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Orion twins show not all are identical

orion nebula

The Orion Nebula and its twin stars, which appear as a single point of light because they are too close to resolve (Source: NASA-JPL-STScI/David James)

A pair of stars in the Orion nebula have shown astronomers that stars born at the same time, in the same parent cloud, don't necessarily have the same physical attributes.

The twins, known as Par 1802, reveal one is developmentally delayed compared to its partner.

"Par 1802 provides direct evidence that birth order in 'identical twin' stars can manifest itself as observable physical differences between the two stars - at least when they are very young," says Vanderbilt University astronomer Professor Keivan Stassun in the journal Nature.

Stassun and colleagues found that the twins' surface temperatures differ by about 300 Kelvin, and that one star is 50% brighter than its sibling. The scientists also strongly suspect one star is up to 10% bigger than the other.

"The easiest way to explain these differences is if one star was formed about 500,000 years before its twin," Stassun says.

"That is equivalent to a human birth-order difference of about half of a day."

Rosetta stones

The discovery is more than a curiosity: Par 1802 is an eclipsing binary star system, meaning the twins periodically orbit in front of one another, relative to earth, temporarily and regularly blocking its partner's light.

Such phenomena are yardsticks for measuring stars' basic physical properties and for testing theories about stellar evolution.

"Very young eclipsing binaries like this are the Rosetta stones that tell us about the life history of newly formed stars," Stassun says.

The finding may mean that thousands of young stars may need to have their ages readjusted by as much as 20% for average-sized stars and 50% for low-mass stars like brown dwarfs.

"The lack of age synchronisation in Par 1802 suggests a precision limit of several hundred thousand years," the scientists say.

Very rare

Eclipsing binaries are very rare - only about one in 1000 stars.

The researchers made the discovery after 15 years of collecting data on stars in Orion, during which time they found three eclipsing binary pairs.

Par 1802 is the only one of the three binary pairs with equal-mass stars, and it is the only eclipsing binary ever found anywhere with same-mass stars young enough - less than a million years old - for physical differences to still be apparent, Stassun says.

"We were pretty surprised when we first discovered the differences in temperature, brightness and diameter, but in truth maybe we should not have been," Stassun says.

"We naively expected stars born at the same time, with the same mass and of the same stuff to look the same, but our current theories of binary star formation don't actually make predictions.

"This is really an important new piece of information."

Tags: astronomy-space, the-universe