Orion's Twin Stars Have Their Differences

Orion's Twin Stars Have Their Differences
Irene Klotz, Discovery News

June 18, 2008 -- Astronomers presumed that stars born at the same time, in the same parent cloud, and with the same mass would, like identical human twins, have the same physical attributes.

So it was a surprise to discover that a pair of twins in the Orion nebula, a popular stellar nursery about 1,500 light-years away, have different temperatures and luminosities. One star is apparently developmentally delayed, relative to its partner.

The twins are known as Par 1802.

"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," writes Vanderbilt University astronomer Keivan Stassun in this week's issue of the journal Nature.

Stassun and colleagues found that the twins' surface temperatures differ by about 10 percent, or 300 degrees Kelvin (80 degrees Fahrenheit) and that one star is 50 percent brighter than its sibling. The scientists also strongly suspect one star is up to 10 percent 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 said. "That is equivalent to a human birth-order difference of about half of a day."

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 said.

Thousands of young stars may need to have their ages re-adjusted by as much as 20 percent for average-sized stars and 50 percent for low-mass stars like brown dwarfs, the scientists estimated.

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

Eclipsing binaries are very rare -- only about one in 1,000 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 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 told Discovery News.

"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 said.

"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," he said. "This is really an important new piece of information."

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