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Cosmic Gatherings: How Stars Get Hitched

Anastronomical survey of what was supposed to be a double star system has uncovered another starlurking behind its thick veils of gas and dust. Scientists think the third starmight have been gravitationally lassoed in by the other two stars as it waspassing by.

Unlike our Sun, many stars in the universe have atleast one stellar companion. How such multiple star systems form is stillan open question, but there are two popular theories.

Accordingto one idea, a large disk of dust, gas and other material swirls around soquickly that it breaks apart to form two or more young stars, called "protostars."

Each protostaris surrounded by its own disk that, in time, can coalesce to form individualsolar systems, complete with planets,moons, comets and asteroids. According to thisso-called "competitive accretion" hypothesis, stars can form in timespans as brief as 1 million years or less.

Analternative idea is that stars are born alone, by a slow accretion of gas anddust that can take 10 million years or more. According to this scenario,multiple star systems form when a star gravitationally ropes in another star asit passes by.

Scientistsled by Jeremy Lim of the Institute of Astronomy & Astrophysics, Academia Sinica,in Taiwan, examined the protostars in L1551 IRS5,a still-forming star system located 450 light-years from Earth. Using the VeryLarge Array (VLA) radio telescope, the team discovered a third star thatwas previously hidden. The close snapshots also revealed features that supportboth formation theories.

"Ournew study shows that the disks of the two main protostars are aligned with eachother, and also are aligned with the larger, surrounding disk. In addition,their orbital motion resembles the rotation of the larger disk," Lim said."This is a 'smoking gun' supporting the fragmentation model."

Alyssa Goodman, an astronomer at Harvard-Smithsonian Center for Astrophysics who was notinvolved in the study, said the findings suggest L1551 IRS5, now a relativelycalm region of the sky, must have once been a much more turbulent place for twoprotostars to form together.

Whether a swirlingdisk gives birth to one star or many depends on how vigorous theinteractions are between its parts. If there is a lot of mixing occurringwithin the disk, it is more likely to fragment and give birth to multiplestars. Goodman likens the chances of interactions between stellar material tothe probability of people living in rural, suburban and urban areas comingacross one another.

Peopleliving in the city are more likely to run into one another than those in thecountryside. The same is true for star-forming disks, Goodman said.

"Thereason [L1551 IRS5] is interesting for me is because it says that even thisregion that looks rural, or possibly even suburban, that something which isthought to happen only in urban regions must have happened," Goodman told

The third protostar,Lim's team discovered, is aligned along a different plane than its twoneighbors, suggesting it might have formed elsewhere. "The misalignment ofthe third protostar and its disk leaves open the possibility that it could haveformed elsewhere and been captured," Lim said.

Themisaligned protostar is not conclusive evidence of the capture scenario,however, since gravitational interactions with its two larger neighbors couldhave skewed the protostar's alignment. Lim's team is planning further studiesto test the two hypotheses.

The studyis detailed in the Dec. 10 issue of Astrophysical Journal.

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Ker Than is a science writer and children's book author who joined as a Staff Writer from 2005 to 2007. Ker covered astronomy and human spaceflight while at, including space shuttle launches, and has authored three science books for kids about earthquakes, stars and black holes. Ker's work has also appeared in National Geographic, Nature News, New Scientist and Sky & Telescope, among others. He earned a bachelor's degree in biology from UC Irvine and a master's degree in science journalism from New York University. Ker is currently the Director of Science Communications at Stanford University.