'Identical twin' stars are about as rare as their human counterparts. As the name suggests, these stars are thought to have been born at the same time and been made from the same materials. They are also equal in mass. It was previously assumed that identical twin stars orbiting one another as part of a binary star system were formed under such conditions. But a team led by Keivan Stassun, an astronomer at Vanderbilt University in Nashville, Tennessee, has discovered a pair of 'newborn' twin stars that aren't all that identical. The finding indicates that the 'twins' may have been born several hundred thousand years apart.

“Our discovery shows that you can't necessarily assume that two stars in a binary star system are identical in age,” says Stassun. He and his co-workers set out to find a young eclipsing binary star system in order to learn more about early stellar evolution. Such systems are hard to find, because they comprise roughly one in every thousand stars. Taking their measurements is also tricky: astronomers can measure properties such as mass, surface temperature, brightness and size only when one of the stars passes directly in front of the other as they swirl in orbit. “You have to stare at thousands of stars and wait for just a couple of them to wink as they eclipse one another,” says Stassun.

The team collected spectra and snapped images of two sibling stars, dubbed Par 1802, within the Orion Nebula, a giant stellar 'nursery'. The stars within the nebula are about a million years old — mere infants for stars, which have a lifetime of tens of billions of years. Although the two stars are almost identical in mass, which qualifies them as identical twins, one is significantly warmer and brighter than the other, and also potentially larger in diameter (see page 1079).

The physical differences between the stars seem to stem from their birth order, says Stassun, who attributes the stars' dissimilarities to a nature-versus-nurture influence similar to that involved in human development. “Nature dictated their birth order — although we have no idea how — and then the physical differences that we see all followed directly from that,” he says. However, the interstellar cloud of gas and dust in which stars form may have fed — or nurtured — these stars equally so that they were born with the same birth weight.

On the basis of models used to predict stellar growth curves, the physical differences between the siblings indicate that the brighter, warmer star is a few hundred thousand years behind its companion in terms of development. Stassun likens this to a half-day difference in age in day-old human twins. When the stars reach maturity in about a hundred million years, the difference in their ages will become almost impossible to detect, he says. “It's only because we caught this pair so early in their lives that we're able to see the manifestations of one being slightly further along than the other.”

But how the twin stars could be born at different times remains largely speculative at this point. “Our theoretical understanding of how binary systems like this one form has a lot of gaps in it,” says Stassun, who is nurturing the careers of some young scientists from minority backgrounds, enlisting them to help collect data from telescopes in Chile for this work. He is optimistic that a closer look at these young sibling stars might reveal how early star systems evolved.