Astronomers have long assumed binary stars, each with the same mass and in orbit around each other, form simultaneously and, therefore, are identical twins. But new evidence gained from twin stars in the Orion Nebula 1,500 light years from Earth indicates they exhibit significant differences in brightness, surface temperature and possibly even size.

The new data, said scientists, suggest one of the stars formed significantly earlier than its twin -- a discovery that might cause astronomers to readjust their estimates for thousands of young stars.

Scientists said the newly formed twin 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.

"The easiest way to explain the observed differences is if one star was fully formed about 500,000 years before its twin," said Keivan Stassun, an associate professor of astronomy at Vanderbilt University.

The discovery by Stassun and Robert Mathieu from the University of Wisconsin-Madison appears in the journal Nature.