Stellar Noise,
Stellar Sounds

NASA’s Kepler space telescope has stopped its initial run of collecting data in its search for new planets, but that doesn’t mean there will be no more new discoveries from that data. A good case in point can be seen from a recent article in Nature that demonstrates a clever way to measure the surface gravity of a star.¹

In order to discover planets, Kepler was pointed at a particular patch of sky and measured the brightness of more than 150,000 stars over long periods of time. When a planet passes in front of a star (from our vantage point), the star will dim measurably. Astronomers have been combing through the Kepler data looking for periodic dips in brightness that indicate a transiting planet. So far we’ve found over 2,700 candidate planets in the data.

Finding evidence of planets from the data is a bit tricky because the data has a lot of variation (called noise) within it. The brightness of a star by itself is not constant. It fluctuates slightly all the time because of things like starspots (sunspots on the star), flares, and even the convective motion of material near the surface of the star, which produces a kind of bubbling porridge effect called granulation.

When searching for planets, all of these fluctuations just get in the way, so you find ways to filter it out to find the planetary signals. But the team behind this article analyzed the noise itself and found it can tell us about the star itself. Since starspots move basically with the rotation of the star, the brightness variations due to the starspots tells us about the rotational speed of the star. The level of granulation is related to the surface gravity of a star, since smaller stars with a higher surface gravity have less granulation than larger stars with a lower surface gravity.

We can analyze this data to determine these properties, but you can get a good idea of how this works by converting the data to sound. In the video above, the brightness variations of the star were sped up and converted to sounds. As a result, the starspot variations sound like low warbles, while the granulation sounds like a static hiss.

These types of observations are useful, because the information we can obtain about a transiting planet is related to the star itself. For example, if we see a star dim by a certain fraction, we know the size of the planet is a certain fraction of the star. But without knowing the size of the star we can’t pin down the planet’s size. So by using the “noise” of the observations to determine the star’s size and mass, we also gain a better measure of the planet’s size and mass.

Since we already have this data from the Kepler mission, we can analyze the noise to further refine our planetary knowledge. Pulling more knowledge from the data we already have.