The surface gravity of a star is a fundamental property, one that depends on the mass and size of the star. But it's also an indication of what's going on in the star's outer layers: higher gravity results in smaller changes in luminosity on a fast time scale. In that way, accurate gravitational measurements reveal much about the processes by which stars evolve—if we can obtain them.

A new analysis of data collected by the Kepler observatory indicated that small fluctuations in a star's brightness are strongly correlated with the surface gravity. Fabienne Bastien, Keivan Stassun, Gibor Basri, and Joshua Pepper compared existing surface gravity estimates with the short-period variations in the same stars' brightness and found a simple relationship. Turning that relationship around, the researchers argue that it should be possible to determine the gravity from the stars' flickering alone, allowing astronomers to determine the surface gravity of stars that are too distant to measure otherwise. This method could vastly increase the toolkit for studying the structure and evolution of stars.

The Sun is necessarily the only star for which we have close-up detailed observations. However, astronomers have learned to identify behaviors in other stars that are similar those on the Sun: starspots that trace magnetic activity, sound waves akin to earthquakes, and small fluctuations in light due to granulation, the bubbles that form and fade over the course of hours.

The study of sound waves in stars is called asteroseismology. (For the Sun, it's known as helioseismology.) These sound waves are more regular than earthquakes, since stars are more uniform than our lumpy Earth. Additionally, their behavior is dictated by the star's gravity; when astronomers can obtain asteroseismic data, they can estimate the star's surface gravity to about 2 percent accuracy. The problem is that these observations require bright stars, which are mostly larger and/or more massive than the Sun. As a result, data isn't available for the vast majority of stars.

Enter the Kepler mission. While Kepler is an observatory designed primarily to hunt for exoplanets, it has proven to be equally valuable for stellar astrophysics. Until its second reaction wheel failure, it monitored more than 150,000 stars for three years, providing a wealth of data on their day-to-day light fluctuations and activity.

The authors of the present study examined variations in the light of individual stars on time scales less than 8 hours. On the Sun, this variation is due to granulation: hot plasma rises from the solar interior, cools, then sinks, resulting in the distinctive bubble-like structures visible in the graphic above. Starspots and other magnetic phenomena produce greater variations in light, but they vary on much longer time scales. Thus, much of the relatively short variations in light from stars in the Kepler data must be due to granulation.

The researchers then compared the granulation activity to the surface gravity obtained via asteroseismology for the stars where such data was available. They found the granulation data strongly tracked the sound-wave measurements, allowing an independent estimate of surface gravity. The accuracy using light fluctuations is lower—25 percent compared with 2 percent from asteroseismology—but it's a vast improvement over other estimates, which can be off by more than 100 percent.

The physical explanation for the relationship is quite straightforward: higher surface gravity in younger, more compact stars yields shorter time scales for granulation, while older giant stars are more sedate, with larger granules.

The surface gravity is a fundamental characteristic of a star, relating to its mass—which changes little over most of the star's lifetime—and its size, which dictates how far the surface is from the center of gravity. A star shrinks and grows as it ages, changing its surface characteristics. The surface gravity therefore is a means to measuring the evolution of stars. Using granulation should greatly increase the number of stars for which surface gravity estimates are available, providing more data for understanding how stars age, individually and as a population.

Nature, 2013. DOI: 10.1038/nature12419 (About DOIs).