Identical twin stars reveal surprising differences
BY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: June 20, 2008

A new study of a pair of seemingly identical stellar twins has revealed such surprising differences that astronomers will need to re-examine the ways in which stars form.

The bewildering binary system is located in the Orion Nebula, a well-known stellar nursery that is 1,500 light years away from Earth and which hosts a horde of young, one million year old stars. According to the new findings, presented in the journal Nature this week by astronomers from the University of St Andrews in the UK and Vanderbilt University and the University of Wisconsin-Madison in the USA, one star of the so-called twin was born 500,000 years before its sibling, the equivalent of a human baby being born half a day before its twin.

"The system we studied is a very special one,” says Eric Stempels from St Andrews. “We've found some peculiar differences between two young stars with identical masses, but according to standard theory of stellar evolution, both stars of such a 'twinned' star system are expected to have exactly the same age. This means that we either do not understand the details of stellar evolution, or that the individual stars have a different history. At the moment we can only guess.”

Stempels speculates that the stars could have originally formed with slightly different masses, but then their masses evened out, possibly when the stars were still accreting material from a circumstellar binary disc of dust and gas that enshrouded the pair.

The arrow points to the location of the identical twin stars in the Orion Nebula, the closest stellar nursery to Earth. The pair are in such close orbit that they appear as a single point of light. Image: Background image from NASA/JPL/STScI; foreground image from D.James, Vanderbilt University.

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The study also shows that not only did one of the stars form significantly earlier than its twin, but that the two stars differ greatly in brightness, surface temperature and surprisingly, even size. This result was achieved by trawling through 15 years’ worth of telescope observations from the Kitt Peak National Observatory in Arizona and the SMARTS telescope at the Cerro Tololo Inter-American Observatory in Chile, and plotting the differences in the amount of light recorded as the stars eclipsed each other. One of the stars is actually twice as bright as the other, with a surface temperature about 300 degrees higher than its twin. Furthermore, one of the stars is about 10 percent larger than the other.

"Our analysis of the light from these stars shows a clear difference in their temperatures and sizes, which indicates the stars have different ages,” explains Stempels. “This modifies the standard view of star formation, and also means we can no
longer assume that 'twinned' stars are born at the same time."

However, there is one similarity between the stars and that is their mass; both weigh in at masses 41 percent that of the Sun.
Stellar physicists currently believe that mass and composition determine a star’s physical appearance and dictate its entire life cycle, and the fact that the two stars condensed from the same cloud of gas and dust meant they should exhibit the same composition. The new result will force scientists to re-examine the theories of stellar evolution, and may even require astronomers to readjust their estimates of the masses and ages of thousands of other young stars less than a few million years old.

“As far as I know this is the only system of its kind discovered so far,” Stempels tells Astronomy Now. “However, it is very important that we continue to search for and identify more young eclipsing binaries, because these systems provide us with strong observable constraints on stellar evolution, which may reveal surprises similar to those we found in this system.”