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Collisions and “vampirism” may make stars look newly young

Dec. 26, 2009
Courtesy NASA, European Space Agency
and World Science staff

Both col­li­sions and “vam­pirism” be­tween stars can make some of them look much young­er than they really are, as­tro­no­mers have found.

Both pro­cesses are at work in a clus­ter of stars called Mess­i­er 30, ac­cord­ing to Fran­ces­co Fer­raro of the Un­ivers­ity of Bo­lo­gna in Italy and col­leagues, who stud­ied the stars us­ing the Hub­ble Space Tele­scope.

This im­age of Mess­i­er 30 (M 30) was tak­en by Hub­ble's Ad­vanced Cam­era for Sur­veys. Mess­i­er 30 formed 13 bil­lion years ago and was dis­cov­ered in 1764 by Charles Mess­i­er. Lo­cat­ed about 28 000 light-years away from Earth, this glob­u­lar clus­ter — a dense swarm of sev­er­al hun­dred thou­sand stars — is about 90 light-years across. (Cred­it: NA­SA/ ESA )
Mess­i­er 30 is a glob­u­lar clus­ter, one of many dense clumps of sev­er­al hun­dred thou­sand stars that sur­round the co­re of the Milky Way and like ga­lax­ies. Stars in glob­u­lar clus­ters are gen­er­ally old—al­most as old as the un­iverse it­self—with ages of 12-13 bil­lion years. 

But a few glob­u­lar clus­ter stars ap­pear much young­er than the rest. They’re dubbed “blue strag­glers” be­cause they are ap­par­ently left be­hind by the stel­lar ev­o­lu­tion pro­cess that turns nor­mal stars in­to so-called red gi­ants to­ward the ends of their lives. Blue strag­glers ap­pear to re­gress from “old age” back to a hot­ter and brighter “y­outh,” gain­ing a new lease on life in the pro­cess. 

Fer­raro and col­leagues stud­ied blue strag­glers in Mess­i­er 30, dis­cov­ered in 1764 by Charles Mess­i­er. Lo­cat­ed about 28,000 light-years away from Earth, this clus­ter—a swarm of sev­er­al hun­dred thou­sand stars—is about 90 light-years across. A light year is the dis­tance light trav­els in a year.

Blue strag­glers have been known since the 1950s, but how they form has re­mained a puz­zle. “It’s like see­ing a few kids in the group pic­ture of a rest-home for re­tired peo­ple. It is nat­u­ral to won­der why they are there,” said Fer­raro, lead au­thor of the stu­dy, whose re­sults are pub­lished in the Dec. 24 is­sue of the re­search jour­nal Na­ture.

Re­search­ers had pre­vi­ously con­clud­ed that blue strag­glers are in­deed old, and had aris­en in sys­tems of two stars mu­tu­ally or­bit­ing at close range. In such pairs, the smaller star acts as a “vam­pire” si­phon­ing fresh hy­dro­gen from its more mas­sive com­pan­ion. The new fu­el supply al­lows the smaller star to heat up, grow­ing blu­er and hot­ter—which is the way a young­er star acts.

The new study found that some of the blue strag­glers have in­stead been re­ju­ve­nat­ed by a sort of facelift, cour­te­sy of cos­mic col­li­sions. These stel­lar en­coun­ters are nearly head-on col­li­sions in which the stars might ac­tu­ally merge, mix­ing their nu­clear fu­el and re-stoking the fires of nu­clear fu­sion, the pro­cess that sup­plies en­er­gy to stars.

“Our ob­serva­t­ions dem­on­strate that blue strag­glers formed by col­li­sions have slightly dif­fer­ent prop­er­ties from those formed by vam­pir­ism. This pro­vides a di­rect demon­stra­t­ion that the two forma­t­ion sce­nar­i­os are val­id and that they are both op­er­at­ing sim­ul­ta­ne­ously in this clus­ter,” said team mem­ber Gia­co­mo Bec­cari from the Eu­ro­pe­an Space Agen­cy.

Us­ing da­ta from the now-re­tired Wide Field Plan­e­tary Cam­era 2 aboard Hub­ble, as­tro­no­mers found that these “s­trag­gling” stars are much more con­cen­trat­ed to­wards the clus­ter cen­ter than the av­er­age star. “This in­di­cates that blue strag­glers are more mas­sive than the av­er­age star in this clus­ter,” be­ing made of merged stars, said Fer­raro. “More mas­sive stars tend to sink deep in­to the clus­ter the way a bil­liard ball would sink in a buck­et of hon­ey.”

