Flicker… A Bright New Method of Measuring Stellar Surface Gravity

A simple, yet elegant method of measuring the surface gravity of a star has just been discovered. These computations are important because they reveal stellar physical properties and evolutionary state – and that’s not all. The technique works equally well for estimating the size of hundreds of exoplanets. Developed by a team of astronomers and headed by Vanderbilt Professor of Physics and Astronomy, Keivan Stassun, this new technique measures a star’s “flicker”.

With an uncertainty ranging from 50 percent to 200 percent, astronomers have been eager to seize on a new way of measuring a star’s surface gravity which will level the playing field. By obtaining improved figures for a wide variety of stars at varied distances, this new method might be able to cut the uncertainty figure in half.

“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,” said Stassun.

“Measuring stellar surface gravities well has always been a difficult business,” added 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.”

Just how do we currently go about measuring stellar surface gravity? Up until now, astronomers relied upon three methods: photometric, spectroscopic and asteroseismic. This new way of measuring, known as the “flicker method”, is much more simplistic than previous ways and is actually more accurate than two of them. Let’s take a look at all three currently accepted methods…

For photometry, one looks at how brightly a star shines in various colors. Like a graph, these patterns reveal chemical composition, temperature and surface gravity. Able to be used on faint stars, the photometric data is easy to observe, but it’s not very accurate. It ranges with an uncertainty of 90 to 150 percent. Similar to photometric observations, the spectroscopic technique takes a look at color, but a much closer look at the elemental emissions of the stellar atmosphere. While it has a lower uncertainty rate of 25 to 50 percent, it is limited to brighter stars. Like a bar code, it measures surface gravity by how wide the spectral lines appear: high gravity is spread apart, while lower gravity is narrow. In asteroseismology, the accuracy sharpens to just a few percent, but the measurements are difficult to obtain and are limited to bright, nearby stars. In this technique, sound traveling through the stellar interior is measured and specific frequencies associated with surface gravity are pinpointed. Giant stars naturally pulse at a low pitch while small stars reverberate at a higher one. Imagine the gong of a large bell as opposed to the jingle of a small one.

So, what is flicker? In the flicker method, the star’s differences in brightness are measured – specifically the variations which occur in eight or less hours. These variations would seem to be tied to surface granulation, the interconnection of “cells” covering the stellar surface. These regions are formed by columns of gas rising from below. For stars which have a high surface gravity, the granulation appears to be finer and they flicker more rapidly, while stars with a low surface gravity display coarse granulation and flicker slowly. Recording flicker is simple process, one which only involves five lines of computer code to create a basic measurement. Thanks to its ease and simplicity, it reduces not only the expense of obtaining data, but also eliminates a great deal of the effort necessary to measure the surface gravity of a large number of stars.

“The spectroscopic methods are like surgery. The analysis is meticulous and involved and very fine-grained,” said Stassun. “Flicker is more like ultrasound. You just run the probe around the surface and you see what you need to see. But its diagnostic power – at least for the purpose of measuring gravity – is as good if not better.”

Is the flicker method accurate? By placing measurements side-by-side with asteroseismology, researchers have determined it to have an uncertainty factor of less than 25 percent – better than both spectroscopic and photometric results. Its only bad feature is that it demands exacting data taken over long time periods. However, a specialty instrument, Kepler, has already provided a vast amount of information that can be recycled. Thanks to its tens of thousands of observations of stars monitored for exoplanets, Kepler data is readily available to future flicker examinations.

“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.”

Just how was flicker discovered? Graduate student Fabienne Bastien was the first to notice something a bit different while using special visualization software to examine Kepler data. This software, developed by Vanderbilt astronomers, was originally intended for investigating large, multi-dimensional astronomy datasets. (The data visualization tool that enabled this discovery, called Filtergraph, is free to the public.)

“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.”

Bastien then reported her discovery to Stassun. Equally curious, the pair then decided to try out the new method on archived Kepler light curves of several hundred sun-like stars. According to the news release, when they mapped out the average brightness of any particular star against its flicker intensity, they noticed a pattern. “As stars age, their overall variation falls gradually to a minimum. This is easily understood because the rate at which a star spins decreases gradually over time. As stars approach this minimum, their flicker begins to grow in complexity – a characteristic that the astronomers have labeled “crackle.” Once they reach this point, which they call the flicker floor, the stars appear to maintain this low level of variability for the rest of their lives, though it does appear to grow again as the stars approach the ends of their lives as red giant stars.”

“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,” said Stassun.

So what is our Sun’s future according to flicker? When the researchers sampled the Sun’s light curve, they found it “hovering just above the flicker floor”. This measurement leads them to hypothesize that Sol will transform to a “state of minimum variability and, in the process, will lose its spots.” Could this be why we don’t see as much activity as expected during current solar maximum time, or is this just a new theory where it’s too early to make any assumptions? We’ll call your flicker and raise you two spots…

Original Story Source: Vanderbilt News Release.