Unexpected curves Kepler tracks changes in the brightness of light emitted by stars over
time, generating what are called light curves. Telltale dips in a star’s light curve offer evidence that
a planet is orbiting and passing in front of the star. But binary star systems, nonplanetary bodies
and internal pulsations can also create signature light curves (as shown for the sources below).
forest of secondary brightness variations continued at a lower level in
between the dramatic brightness peaks.
One hypothesis was that the star is
orbited by a black hole, its strong gravity
lensing the star’s light into a beam that
briefly aims at Earth once every orbit.
A platoon of astronomers got time on
NASA’s Swift telescope to seek signs of
a black hole’s telltale X-rays, but no luck
there. A spectroscopic examination with
half a dozen ground-based telescopes
solved the puzzle.
HD 187091, now known as KOI-54 for
“Kepler Object of Interest No. 54,” is not
one A-star but two of nearly the same size,
one 2. 19 and the other 2.33 times the sun’s
1.39-million-kilometer diameter. They are
in a wildly stretched-out, elliptical orbit.
Every 41 day, 19 hour orbit sends them
racing nearly toward each other and zipping around one another at a separation
equal to only about three times each one’s
diameter, and then flings them almost
120 million kilometers apart.
The brightening occurs as the stars,
tidally warped by their gravity at closest
approach into slight egg shapes, roast
one another on their facing sides and
heat up. And that explains the spike in
brightness, the team reported online
in February at arXiv.org. The more surprising revelation of Kepler’s data is that
one, and perhaps both, pulsate furiously
at rates that are precise multiples of
their rate of close encounters, in some
cases pulsing exactly 90 and 91 times for
each orbit. “Nobody had ever seen, or
even thought, something like this could
happen,” Welsh says. Discovering that
a star’s rapid pulsations are not always
driven by internal processes, but can be
paced by a tidal metronome from a partner star, offers a new window into stellar
dynamics and structure.
Relative brightness Relative brightness Relative brightness
20 40 60 80 Days
Amateur astronomer Kevin Apps helped researchers discover that LHS 6343 is actually a pair
of stars. Abrupt dips in light (thin vertical lines) are caused by an orbiting brown dwarf.
20 40 60 80 Days
Kepler data have revealed a star that behaves in a way unlike anything astronomers have seen
before, pulsating on two axes and exhibiting torsional modes in its rotation.
SOURCE: M. STILL
20 40 60 80 Days
KOI-54 is actually two stars of roughly the same size that circle each other in an elliptical orbit.
The two stars heat up when they face each other, explaining the brightness spikes.
In the next year or two, and longer if the
Kepler mission is extended, myriad more
discoveries like these are likely. Eventually the craft will go out of commission; at
the outside it could keep working for four
or five years, says deputy Kepler project
scientist Steve Howell, also at Ames.
But the telescope’s abilities may be
surpassed by another in the works. The
European Space Agency is considering a
mission called Plato, for Planetary Transits and Oscillations of stars. It could go up
sometime between 2015 and 2025 with a
prime mission much like Kepler’s — to
study stars with planetary systems. The
craft is to be parked in a libration point,
a sort of kink in the sun’s and Earth’s
combined gravity that will hold it hovering near Earth but far enough away to be
able to stare constantly for long periods
at one region.
But unlike Kepler, Plato will be able to
swivel around and look at different parts
of the sky, and will use bundles of telescopes to stare at larger pieces with even
greater precision. Undoubtedly it, too,
will find yet more stars whose wonders
have nothing to do with planets. Who
knows: It may even be able to check on
some that Kepler found first, just to see
how they’re doing. s
s Visit Kepler’s guest observer website:
June 4, 2011 | SCIENCE NEWS | 21