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Exiled stars may have merged to form speeding giant

July 23, 2010
Courtesy of NASA
and World Science staff

NASA’s Hub­ble Space Tel­e­scope has de­tected a hy­pe­r­ve­locity star, a rare ob­ject mov­ing three times faster than our Sun.

The star may have been cre­at­ed in a cos­mic mis­step, as­tro­no­mers say. In the proposed scenario, a triple-star sys­tem was drift­ing through the bustling cen­ter of our Milky Way gal­axy a hun­dred mil­lion years ago when it came too close to the gal­ax­y’s gi­ant black hole. This object cap­tured one of the stars and hurled the oth­er two out of the Milky Way in a sort of gravita­t­ional bil­liards game. The two ex­iles merged to form one in­credibly speedy, super-hot blue star.

This tale may seem like sci­ence fic­tion, but Hub­ble as­tro­no­mers call it the most likely or­i­gin of a so-called hy­pe­r­ve­locity star, known as HE 0437-5439. It is one of the fastest ev­er de­tected with a speed of 1.6 mil­lion miles (2.5 mil­lion km) per hour.

Hub­ble ob­serva­t­ions con­firm that the stel­lar speed­ster hails from the Milky Way’s co­re, set­tling some con­fu­sion about its or­i­ginal home, re­search­ers said.

Most of the roughly 16 known hy­pe­r­ve­locity stars, all dis­cov­ered since 2005, are thought to hail from the heart of our gal­axy. But this Hub­ble re­sult is said to be the first di­rect ob­serva­t­ion link­ing such a star to an or­i­gin in the ga­lac­tic cen­ter.

“Us­ing Hub­ble, we can for the first time trace back to where the star came from by meas­ur­ing the star’s di­rection of mo­tion on the sky,” said as­tron­o­mer War­ren Brown of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics in Cam­bridge, Mass. “Our mea­sure­ments point di­rectly to the Milky Way cen­ter.” Brown, a mem­ber of the Hub­ble team that ob­served the star, is the lead au­thor of a paper on the find­ing pub­lished on­line July 20 in the As­t­ro­phys­i­cal Jour­nal Let­ters.

“These ex­iled stars are rare in the Milky Way’s popula­t­ion of 100 bil­lion stars. For ev­ery 100 mil­lion... there lurks one hy­pe­r­ve­locity star,” Brown said. The stel­lar out­cast al­ready is cruis­ing in the Milky Way’s dis­tant out­skirts about 200,000 light-years from the cen­ter. A light-year is the dis­tance light trav­els in a year. 

Us­ing Hub­ble to meas­ure the run­a­way star’s di­rection and de­ter­mine the Milky Way’s co­re as its start­ing point, Brown and col­leagues cal­cu­lat­ed how fast the star had to have been ejected to reach its cur­rent loca­t­ion.

“S­tudy­ing these stars could pro­vide more clues about the na­ture of some of the uni­verse’s un­seen mass, and it could help as­tro­no­mers bet­ter un­der­stand how ga­lax­ies for­m,” said team lead­er Oleg Gnedin of the Uni­vers­ity of Mich­i­gan in Ann Ar­bor.

The star’s age is anoth­er mys­tery. Based on the speed and po­si­tion of HE 0437 5439, it would have to be 100 mil­lion years old to have jour­neyed from the Milky Way’s co­re. Yet its mass — nine times that of our Sun — and blue col­or imply it should have burned out af­ter only 20 mil­lion years of age.

As­tro­no­mers have pro­posed two pos­si­bil­i­ties to solve the age prob­lem. The star ei­ther dipped in­to a Foun­tain of Youth by be­com­ing a so-called “blue strag­gler,” or it was flung out of a near­by neigh­bor­ing gal­axy, the Large Mag­el­lanic Cloud.

In 2008 a team of as­tro­no­mers thought they had solved the mys­tery. They matched the ex­iled star’s chem­i­cal make­up to char­ac­ter­is­tics of Large Mag­el­lanic Cloud stars. The rogue star’s po­si­tion al­so is close to the neigh­bor­ing gal­axy, only 65,000 light-years away. 

The new Hub­ble re­sult, though, would place its native home in the Milky Way. The best ex­plana­t­ion for its col­or and speed, Hub­ble as­tro­no­mers claim, is that it was part of a triple-star sys­tem that was in­volved in a vi­o­lent gravita­t­ional dance with the gal­ax­y’s mon­ster black hole. The con­cept be­hind such an event was first pro­posed in 1988 in a the­o­ry that pre­dicted the Milky Way’s black hole should eject a star about once ev­ery 100,000 years.

The three-star sys­tem would have con­tained a pair of closely or­bit­ing stars, and a much further-out mem­ber cir­cling this pair. The black hole, an ob­ject with gravita­t­ional pull so strong that not even light can es­cape it, would have sucked in this out­er star, al­low­ing its mo­men­tum to pass to its com­pan­ions, hurl­ing them out of the gal­axy. 

As they sped away, they went on with nor­mal stel­lar ev­o­lu­tion. The heav­i­er one in its old age be­came a so-called red gi­ant and en­vel­oped its part­ner, as­tro­no­mers pro­pose. The two then spi­raled to­geth­er, merg­ing in­to one supe­rstar, the blue strag­gler de­tected by Hub­ble. A blue strag­gler is a rel­a­tively young, heavy star born of the merg­er of two lighter ones.

