"Long before it's in the papers"
January 28, 2015


“Cannibalism” creates huge stars

Jan. 19, 2009
Courtesy University of Southampton
and World Science staff

Oddly bloat­ed stars known as blue strag­glers are prod­ucts of “stel­lar can­ni­bal­ism,” as­tro­no­mers claim.

The sci­en­tists at the Uni­ver­s­ity of South­amp­ton, U.K. and Mc­Mas­ter Uni­ver­s­ity in Can­a­da pub­lished the find­ings in the Jan. 15 is­sue of the re­search jour­nal Na­ture.

In this artist's con­cep­tion, a star swal­lows mat­ter from a sec­ond star (the nearer one in the pic­ture). Due to the ac­tion of grav­i­ty, ma­te­ri­al from the can­ni­bal­ized star spir­als into the oth­er one, form­ing a disc shape, be­fore fall­ing in­side. (Im­age cour­te­sy NA­SA) 

Blue strag­glers are found in groups of stars known as glob­u­lar clus­ters, col­lec­tions of about 100,000 stars tightly bound by gra­vity. With­in these groups, blue strag­glers are heav­i­er and ap­pear young­er in sci­en­tif­ic anal­y­ses than the bulk of their coun­ter­parts.

This vi­o­lates stand­ard the­o­ries of stel­lar ev­o­lu­tion, in which all stars in a clus­ter are born at the same time. Stars as mas­sive as blue strag­glers should have died long ago, ac­cord­ing to these the­o­ries, be­cause more mas­sive stars burn out soon­er. Yet vir­tu­ally eve­ry ob­served clus­ter con­tains some of these over­weight stars.

“The or­i­gin of blue strag­glers has been a long-stand­ing mys­tery. 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,” said the Uni­ver­s­ity of South­amp­ton’s Chris­tian Knigge, who led the stu­dy.

“We’ve known of these stel­lar anoma­lies for 55 years,” added col­la­bo­ra­tor Al­i­son Sills of Mc­Mas­ter. “Two main the­o­ries have emerged.” One is that blue strag­glers were cre­at­ed through col­li­sions with oth­er stars. The other has to do with stars in bi­nary, or paired, systems: one star, us­ing gravita­t­ional force, might have pulled ma­ter­ial away from its part­ner and in­cor­por­ated it.

The re­search­ers looked at blue strag­glers in 56 glob­u­lar clus­ters. They found that the num­ber of blue strag­glers in a giv­en clus­ter did not seem to cor­re­late with the pre­dicted col­li­sion rate in the clus­ter—dispelling the­o­ry num­ber one.

But they did find a con­nec­tion be­tween the num­ber of strag­glers and the mass of the clus­ter co­re, and from this in­ferred a con­nec­tion to the num­ber of bi­na­ry stars in a clus­ter co­re. This con­nec­tion is sup­ported by pre­lim­i­nar­y ob­serva­t­ions of bi­na­ry stars in clus­ters, and points to “stel­lar can­ni­bal­ism” as the main driv­er of blue strag­gler or­i­gin, the re­search­ers said.

“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 bi­na­ry stars trans­fer­ring mat­ter,” Knigge said. “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.”

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Oddly bloated stars known as blue stragglers are products of “stellar cannibalism,” astronomers claim. The scientists at the University of Southampton, U.K. and McMaster University in Canada published the findings in the Jan. 15 issue of the research journal Nature. Blue stragglers are found in groups of stars known as globular clusters, collections of about 100,000 stars tightly bound by gravity into a vast sphere. Within these groups, blue stragglers are heavier and appear younger in scientific analyses than the bulk of their counterparts. This violates standard theories of stellar evolution, in which all stars in a cluster are born at the same time. Stars as massive as blue stragglers should have died long ago, according to these theories, because more massive stars burn out sooner. Yet virtually every observed cluster contains some of these overweight stars. “The origin of blue stragglers has been a long-standing mystery. 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,” said the University of Southampton’s Christian Knigge, who led the study. “We’ve known of these stellar anomalies for 55 years now,” added collaborator Alison Sills of McMaster. “Two main theories have 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” using its gravitational force. The researchers looked at blue stragglers in 56 globular clusters. They found the total number of blue stragglers in a given cluster did not seem to correlate with the predicted collision rate in the cluster—dispelling theory number one. But they did find a connection between the number of stragglers and the mass of the cluster core, and from this inferred a connection to the number of binary stars in a cluster core. This connection is supported by preliminary observations of binary stars in clusters, and points to “stellar cannibalism” as the main driver of blue straggler origin, the researchers said. “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 binary stars transferring matter,” Knigge said. “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.”