Oddball 'Blue Stragglers' Are Stellar Cannibals
The core of globular cluster 47 Tucanae is home to many blue stragglers, rejuvenated stars that glow with the blue light of young stars. A ground-based telescope image (on the left) shows the entire crowded core of 47 Tucanae, located 15,000 light-years away in the constellation Tucana. Peering into the heart of the globular cluster's bright core, the Hubble Space Telescope's Wide Field and Planetary Camera 2 separated the dense clump of stars into many individual stars (image on right).
Credit: NASA Goddard Space Flight Center (NASA-GSFC)

Astronomers have found what they say is the strongest evidence yet that a mysterious class of stars known as "blue stragglers" are the result of stellar cannibalism.

Blue stragglers are found throughout the universe in globular clusters ? which typically are collections of about 100,000 stars, tightly bound by gravity. Because all the stars in these clusters are thought to have been born at the same time, they should all be the same age, but blue stragglers appear to be younger than their cluster peers.

The origin of these strange, massive stars has been a longstanding mystery, said study leader Christian Knigge of Southampton University in England.

"The only thing that was clear is that at least two stars must be involved in the creation of every single blue straggler, because isolated stars this massive simply should not exist in these clusters," Knigge added.

Since these oddball stars were first discovered more than half a century ago, two competing explanations for their formation emerged: "that blue stragglers were created through collisions with other stars; or that one star in a binary system was 'reborn' by pulling matter off its companion," said study team member Alison Sills of the McMaster University in Canada.

A 2006 study examined the chemical signatures of 43 blue stragglers in the globular cluster 47 Tucanae, and found that six of the unusual stars had less carbon and oxygen than the others. The anomaly indicated that their surface material had been sucked from the deep interior of a parent star in a binary system.

The new study, detailed in the Jan. 15 issue of the journal Nature, provides even more evidence in favor of the stellar cannibalism idea.

The researchers looked at blue stragglers in 56 globular clusters and found that the total number of blue stragglers in a given cluster didn't match the predicted collision rate ? dispelling the theory that blue stragglers are created through collisions with other stars.

But there was a connection between the total mass contained in the core of the globular cluster and the number of blue stragglers observed within it. Since more massive cores also contain more binary stars, the researchers could infer a relationship between blue stragglers and binaries in globular clusters. This conclusion is also supported by preliminary observations that directly measured the abundance of binary stars in cluster cores.

"This is the strongest and most direct evidence to date that most blue stragglers, even those found in the cluster cores, are the offspring of two binary stars," Knigge said. Though there is still plenty of research on these stars to do.

"In our future work we will want to determine whether the binary parents of blue stragglers evolve mostly in isolation, or whether dynamical encounters with other stars in the clusters are required somewhere along the line in order to explain our results," Knigge said.

The study was funded in part by the United Kingdom's Science and Technology Facilities Council.

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