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To determine stars' physical traits, Kepler sees the light

Brightness measurements can help gauge size, stage of evolution

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The key to learning about distant stars is as simple as watching them flicker.

Stellar brightness measurements from NASA’s planet-hunting Kepler space telescope can pin down the once-elusive physical properties of about 170,000 stars and the planets orbiting them, scientists report in the Aug. 22 Nature. The new technique can translate modest fluctuations in starlight over several hours into a star’s size, surface gravity and stage of life.

“All this information is encoded in a star’s brightness in such a wonderfully simple way,” says Vanderbilt astrophysicist and study coauthor Keivan Stassun.

For four years, the now-moribund Kepler (SN Online: 8/15/13) stared at the stars within a patch of sky, looking for eclipsing planets. Kepler homed in on several hundred for meticulous scrutiny. By studying subtle changes in the brightness of these select stars, astronomers have been able to tease out vibrations in the stars’ interiors — analogous to seismic waves that cause earthquakes on Earth — and precisely determine a slate of physical properties for each star.

In contrast, astronomers have only rough estimates of the size, mass and other attributes of most other Kepler stars, and stars in general. “We know [the estimates] are not very good,” says Ronald Gilliland, an astronomer at Penn State University in University Park, Pa.

One of the authors of the study, Fabienne Bastien, an astrophysicist at Vanderbilt University in Nashville, was sifting through the brightness data of the carefully studied stars when she noticed a clear-cut connection: The more a star flickered over the course of several hours, the weaker its surface gravity was. Surface gravity reveals a lot about a star because it decreases as the star ages and swells up. “It tells you how evolved the star is and how big it should be,” Bastien says.

While no one had noticed the connection between flickering and surface gravity before, Stassun says it makes sense. The surfaces of stars consist of giant pockets called granules that form when hot gas rises and cooler gas sinks. Some granules are hotter and brighter than others, and their temperatures can change over the course of minutes. As a star swells up later in its life, the volatile granules also increase in size, leading to greater variations in the overall surface brightness.

Bastien teamed up with Stassun and two colleagues to create a scale of stellar flicker that can be applied to any of the stars in Kepler’s field of view. By simply knowing how much a star’s brightness fluctuates over an eight-hour period, the researchers say, astronomers can determine the surface gravity of the star with an uncertainty of only about 20 percent. The size measurements of most Kepler stars have uncertainties of more than 90 percent.

The quick, easy scale will also be a valuable tool for astronomers using Kepler for its main purpose: finding and describing planets, Gilliland says. The magnitude of the shadow cast by an eclipsing planet reveals only the ratio of a planet’s size to that of its star. To determine whether a planet is a giant, a runt or something in between, astronomers first need to know the size of the star. Bastien says her team’s new scale will allow astronomers to attain a reliable estimate quickly without having to resort to time-consuming, expensive observations with ground-based telescopes.

Astronomers with the Transiting Exoplanet Survey Satellite could also use the technique once the spacecraft starts hunting planets around another set of stars in 2017.

Stassun emphasizes that the usefulness of this technique goes beyond planet hunting. Now, from the Kepler catalog, astronomers can pick out a star and almost immediately determine whether it is small and middle-aged like the sun, or bloated and on its last legs.