The cen­tral re­gions of high dens­ity glob­u­lar clus­ters are crowd­ed neigh­bour­hoods where in­ter­ac­tions be­tween stars are nearly in­ev­i­ta­ble. Re­search­ers con­jec­ture that one or two bil­lion years ago, Mess­i­er 30 un­der­went a ma­jor “co­re col­lapse” that started to throw stars to­wards the cen­tre of the clus­ter, lead­ing to a rap­id in­crease in the dens­ity of stars. This boosted the num­ber of col­li­sions among stars, fa­vor­ing the forma­t­ion of one type of blue strag­gler. But this crowd­ing would have al­so per­turbed the twin sys­tems, en­cour­ag­ing the vam­pir­ism phe­nom­e­non and thus form­ing the oth­er family of blue strag­glers. 

“Al­most ten per­cent of ga­lac­tic glob­u­lar clus­ters have ex­pe­ri­enced co­re col­lapse, but this is the first time that we see the ef­fect of the co­re col­lapse im­printed on a stel­lar popula­t­ion,” said Bar­ba­ra Lan­zoni of the Un­ivers­ity of Bo­lo­gna, anoth­er of the re­search­ers. “Our dis­cov­ery is di­rect ev­i­dence of the im­pact of star clus­ter dy­nam­ics on stel­lar ev­o­lu­tion,” Fer­raro added. “We should now try to see if oth­er glob­u­lar clus­ters pre­s­ent this dou­ble popula­t­ion of blue strag­glers.” 

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Both collisions and “vampirism” between stars can make some stars look much younger than they really are, astronomers have found. Both processes are at work in a cluster of stars called Messier 30, according to Francesco Ferraro of the University of Bologna in Italy and colleagues, who studied the stars using the Hubble Space Telesocpe. Messier 30 is a globular cluster, one of many dense clumps of several hundred thousand stars that surround the core of the Milky Way and like galaxies. Stars in globular clusters are generally old—almost as old as the universe itself—with ages of 12-13 billion years. But a few globular cluster stars appear much younger than the rest. They’re dubbed “blue stragglers” because they are apparently left behind by the stellar evolution process that turns normal stars into so-called red giants toward the ends of their lives. Blue stragglers appear to regress from “old age” back to a hotter and brighter “youth,” gaining a new lease on life in the process. Ferraro and colleagues studied the blue straggler star content in Messier 30, discovered in 1764 by Charles Messier. Located about 28,000 light-years away from Earth, this cluster—a swarm of several hundred thousand stars—is about 90 light-years across. A light year is the distance light travels in a year. Although blue stragglers have been known since the early 1950s, their formation process is still an unsolved puzzle. “It’s like seeing a few kids in the group picture of a rest-home for retired people. It is natural to wonder why they are there,” said Ferraro, lead author of the study, whose results are published in the Dec. 24 issue of the research journal Nature. Researchers had previously concluded that blue stragglers are indeed old, and had arisen in systems of two stars mutually orbiting at close range. In such pairs, the smaller star acts as a “vampire”, siphoning fresh hydrogen from its more massive companion. The new fuel supply allows the smaller star to heat up, growing bluer and hotter—which is the way a younger star acts. The new study found that some of the blue stragglers have instead been rejuvenated by a sort of “cosmic facelift”, courtesy of cosmic collisions. These stellar encounters are nearly head-on collisions in which the stars might actually merge, mixing their nuclear fuel and re-stoking the fires of nuclear fusion, the process that supplies energy to stars. “Our observations demonstrate that blue stragglers formed by collisions have slightly different properties from those formed by vampirism. This provides a direct demonstration that the two formation scenarios are valid and that they are both operating simultaneously in this cluster,” said team member Giacomo Beccari from the European Space Agency. Using data from the now-retired Wide Field Planetary Camera 2 aboard Hubble, astronomers found that these “straggling” stars are much more concentrated towards the cluster center than the average star. “This indicates that blue stragglers are more massive than the average star in this cluster,” being made of merged stars, said Ferraro. “More massive stars tend to sink deep into the cluster the way a billiard ball would sink in a bucket of honey.” The central regions of high density globular clusters are crowded neighbourhoods where interactions between stars are nearly inevitable. Researchers conjecture that one or two billion years ago, Messier 30 underwent a major “core collapse” that started to throw stars towards the centre of the cluster, leading to a rapid increase in the density of stars. This boosted the number of collisions among stars, favoring the formation of one type of blue straggler. But this crowding would have also perturbed the twin systems, encouraging the vampirism phenomenon and thus forming the other family of blue stragglers. “Almost ten percent of galactic globular clusters have experienced core collapse, but this is the first time that we see the effect of the core collapse imprinted on a stellar population,” said Barbara Lanzoni of the University of Bologna, another of the researchers. “Our discovery is direct evidence of the impact of star cluster dynamics on stellar evolution,” Ferraro added. “We should now try to see if other globular clusters present this double population of blue stragglers.”