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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