In the News this month: stellar canibalism in globular clusters
The globular cluster M30 as seen by the Hubble Space Telescope CREDIT: NASA/ESA
Globular clusters contain some of the oldest known stars. Formed billions of years ago in the halos of what eventually become the galaxies we see today, globular clusters are roughly spherical collections of stars bound together by their own gravity. Our own Milky Way contains many such clusters, several of which were catalogued by Charles Messier in the 18th Century.
While most of the stars in globular clusters have ages of 12 to 13 billion years, some of them appear to be much younger. Most stars in globular clusters are reddish in colour, but there are some which appear bluer, as if they were much younger. Because they appear to have been left behind by the general evolution of the rest of the stars in the cluster, many of which have evolved into red giants, this population of stars is known as the "blue stragglers". First identified in the 1950s, these peculiar stars appear to regress from old age back to a more youthful appearance, gaining a second lease of life, but there is more than one theory that explains this phenomenon. In research published in Nature on Christmas Eve, two teams describe observations that provide important clues that may explain the process.
One team, led by Francesco Ferraro at the University of Bologna, used the Hubble Space Telescope to examine stars in the globular cluster M30. What they found was that the blue stragglers in this tightly bound cluster are, on average, more massive than the rest of the stars in the cluster, and actually comprise two distinct populations, suggesting that they formed in two different ways. One method is via stellar collisions: globular clusters are very dense objects where stars are comparatively closely packed, especially near the centre, so collisions between stars are far more likely than they are in the more sparsely populated region of space where our own Sun exists. A collision between two stars would result in the creation of a much more massive star, with plenty of fuel being drawn in to the star's core, effectively regenerating it. Collisions are not the entire picture, however. Another way to rejuvenate a star is by mass transfer. Most stars do not exist in isolation like the Sun, but in binary or multiple systems. In a binary system, when one star reaches the red giant phase and begins expanding, some of that material can be drawn onto the surface of the companion star if they are close enough together. When this happens, more fuel is available for the second star, allowing it to burn bluer for longer, and the end result is a blue straggler with a white dwarf companion, the end point of stellar evolution for a red giant.
In the second paper published in the same issue of Nature, two researchers from the University of Wisconsin–Madison, examined the population of blue stragglers in the less dense open cluster NGC 188. Open clusters are much less dense than globular clusters and their stellar populations generally have a younger average age than those of globular clusters. What they found in NGC 188 was that 16 of the cluster's 21 blue stragglers are in binary systems with white dwarf companions, three times as many as expected based on the numbers of normal stars found in binary systems.
Together, these results show a consistent picture where, in the less dense open clusters blue stragglers form mainly through accretion from a companion star, while in the denser environment of globular clusters where collisions between stars are far more likely, blue stragglers form via both mechanisms.
Ferraro, F., Beccari, G., Dalessandro, E., Lanzoni, B., Sills, A., Rood, R., Pecci, F., Karakas, A., Miocchi, P., & Bovinelli, S. (2009). Two distinct sequences of blue straggler stars in the globular cluster M 30 Nature, 462 (7276), 1028-1031 DOI: 10.1038/nature08607
Mathieu, R., & Geller, A. (2009). A binary star fraction of 76 per cent and unusual orbit parameters for the blue stragglers of NGC 188 Nature, 462 (7276), 1032-1035 DOI: 10.1038/nature08568