Asteroseismologists delving into the Kepler mission’s data have found a star that appears
to be more spherical than any natural object.
Astronomers were excited to now find a star
that’s round to within 0.0002% — the authors called it “the most spherical natural object ever measured.”
A Little More Than Just Points of Light
For most astronomers, stars are little more than points of light. Truly seeing a star and resolving its
shape is a impressive feat — stars may be enormous, but they’re also incredibly far away.
Nevertheless, it’s been done: astronomers using infrared interferometry managed to directly
image the shapes of Altair and Vega, both rapidly rotating stars. (Altair has a significant bulge of 14%,
while Vega turned out to be spinning with its pole pointed at Earth, making its oblateness too difficult to measure.)
Laurent Gizon (Max Planck Institute for Solar System Research and Georg-August University,
both in Göttingen, Germany; National Astronomical Observatory of Japan, Tokyo; and New York University Abu Dhabi,
United Arab Emirates) and colleagues tried a different tack on a star with the
call sign KIC 11145123 in the primary Kepler mission’s field of view.
“This is in many regards a very surprising star,” says Jorgen Christensen-Dalsgaard (Aarhus University, Denmark),
who was not involved in the study. As an A-class star, it’s more than twice as wide as the Sun. But unlike fellow
A stars Altair and Vega, it spins slowly, completing a rotation every 100 days, compared to the Sun’s roughly
26-day equatorial period. The slow rotation may be a sign of its age.
Rather than image this star directly, Gizon’s team followed its variations in brightness over four years,
watching the star expand and contract ever so slightly as sound waves rang through its interior.
The study of those celestial sounds is known as asteroseismology. That field really took off when Kepler
began monitoring thousands of ordinary stars with exquisite precision over the four years of the spacecraft’s primary mission.
Is It Really the Roundest Object?
Is KIC 11145123 really the roundest celestial object we’ve ever measured?
If we’re counting solid objects, then we shouldn’t forget Mercury and Venus,
both of which are spherical to within one-tenth of a kilometer due to their slow rotations.
Unfortunately, NASA’s measurements are one significant figure short of the precision needed to call the contest.
The planets’ place at the round table will have to await future planetary missions.
If we limit the comparison to other stars, then the answer — for now, at least — becomes more straightforward:
yes, this is the roundest star known. Don’t expect it to stay that way though.
The Kepler data have been combed pretty thoroughly for transiting exoplanets,
but thousands of transitless stars are a treasure trove waiting for curious asteroseismologists.
“Since we do not fully understand this star it is hard to predict whether
it is almost unique or whether other examples could be found in the galaxy,” Christensen-Dalsgaard.
“There is certainly potential for further discoveries of a similar nature as we continue the work on these remarkable data.”