As­tro­no­mers used the sharp vi­sion of Hub­ble’s Ad­vanced Cam­era for Sur­veys to make two sep­a­rate ob­serva­t­ions of the way­ward star 3.5 years apart. Team mem­ber Jay An­der­son of the Space Tel­e­scope Sci­ence In­sti­tute in Bal­ti­more de­vel­oped a tech­nique to meas­ure the star’s po­si­tion rel­a­tive to each of 11 dis­tant back­ground ga­lax­ies. These back­ground ga­lax­ies form a ref­er­ence frame in which An­der­son com­pared the star’s po­si­tion in 2006 and 2009 to cal­cu­late how far it had moved.

“Hub­ble ex­cels with this type of meas­urement,” An­der­son said. “This ob­serva­t­ion would be chal­leng­ing to do from the ground.” The team is try­ing to de­ter­mine the homes of four oth­er un­bound stars, all lo­cat­ed on the fringes of the Milky Way.

“We are tar­get­ing mas­sive ‘B’ stars, like HE 0437-5439,” said Brown, who has dis­cov­ered 14 of the 16 known hy­pe­r­ve­locity stars. “These stars should­n’t live long enough to live in the dis­tant out­skirts of the Milky Way, so we should­n’t ex­pect to find them there. But the quantity of stars in the out­er re­gion is much less than in the co­re, so we have a bet­ter chance of find­ing these un­usu­al ob­jects.” 


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NASA’s Hubble Space Telescope has detected a hypervelocity star, a rare object moving three times faster than our Sun. The star may have been created in a cosmic misstep, astronomers say. A hundred million years ago, a triple-star system was traveling through the bustling center of our Milky Way galaxy when it wandered too close to the galaxy’s giant black hole. The black hole captured one of the stars and hurled the other two out of the Milky Way in a sort of gravitational billiards game. The two exiles merged to form an incredibly speedy, super-hot blue star. This tale may seem like science fiction, but Hubble astronomers call it the most likely origin of a so-called hypervelocity star, known as HE 0437-5439. It is one of the fastest ever detected with a speed of 1.6 million miles (2.5 million km) per hour. Hubble observations confirm that the stellar speedster hails from the Milky Way’s core, settling some confusion about its original home, researchers said. Most of the roughly 16 known hypervelocity stars, all discovered since 2005, are thought to hail from the heart of our galaxy. But this Hubble result is said to be the first direct observation linking such a star to an origin in the galactic center. “Using Hubble, we can for the first time trace back to where the star came from by measuring the star’s direction of motion on the sky,” said astronomer Warren Brown of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “Our measurements point directly to the Milky Way center.” Brown, a member of the Hubble team that observed the star, is the lead author of a paper on the finding published online July 20 in the Astrophysical Journal Letters. “These exiled stars are rare in the Milky Way’s population of 100 billion stars. For every 100 million stars in the galaxy, there lurks one hypervelocity star,” Brown said. The stellar outcast already is cruising in the Milky Way’s distant outskirts about 200,000 light-years from the galaxy’s center. A light-year is the distance light travels in a year. Using Hubble to measure the runaway star’s direction and determine the Milky Way’s core as its starting point, Brown and colleagues calculated how fast the star had to have been ejected to reach its current location. “Studying these stars could provide more clues about the nature of some of the universe’s unseen mass, and it could help astronomers better understand how galaxies form,” said team leader Oleg Gnedin of the University of Michigan in Ann Arbor. The star’s age is another mystery. Based on the speed and position of HE 0437 5439, it would have to be 100 million years old to have journeyed from the Milky Way’s core. Yet its mass — nine times that of our Sun — and blue color imply it should have burned out after only 20 million years of age. Astronomers have proposed two possibilities to solve the age problem. The star either dipped into a Fountain of Youth by becoming a so-called “blue straggler,” or it was flung out of a nearby neighboring galaxy, the Large Magellanic Cloud. In 2008 a team of astronomers thought they had solved the mystery. They matched the exiled star’s chemical makeup to characteristics of Large Magellanic Cloud stars. The rogue star’s position also is close to the neighboring galaxy, only 65,000 light-years away. The new Hubble result, thought, would place it in the Milky Way. The best explanation for its color and speed, Hubble astronomers claim, is that it was part of a triple-star system that was involved in a violent gravitational dance with the galaxy’s monster black hole. The concept behind such an event was first proposed in 1988 in a theory that predicted the Milky Way’s black hole should eject a star about once every 100,000 years. The three-star system would have contained a pair of closely orbiting stars, and a much further-out member circling this pair. The black hole, an object with gravitational pull so strong that not even light can escape it, would have sucked in this outer star, allowing its momentum to pass to its companions, hurling them out of the galaxy. As they sped away, they went on with normal stellar evolution. The heavier one in its old age became a so-called red giant and enveloped its partner, astronomers theorized. The two then spiraled together, merging into one superstar, the blue straggler detected by Hubble. A blue straggler is a relatively young, massive star produced by the merger of two lighter-weight stars. Astronomers used the sharp vision of Hubble’s Advanced Camera for Surveys to make two separate observations of the wayward star 3.5 years apart. Team member Jay Anderson of the Space Telescope Science Institute in Baltimore developed a technique to measure the star’s position relative to each of 11 distant background galaxies. These background galaxies form a reference frame in which Anderson compared the star’s position in 2006 and 2009 to calculate how far it had moved. “Hubble excels with this type of measurement,” Anderson said. “This observation would be challenging to do from the ground.” The team is trying to determine the homes of four other unbound stars, all located on the fringes of the Milky Way. “We are targeting massive ‘B’ stars, like HE 0437-5439,” said Brown, who has discovered 14 of the 16 known hypervelocity stars. “These stars shouldn’t live long enough to live in the distant outskirts of the Milky Way, so we shouldn’t expect to find them there. But the quantity of stars in the outer region is much less than in the core, so we have a better chance of finding these unusual objects.”