Identical twin stars reveal surprising differences

Wednesday 18 June 2008

The arrow points to the location of the identical twin stars in the Orion Nebula, the stellar nursery that is closest to Earth. The pair are in such a close orbit that they appear as a single point of light. (Credit: NASA/JPL, HST, David James)

The youngest identical twin stars discovered to date are not identical at all, according to scientists.

The new study of a pair of stars discovered nestling in a stellar ‘nursery’ revealed such surprising differences that astronomers will need to re-examine the ways in which stars form.

An astronomer from the University of St Andrews was part of the international research project that suggests not only that one of the stars formed significantly earlier than its twin, but that the two stars differ significantly in brightness, surface temperature and possibly even size.

The study was carried out by St Andrews researcher Eric Stempels alongside project leader Keivan Stassun of Vanderbilt University and Robert D. Mathieu from the University of Wisconsin-Madison.

Eric Stempels, from the University’s School of Physics & Astronomy, said, “We’ve found some peculiar differences in a system of two young stars with identical masses (a stellar ‘twin’). According to standard theory of stellar evolution both stars of such a ‘twinned’ star system are expected to have exactly the same age.

“Our analysis of the light from these stars shows a clear difference in their temperatures and sizes, which indicates the stars have different ages. 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.”

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 – the equivalent of one-day-old human babies – with a full lifespan of about fifty billion years.

The new report suggests that one star was formed about 500,000 years before its twin, the equivalent to a human baby being born half of a day before its twin.  Until now, astrophysicists have assumed that twins, known as ‘binary’ stars, form simultaneously.

The research team sifted 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. By measuring the difference in the amount that the light dipped during the stars’ eclipses, the astronomers were able to determine that one of the stars is twice as bright as the other and 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.

However, the study did confirm that the twins were of a similar mass.  Using Newton’s laws of motion they found that the twins have nearly identical masses, around 41 percent 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.

Eric concluded, “We were surprised to discover that the twins exhibited significant differences because having the same mass and composition they should be identical in every other way.”

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

The study is published in the journal Nature.



Eric Stempels is available for interview on mobile 07783 525 925 or email [email protected]


A multimedia version of this story is available on Exploration, Vanderbilt University’s online research magazine at

Issued by the Press Office, University of St Andrews
Contact Fiona Armstrong, Press Officer on 01334 462530 / 467227, mobile 07730 415 015 or email [email protected]
Ref:  Twin Stars 17/06/08
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