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Stars' Twinkling Light Reveals Gravity Strength

Granulation Simulations for the Sun, Subgiant Star, and Giant Star
This image shows snapshots of granulation simulations for the Sun, a subgiant star, and a giant star with the stellar sizes scaled relative to each other by the square root of their true sizes. The Earth in each case is shown to scale.
(Image: © Regner Trampedach, JILA/CU Boulder, CO.)

The way a star flickers can shed light on the strength of gravity's pull on its surface, researchers say. And discovering more about a star's pull can yield key insights on its evolutionary state and on any planets that might orbit it, scientists added.

Fluctuations in the light of sun-like stars are driven by many factors, such as the presence of darker, cooler areas on its surface. This spottiness or granulation results from the way the material that makes up the stars rises and falls. The strength of the gravitational pull on the surface of that star can in turn influence how great this churning gets.

The surface gravity of a star can in principle shed light on many of its other properties, such as temperature and chemical makeup. [6 Weird Facts About Gravity]

"Once you know a star's surface gravity, then you only need one other measurement, its temperature, which is pretty easy to obtain, to determine its mass, size and other important physical properties," study author Keivan Stassun, an astrophysicist at Vanderbilt University in Nashville, said in a statement.

The surface gravity of most stars has often proven difficult to measure accurately, with estimates potentially being as much as 150 percent off. Astronomers can deduce the surface gravity of some bright stars with an uncertainty of just 2 percent by analyzing rhythmic fluctuations in their light due to sound waves zipping inside them, a strategy known as asteroseismology. However, this method only works on several hundred of the closest, brightest stars, a paltry fraction of all the stars in the sky.

"Measuring stellar surface gravities well has always been a difficult business," study author Gibor Basri at the University of California, Berkeley, said in a statement. "So it is a very pleasant surprise to find that the subtle flickering of a star's light provides a relatively easy way to do it."

The researchers discovered they could deduce the surface gravity of stars by looking at how their brightness varied by analyzing high-precision measurements of more than 150,000 stars gathered by NASA's Kepler space telescope. They compared their results with the surface gravity values of a few stars calculated independently via asteroseismology

The scientists confirmed that granulation caused flickering in stellar brightness on the scale of less than eight hours. This granulation was in turn linked with surface gravity. The lower the surface gravity, the more change in brightness was seen in the flicker, probably because of the greater amount of churning by the star's hotter, brighter matter and colder, darker material.

"It turns out you can accurately measure a key fundamental property of a star in a really straightforward and conceptually simple manner," Stassun told

This new technique apparently enables scientists to calculate the surface gravity of sun-like stars with as little as 25 percent uncertainty. Its major limitation is that it requires very high quality data taken over long time periods, but this is exactly the kind of information collected by Kepler while it was searching for regular dips in starlight caused when alien planets crossed in front of their stars.

This new technique could also help reveal clues about stellar evolution. The researchers noted that as stars got older and swelled to become red giants, their flickering became slower, probably because their matter churned more slowly.

"The flickering of these older stars is slow and it's loud," Stassun said. "This is an interesting new way to look at stellar evolution and a way to put our sun's future evolution into a grander perspective."

This method could shed light on any worlds encircling these distant stars. A common strategy for detecting and learning more about such exoplanets is to look at the effects of their gravitational pull on their stars. Calculating the strength of the star's pull can therefore help significantly improve estimates of the sizes of the hundreds of exoplanets discovered in the last 20 years. Current estimates have uncertainties ranging from 50 to 200 percent, and the improved figures for the surface gravity of stars via this new method should reduce these uncertainties by at least half, researchers said.

"This actually could be the breakthrough we've needed to pin down the sizes of hundreds more stars and exoplanets," Maria Womack, the program director at the National Science Foundation, which funded the research, said in a statement. "Getting accurate sizes is critical to measuring exoplanet density, which has been a missing puzzle piece for so many planets. So, in addition to having implications for stellar evolution, this innovative work will be invaluable for identifying hundreds of exoplanets as either rocky or gaseous."

Stassun added, "We're now working to, using our method, recalculate the surface gravitiesof all the stars that Kepler has observed planets around. We want to do a reassessment en masse of all the planets found by Kepler to more accurately measure their properties."

"We have a whole new perspective now on sizing up stars, on how they live out their lives, and all you have to do is watch the stars twinkle," Stassun said.

The scientists detailed their findings in the Aug. 22 issue of the journal Nature.

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