Singing stars – how astronomers turn starlight into sound
Fluttering, sizzling hiss, alien music. These are the sounds the stars make.
NASA’s Kepler space telescope observed 150,000 stars looking for telltale dips in their light that would indicate a planet cycling in front of a star. Knowing the distance and diameter of the star and the length of time the passing object dimmed the star’s light, astronomers can determine the planet’s size and mass.
Using the ‘transit method’, scientists have mined Kepler data to uncover 4,229 candidate extra-solar planets 981 of which are confirmed. Finding planets, which are typically much smaller than their host stars, is no easy business. Any noise in the data can hide the weak change in light caused by an orbiting object. But sometimes the noise itself can yield useful new information.
Star brightness is not constant, especially if you look closely over short intervals of time. Starspots (stellar versions of sunspots), flares and even the bubbles of heated gas called rising from the star’s hotter interior to the surface cause subtle changes in its brightness.
The last is called granulation and gives the sun’s surface a grainy or cellular texture. Plumes of rising gas are hot and bright but soon darken, cool and sink back down only to be re-heated and rise again. Seen in sped-up time, they cause a star to flicker.
In a recent paper that appeared in Nature, Fabien Bastienne of Vanderbilt University in Nashville and team analyzed the noise from the Kepler data and discovered that brightness changes under 8 hours directly relate to a sun-like star’s surface gravity. Smaller stars with higher surface gravity have less granulation than big ones with less gravity.
Turning sound into light and back again. How it’s done
Now here’s the beautiful thing. You can take these slight but continuous changes in a star’s brightness and convert them into sound. The accompanying video shows how an audio signal from a pocket radio, for instance, can be turned into light and then transformed back into sound using little more than an LED and solar cell.
With Kepler, the brightness variations were sped up and converted into sound, giving stars a ‘voice’. Starspots create the fluttery sounds, granulation the hiss. Small stars flutter a lot; red giants are big hissers.
Being a dwarf star in the grand scheme of things, the sun flutters much like the dwarf star in the video. To my ear, it sounds ominous, like background music for a dystopian sci-fi flick. Rotation speed also causes variations in sound.
Scientists use these curious hisses and flutters to help nail down a star’s size, mass and stage of its evolution. The more precisely we know those details, the more precisely we also know a transiting planet’s size and mass.
Motto of the story: Don’t throw away your garbage. There’s valuable data in there!