Sprinkled throughout every galaxy are tightly packed balls of thousands or even millions of stars. And deep within the heart of just about all of these stellar clusters are strange populations of stars called blue stragglers, which appear much younger than their companions, even though they should all be the same age. Now astronomers think they have identified the secret of these stars, which, like Dorian Gray, refuse to grow old.
As low-mass stars approach the end of their life cycles--at around 10 billion years old--most evolve first into red giants and then into white dwarfs. As they pass through those stages, the stars grow dimmer and cooler. But blue stragglers seem to follow a different path. Despite their advanced age--perhaps as old as 13 billion years--they burn hot and bright, just like a young star. Astronomers have hypothesized that these stars may owe their youthful looks to leaching from their neighbors. They may suck hydrogen away from nearby stars in a process called "mass transfer," or collide with them and absorb them whole.
In two papers appearing this week in Nature, astronomers report evidence for both processes. One team observed blue stragglers in NGC 188, a cluster located about 5000 light-years away in the northern constellation Cepheus. The other group studied Messier 30, about 26,000 light-years away in Capricorn.
First, in the observation of Messier 30, astronomer Francesco Ferraro of the University of Bologna in Italy and colleagues confirmed that, in spite of their youthful appearance, blue stragglers are most likely quite old. Using data involving the temperature and brightness of the stars collected by the Hubble Space Telescope, they observed 44 blue stragglers among the cluster's many thousands of red-giant and white-dwarf stars. The data revealed that the stragglers are bright and hot, as would be expected for young suns. But these stars must be old, as they formed in the same place and, thus, at the same time as the other stars in the cluster. The absence of protostellar cloud material and the low velocity of the stars shows they could not have been born relatively recently or sailed in from elsewhere in the galaxy.
Meanwhile, Robert Mathieu and Aaron Geller of the University of Wisconsin, Madison, used the telescope at the WIYN Observatory, near Tucson, Arizona, to track the motion and brightness of stars in a cluster called NGC 188 for nearly 15 years. The two astronomers found that about 75% of the blue stragglers in the cluster have partners that typically orbit at a distance similar to that between the sun and Jupiter. Such close proximity strongly suggests "that blue stragglers result from mass transfers, collisions, and mergers between two or more stars," Mathieu says.
Together, the results confirm suspicions that the blue stragglers emerged from collisions and mass transfers, says astronomer Alison Sills of McMaster University in Hamilton, Canada. But there's still no direct evidence that shows exactly how an individual blue straggler was formed. The definitive answer, Sills says, could take another 10 to 15 years of observations.