Identical twin stars are remarkably different

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Washington, June 19 : The analysis of the youngest pair of identical twin stars yet discovered has revealed remarkable differences in brightness, surface temperature and possibly even the size of the two.

The identical twins were discovered in the Orion Nebula, a well-known stellar nursery, that is 1,500 light years away.

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

Their study 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 test for successful star formation theories, forcing theorists back to the drawing board to determine if their models can produce binaries with stars that form at different times.

"Very young eclipsing binaries like this are the Rosetta stones that tell us about the life history of newly formed stars," said Keivan Stassun, associate professor of astronomy at Vanderbilt University.

Eclipsing binaries are pairs of stars that revolve around an axis at a right angle to the direction to Earth.

This orientation allows astronomers to determine the rate that the two stars orbit around each other even when they cannot resolve the individual stars by measuring the periodic variations in brightness that result when the stars pass in front of each other.

With this information, astronomers can determine the masses of the two stars using Newton's laws of motion.

In this fashion, the astronomers calculate that the newly discovered twins have nearly identical masses 41 percent that of the sun.

Because the two stars condensed from the same cloud of gas and dust they should have the same composition. With identical mass and composition, they should be identical in every way.

So, the astronomers were surprised when they discovered that the twins exhibited significant differences in brightness, surface temperature and possibly size.

By measuring the difference in the amount that the light dipped during the eclipses, the astronomers were able to determine that one of the stars is two times brighter than the other and calculate that the brighter star has a surface temperature about 300 degrees higher than its twin.

An additional analysis of the light spectrum coming from the pair also suggests that one of the stars is about 10 percent larger than the other, but additional observations are needed to confirm it.

In addition to causing theorists to re-examine star-formation models, the new discovery may cause astronomers to readjust their estimates of the masses and ages of thousands of young stars less than a few million years old.

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