Celestial Twins Have Their Differences

Binary stars, as every science-fiction aficionado knows, are pairs of stars that orbit around their center of mass. In the world of astrophysics, binary stars are important because observing their mutual orbits not only helps determine the mass of the binaries, but also, by extrapolation, the mass of many single stars.

An estimated one-third of stars in the Milky Way are binary or multiple stars. Far rarer—approximately one in every thousand stars—are what astronomers call “identical twin” stars, two stars that condensed from the same cloud of gas and dust and should have the same composition. According to current theories, mass and composition are the two factors that determine a star’s physical characteristics and dictate its entire life cycle.

But analysis of the youngest pair of identical-twin stars ever discovered reveals surprising differences in brightness, surface temperature and possibly even size. A study 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 test for successful star-formation theories, sending theorists back to the drawing board to determine if their models can produce binaries with stars that form at different times.

The newly discovered identical twins, dubbed Par 1802, were discovered in the Orion Nebula, a well-known stellar nursery that is 1,500 light-years away. The young stars are about 1 million years old. With a full life span of about 50 billion years, that makes them equivalent to 1-day-old human babies.

“Young binary stars are astrological Rosetta stones.”

~Keivan Stassun

“Very young eclipsing binaries like this are the Rosetta stones that tell us about the life history of newly formed stars,” says Keivan Stassun, associate professor of astronomy at Vanderbilt. He and Robert D. Mathieu from the University of Wisconsin-Madison headed up the project.

Eclipsing binaries are pairs of stars that revolve around an axis at a right angle to the direction of Earth. This orientation allows astronomers to determine the rate at which the two stars orbit around each other by measuring periodic variations in brightness that result when the stars pass in front of each other. With this information astronomers determine the masses of the two stars using Newton’s laws of motion.

By measuring Par 1802’s differences in light during eclipses, astronomers determined that one of the stars is two times brighter than the other and calculated that the brighter star has a surface temperature about 300 degrees higher than its twin. 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 for confirmation.

“The easiest way to explain these differences is for one star to have been formed about 500,000 years before its twin,” says Stassun. “That is equivalent to a human birth-order difference of about half a day.”

In addition to causing theorists to re-examine star formation models, the new discovery may cause astronomers to readjust estimates of the masses and ages of thousands of young stars.

Astronomers made initial measurements of eclipses of the two stars by sifting through nearly 15 years’ worth of observations of several thousand stars using a telescope at the Kitt Peak National Observatory in Arizona and the SMARTS telescopes at the Cerro Tololo Inter-American Observatory in Chile. They made additional measurements using the Hobby–Eberly Telescope in Texas.

Other participants in the study are doctoral students Phillip Cargile and Alicia Aarnio from Vanderbilt, Aaron Geller from the University of Wisconsin-Madison, and Eric Stempels from the University of St. Andrews in Scotland.

The research is part of the Vanderbilt Initiative in Data-Intensive Astrophysics and was supported by grants from the National Science Foundation and the Research Corp.

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