Stars known as “blue stragglers” have stumped astronomers for years. Blue stragglers are found in open or globular clusters, and are hotter, bigger and bluer than other stars in the same vicinity. According to conventional theories, these massive stars should have died long ago because all stars in a cluster are born at the same time and should therefore be at a similar phase. Instead of being older, however, these massive rogue stars appear to be much younger than the other stars and are found in virtually every observed cluster. But now researchers have discovered these mysterious overweight stars are the result of ‘stellar cannibalism’ where plasma is gradually pulled from one star to another to form a massive, unusually hot star that appears younger than it is. The process takes place in binary stars – star systems consisting of two stars orbiting around their common center of mass. This helps to resolve a long standing mystery in stellar evolution.
Two theories for Blue Stragglers were that blue stragglers were either created through collisions with other stars or that one star in a binary system was ‘reborn’ by pulling matter off its companion.
The researchers, led by Dr. Christian Knigge from Southampton University and Professor Alison Sills from the McMaster University, looked at blue stragglers in 56 globular clusters. They found that the total number of blue stragglers in a given cluster did not correlate with predicted collision rate – dispelling the theory that blue stragglers are created through collisions with other stars.
They did, however, discover a connection between the total mass contained in the core of the globular cluster and the number of blue stragglers observed within in. Since more massive cores also contain more binary stars, they were able to infer a relationship between blue stragglers and binaries in globular clusters. They also showed that this conclusion is supported by preliminary observations that directly measured the abundance of binary stars in cluster cores. All of this points to “stellar cannibalism” as the primary mechanism for blue straggler formation.
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“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,” said Dr. Knigge. “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.”
The research, which is part funded by the UK’s Science and Technology Facilities Council (STFC) will be published in the journal Nature on Thursday January 15.