Twin Stars Born 500,000 Years Apart
The two identical twin stars (inset) called Par 1802 appear as a single point of light, because they are so close to each other in the Orion Nebula (background).
Credit: NASA-JPL-STScI/David James.

Bouncing baby stars considered identical twins were oddly born 500,000 years apart, a new study finds.

The newly discovered star pair is located in the Orion nebula, a nearby "maternity ward" bustling with stellar-birth activity and located 1,500 light-years from Earth. One light year is the distance light will travel in a year, or about 6 trillion miles (10 trillion kilometers).

The astronomers who discovered the binary found that the stars show identical masses and compositions, elevating the pair to identical twin status. However, their relative brightness and other physical features differ, suggesting one star in Par 1802 formed earlier than the other.

Until now, astrophysicists had assumed binary stars form at about the same time. And so the discovery of these not-so-similar twin stars, detailed in the June 19 issue of the journal Nature, puts a new wrinkle in star formation theories.

Winking baby stars

Astronomers Keivan Stassun of Vanderbilt University and Robert Mathieu of the University of Wisconsin-Madison headed up the study.

They sifted through 15 years' worth of observations of thousands of stars, looking for stellar winks, which suggest a star has just eclipsed its partner. In eclipsing binaries, the two stars as viewed from Earth revolve around an axis that is edge-on to us. Periodically, the stars eclipse, or pass in front of, each other.

Sure enough, they saw a dip in light where the new eclipsing binary resides.

Both stars are about 41 percent that of the sun's mass. Each star is about 1.7 to 1.8 times the size of the sun, though the astronomers estimate one could be about 10 percent larger than the other.

The observations come from the Kitt Peak National Observatory in Arizona and the SMARTS telescopes at the Cerro Tololo Inter-American Observatory in Chile.

Stellar genetics

Like humans' genetic material, mass and composition for stars can be akin to destiny, according to Stassun. Human identical twins, which come from the same egg, carry matching DNA and everything that comes with it, including spitting-image looks.

And that's what astronomers would've assumed about two same-mass stars orbiting each other.

"The mass of a star is the physical characteristic that, more than anything else, determines how the star will go through its life. Mass is destiny for a star," Stassun told SPACE.com. "If you have two stars with the same mass, their destinies ought to be the same. Or so we thought."

Stassun and Mathieu found some glaring differences. One star is twice as bright as its sister. This showy twin also has a surface temperature that's about 300 degrees higher than its twin's.

Birth order

These features make sense with a birth-order scenario suggested by the astronomers.

Here's how they think it works:

Within Orion's stellar nursery, cloudlets of gas and dust are slowly collapsing in under their own weight, i.e. celestial conception. As the clumps of gas and dust condense and get smaller and smaller, they heat up until eventually they light up as full-fledged stars.

Most cloudlets pop out newborn stars at roughly the same time, Stassun said. But for the newly discovered twins, one star likely emerged roughly a half million year before its sibling.

That means the elder star would be slightly ahead of its sibling in the star-formation process and hence would have shed some of its heat and contracted more, explaining why one star (the elder) has a smaller size, lower temperature and glows fainter than the younger one. Since the older star has contracted more, it just crams the same mass into a smaller package.

"Our best interpretation at this point is that the reason we're seeing these physical differences is because there was a birth order between the twins," Stassun said. "One of them was born a little before the other one."

Stassun said they aren't sure "what was going on when the two stars were still in the womb; was one being fed more than the other?"

Astronomers speculate stars tend to come in pairs, though eclipsing binaries are less common. And finding identical twins eclipsing, to boot, is like spotting that needle in a haystack, Stassun said.

The study was funded by the National Science Foundation and a Cottrell Scholar award from the Research Corporation.

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