Bologna

Blue stragglers are stars that stay on the main sequence longer than expected. They even appear to regress from “old age” back to a hotter and brighter “youth,” gaining a new lease on life in the process. Astronomers have thought blue stragglers were “vampires” that suck fresh hydrogen from companion stars to heat up and maintain their youthfulness. But now there appears to be two kinds of blue stragglers. In addition to the vampires, there are also the bad-boys: these blue stragglers steal mass from companion stars by crashing into their neighbors, as if they were in a stellar mosh pit. A team of astronomers used data from the Hubble Space Telescope to study the blue straggler star content in Messier 30, a swarm of several hundred thousand stars located about 28,000 light-years from Earth.

Blue stragglers have been known since the early 1950s, but how they formed remains an astrophysical puzzle. Of all the stars in Messier 30, which formed about 13 billion years ago, a small fraction of them appeared to be significantly younger.

“It’s like seeing a few kids in a group photo of residents of a retirement home, and ask, ‘How did they get there?'” said Alison Sills, assistant professor at McMaster University. “In short, we seem to have found that there are two fountains of youth for stars.”

Previously, it was thought that that the less massive star in a binary system acts as a “vampire”, siphoning fresh hydrogen from its more massive companion star that allows the smaller star to heat up, growing bluer and hotter. However, the new study shows that some of the blue stragglers have instead been rejuvenated by a sort of “cosmic facelift”, courtesy of cosmic collisions. These stellar encounters are nearly head-on collisions in which the stars actually merge, mixing their nuclear fuel and re-stoking the fires of nuclear fusion. Merged stars and binary systems would both be about twice the typical mass of individual stars in the cluster.

“The observations, which agree with our models, demonstrate that blue stragglers formed by collisions have slightly different properties from those formed by vampirism. This provides a direct demonstration that the two formation scenarios are valid and that they are both operating simultaneously in this cluster,” said Sills, who was part of an international steam that made the findings.

Using data from the now-retired Wide Field Planetary Camera 2 (WFPC2) aboard Hubble, astronomers found that these “straggling” stars are much more concentrated towards the center of the cluster than the average star.

The central regions of high density globular clusters are crowded neighborhoods where interactions between stars are nearly inevitable. Researchers conjecture that one or two billion years ago, Messier 30 underwent a major “core collapse” that started to throw stars towards the centre of the cluster, leading to a rapid increase in the density of stars. This event significantly increased the number of collisions among stars, and favored the formation of one of the families of blue stragglers. On the other hand, the increase of stellar crowding due to the collapse of the core also perturbed the twin systems, encouraging the vampirism phenomenon and thus forming the other family of blue stragglers.

The study will be published in the Dec. 24 issue of Nature.

Sources: ESA Hubble Information, Center, McMaster University University of Wisconsin/Madison