Star's Bubbly Surface Reveals Cosmic Cooking Below

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Staring at the seething surface of a star is providing
astronomers with clues as to its interior. The bubbling of the
outer layers reveals a great deal about what's going on
underneath.

Delta Scuti is a special class of
pulsating stars that are twice the size of the sun.
Astronomers have long known about the primary force behind their
variations, called the kappa mechanism, in which ionization
regions deep in the star store energy and release it as a valve.

However, new research reveals that these pulsing stars also
utilize a second, sunlike option to brighten and dim, an option
that can help determine more about what's happening in the unseen
middle of the star.

Using NASA's Kepler
space observatory, a team of international astronomers was
able to examine the star — known as HD 187547 — for 30 days. They
concluded that, despite its larger size, the convective forces at
work in Earth's nearest star still function on more massive
bodies, and even help drive their changes in brightness.
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"In the earlier days, astronomers measured variable stars by
looking through telescopes and writing down the intensity,
comparing it to standard stars, but those stars had large
brightness changes," Victoria Antoci, of the University of Vienna
in Austria, told SPACE.com in an email.

The changes in brightness for HD 187547 were almost one-millionth
that of what could be seen by the naked eye.

"Only measurements from space telescopes such as Kepler are
capable of providing the needed accuracy," Antoci said.

Stars like the sun pulsate because of convection at their
surface. As gas is heated, it bubbles up from beneath, where it
expands, causing the star to very slightly brighten. This
expansion cools the gas, which sinks back down, and the star dims
again.

Gerald Handler, of the Nicolaus Copernicus Astronomical Center in
Warsaw, Poland, compared the process to a pot of boiling water.

"When the water is deep, you have bubbles — or convection cells —
rising rather powerfully, and the whole pot can shake," he
told SPACE.com via email.

Handler worked with Antoci on the paper, which was published
online in today's (Sept. 14) edition of the journal Nature.

But astronomers weren't sure they would see the expected
brightness fluctuations on Delta Scuti stars, which can be as
much as four times the radius of the sun. The convection zone on
these stars is significantly smaller — only 1 percent of their
radius, compared to zones almost one-third of the radius of
sunlike stars.

On HD 187547, that's exactly what is happening.

"We have now discovered the first star that shows both types of
oscillation at the same time, slow and fast," said Handler.