Measuring Stellar Gravity
August 23, 2013

A New Way To Measure A Star’s Surface Gravity

[ Watch The Video: Star Flickering Discovery ]

John P. Millis, PhD for - Your Universe Online

A star’s gravitational strength is key in understanding and modeling other properties of a particular star, to say nothing of understanding the planets that orbit around it. But making accurate measurements of a star’s surface gravity is difficult, and measurements are often only good to within about a 50% estimation of the actual value, sometimes worse.

Now, however, researchers at Vanderbilt University, led by physics and astronomy professor Keivan Stassun, have found a clever, simple way to estimate this important metric much more accurately. “Measuring stellar surface gravities well has always been a difficult business,” reports Gibor Basri, professor of astronomy at the University of California, Berkeley who contributed to the study. “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 simplest method, known as photometric analysis, uses color data from stars to infer gravitational strength. It is applicable to virtually any star, but yields high uncertainty. Other procedures, such as spectroscopic techniques and asteroseismology, are far more accurate – asteroseismology is good to within a few percent – but require such detailed data and are only technically possible on the closest and brightest stars.

By contrast, the new “flicker” method analyzes the fluctuations of a star’s brightness over a very short time period – some eight hours or less. The tiny variations arise from granulation – the flow of hot gas and plasma across the stellar surface – which itself is tied to the gravitational strength of the star. The greater the surface gravity the finer the granulation, leading to a higher frequency of brightness flicker.

While not quite as precise as the asteroseismology method, this new technique can be applied to virtually any observable star. There is still one challenge however; the technique requires very high quality data taken over long periods of time. Luckily, such data is readily available, and still being acquired. NASA’s Kepler mission has already catalogued tens of thousands of stars, the data from which can be fed directly into the flicker method.

[ Watch The Video: Sound To Light To Sound ]

“The exquisite precision of the data from Kepler allows us to monitor the churning and waves on the surfaces of stars,” said team member Joshua Pepper, assistant professor of physics at Lehigh University. “This behavior causes subtle changes to a star’s brightness on the time scale of a few hours and tells us in great detail how far along these stars are in their evolutionary lifetimes.”

One of the interesting stories behind this discovery is that the key correlation between brightness variation and surface gravity was discovered by Vanderbilt University graduate student Fabienne Bastien while she was “playing around” with a special data visualization tool developed by her research group to analyze Kepler data.

"I was plotting various parameters looking for something that correlated with the strength of stars' magnetic fields," said Bastien. "I didn't find it, but I did find an interesting correlation between certain flicker patterns and stellar gravity."

The power of this discovery is still being realized, as scientists are beginning to see it as a way of predicting the evolution of stars. Soon, the new flicker method could be employed as an important tool in stellar astronomy.