NASA/JPL/STScI/David James

Cosmic conundrum. The binary star system Par 1802 within the Orion Nebula poses a riddle in stellar evolution.

Twin Stars Not So Identical

It's the stellar equivalent of identical twins being born weeks apart. A U.S.-British team has found a closely orbiting pair of twin stars in the Orion Nebula, where one star is hundreds of thousands of years older than the other. This head-scratcher of a discovery could force theorists to reconsider their ideas on how stars form.

Our solitary sun--even the word "solitary" derives from it--is in the minority among stars in the galaxy. A bit more than half of the hundreds of billions of stars in the Milky Way travel in pairs, nearly all of them orbiting so close that they can't be distinguished individually except by powerful telescopes. Current theory states that baby stars hatch by the bushel in stellar nurseries, vast clouds of dust and gas that congeal into stars. Often, two of those stars will form out of the same dust cloud at the same time, driven into very close orbits by gravity and their initial proximity. A small proportion of those pairs--about 0.2%--become identical twins, nearly equal in mass.

But the newly identified twins don't quite fit the mold. Typical stellar twins, because they are the same age, emit the same intensity and color of light. But the new twins, known collectively as Par 1802 and located 1500 light-years away, contain one member that is brighter and hotter than the other, a team of astronomers led by Keivan Stassun of Vanderbilt University in Nashville, Tennessee, reports today in Nature. The difference means that for several hundred thousand years, one twin was fully formed whereas the other remained a loose lump of still-condensing protostellar material.

Why would one star from an identical pair coalesce and ignite so far ahead of the other? Stassun's team says that it's extremely unlikely the two stars could have been born in distant gas clouds and paired up later; there would not have been time for them to move so close together. Instead, the researchers think some as-yet-unknown factor separated the twins in time. But because stellar-formation models assume that identical twins form more or less simultaneously, the discovery "tells us that there may be something fundamental about the stellar birth process that we still don't understand," Stassun says.

The paper has gotten other teams interested in tracking down identical binaries to see what aspects of the mystery can be unfolded. Astronomer Ray Jayawardhana of the University of Toronto in Canada is working on one such team. He says the finding is "definitely an intriguing result," because it "shows that our understanding of the birth and early evolution of multiple star systems is far from complete."

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