Twin stars observed in the Orion Nebula, a stellar nursery 1,500 light years from Earth. At this distance the twin stars appear as a single point of light. The observations were made with the NSF-supported SMARTS telescopes at the Cerro Tololo Inter-American Observatory in Chile, and with access provided by NSF to the Hobby-Eberly Telescope at the MacDonald Observatory in Texas. Click on image for full size (134 Kb) Image Courtesy of NASA-JPL/HST and David James (Vanderbilt)

Two stars, each with the same mass and in orbit around each other, are twins that one would expect to be identical. So astronomers were surprised when they discovered that twin stars in the Orion Nebula, a well-known stellar nursery 1,500 light years away, were not identical at all. In fact, these stars exhibited significant differences in brightness, surface temperature and possibly even size.

The study, which is published in the June 19 issue of the journal Nature, suggests that one of the stars formed significantly earlier than its twin. Because astrophysicists have assumed that binary stars form simultaneously, the discovery provides an important new challenge for today's star formation theories, forcing theorists to reexamine their models to see if the models can indeed produce binaries with stars that form at different times.

Because mass and age estimates for stars less than a few million years old are based on models that were calibrated with measurements of binary stars presumed to have formed simultaneously, this new discovery may cause astronomers to readjust their estimates for thousands of young stars.

The newly formed twin stars are about 1 million years old. With a full lifespan of about 50 billion years, that makes them equivalent to one-day-old human babies.

"The easiest way to explain the observed differences is if one star was fully formed about 500,000 years before its twin. That would be equivalent to a human birth-order difference of about half a day," said Keivan Stassun, associate professor of astronomy at Vanderbilt University.

Stassun and Robert D. Mathieu from the University of Wisconsin-Madison headed up the project, with significant funding from the National Science Foundation.

Text above is courtesy of the National Science Foundation

Twin stars observed in the Orion Nebula. At this distance the twin stars appear as a single point of light. Click on image for full size (134 Kb) Image Courtesy of NASA-JPL/HST and David James (Vanderbilt)

Two stars, each with the same mass and in orbit around each other, are twins that one would expect to be identical. So astronomers were surprised when they discovered that twin stars in the Orion Nebula were not identical at all. In fact, these stars showed significant differences in brightness, surface temperature, and possibly even size.

It seems that one of the stars formed significantly earlier than its twin. Because astrophysicists have assumed that twin stars form at the same time, this new discovery adds an important new challenge for today's star formation theories and will force scientists to look back at their models to see if their models can produce twin stars that have formed at different times.

This new discovery may cause astronomers to readjust their estimates for thousands of young stars.

The newly formed twin stars are about 1 million years old. With a full life span of about 50 billion years, that makes them equivalent to one-day-old human babies.

"The easiest way to explain the observed differences is if one star was fully formed about 500,000 years before its twin. That would be equivalent to a human birth-order difference of about half a day," said Keivan Stassun, associate professor of astronomy at Vanderbilt University.

Twin stars observed in the Orion Nebula. At this distance the twin stars look like a single point of light. Click on image for full size (134 Kb) Image Courtesy of NASA-JPL/HST and David James (Vanderbilt)

Astronomers have always thought that twin stars, which are stars that have the same mass and orbit around each other, are identical. So they were surprised when they discovered that twin stars in the Orion Nebula were not identical at all. These stars actually have major differences in their brightness, surface temperature, and possibly even their size.

The newly formed twin stars are about 1 million years old. Since their full life span is about 50 billion years, they are equal to one-day-old human babies.

"The easiest way to explain the observed differences is if one star was fully formed about 500,000 years before its twin. That would be equivalent to a human birth-order difference of about half a day," said Keivan Stassun, one of the scientists who worked on this project.