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> Oddball 'Blue Stragglers' Are Stellar Cannibals - Yahoo! News
>Oddball 'Blue Stragglers' Are Stellar Cannibals
SPACE.com Wed Jan 14, 1:19 pm ET
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
"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.