Newly born twin stars show surprising differences

June 18, 2008

by Terry Devitt

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

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 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.

“It’s a bit like finding identical twins who aren’t identical. They have the same DNA, but they don’t look the same.”

Robert Mathieu, professor of astronomy

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

"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 University. He and Robert Mathieu from the University of Wisconsin-Madison headed the project.

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. According to current theories, mass and composition are the two factors that determine a star's physical characteristics and dictate its entire life cycle. 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.

"It's a bit like finding identical twins who aren't identical," Mathieu explains. "They have the same DNA, but they don't look the same."

The astronomers made the initial measurements of the 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. To gain more information about the two stars, they made additional measurements using the Hobby Eberly Telescope in Texas. 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.

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

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. Current estimates are based on models that were calibrated with measurements of young binary stars that were presumed to have formed simultaneously. The recalibration required could be as much as 20 percent for the mass of a typical young star and as much as 50 percent for very low-mass stars like brown dwarfs, Stassun estimates.

Other participants in the study are doctoral students Phillip Cargile and Alicia Aarnio from Vanderbilt and Aaron Geller from UW-Madison, along with 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.