full article in PLASMA 6
Jim Fuller is a professor of theoretical astrophysics as CalTech. Jim studies the physics of ‘vibrating fluid spheres’ and is currently applying this knowledge to studying plasma movements in stars. Internal gravity waves in stars cause constant vibrations, which can be detected through brightness changes. These vibrations can be sonified and turned into sounds! We spoke to Jim about his research and how ‘listening’ to the stars is helping researchers determine the composition and magnetic fields:
PLASMA: Could you tell us a bit more about your research?
Jim: I work on asteroseismology, which is a technique to determine the internal structure and composition of stars. We do this by “listening” to the sounds of stars, and then using physical models to invert these sounds to determine a star’s structure. This is similar to terrestrial seismology, in which scientists measure the sounds of the earth with a seismograph that records the rumblings of the Earth. Because the Earth’s shaking is caused by compressional waves, i.e., sound waves, that travel through the interior of the Earth and back out, the waves carry information about the internal structure of the Earth. In the same way, stellar pulsations carry information about the interiors of stars.
PLASMA: What tools do you use to record and listen to the stars?
Jim: We cannot put a seismograph on other stars, and sound cannot travel through space, so our techniques are more limited than Earth seismology. Fortunately, the pulsations of many stars cause their brightness to change by detectable amounts (often less than one part in a thousand), and we measure this with space telescopes such as the Kepler and TESS satellites.
PLASMA: How do stars make sounds?
Jim: Stars are gigantic nuclear furnaces that carry enormous amounts of energy from their cores to their surfaces, making them shine. The energy drives convection in stars, similar to the turbulent motions of boiling water. And like a vigorously boiling teapot, this turbulent energy excites sound waves that propagate through the stars. The sound waves cause the star’s surface to pulsate and fluctuate in brightness, which is what we measure.
PLASMA: How does the sound of our Sun compare to other stars?
Jim: The sounds of the Sun are very ordinary, many other stars like it pulsate in a very similar manner. However, we have also detected pulsations in thousands of red giant stars. Because these stars have radii several times larger than the Sun, it takes sound waves longer to propagate through them. Hence, these stars pulsate much slower than the Sun, i.e., they “sing” in lower octaves. In this way, the pitch of a star allows us to measure its size.
PLASMA: What is your favourite sound? And the hardest to record?
Jim: My favorite stellar sounds are heartbeat stars. In these binary stars, the pulsations are not excited by convection, but rather by the gravity of the orbiting companion star. For this reason, the pulsation frequencies are all integer multiples of the binary orbital frequency, making these stars more harmonious than other stars. The “sound” of many of these stars is a tone created by the star’s pulsations, combined with a “drumbeat” as the stars make their closest approach in the orbit.
Some of the hardest stellar sounds to record are those from low-mass stars (and even planets). These stars have much smaller energy generation rates, so they put very little power into sound waves. Their brightness fluctuations are thus very small and extremely difficult to detect. If we can develop more sensitive telescopes to measure their pulsations, we can learn more about them. We have detected the “sound” of Saturn by essentially using its rings as a giant record player!
PLASMA: Do you make music?
Jim: I wish I made music! The closest thing I do is to sonify the pulsations of stars, so that we can listen to the sounds of stars. I did send some of these sounds to a DJ, but I haven’t gotten any royalties for my dope beats… I love music and sound, though I must confess I’m more of a visual learner. lol