Ask anyone what a star looks like on the outside and you’re most likely to have a deluge of answers from a wide range of people. Some may say that their surfaces are bright, throwing out light into outer space, while others may make references to our Sun and go into detail about the sunspots and solar prominences that erupt from its bubbling surface. Some might refer to the types of stars – from the massive red supergiant all of the way down to the glowing hot stellar remnant that we recognise as the white dwarf – and how their colours compare. The point is that the majority of individuals that you end up asking will have some idea of what a star’s surface looks like.
What does the inside of a star look like? Ask this, and you might not get such a flurry of answers, so this is where asteroseismology is attempting to plug the knowledge gap. Put simply, this is the study of a star’s pulsations to define just what’s going on inside these gaseous structures. The discipline is very similar to how seismologists probe the interior of our planet. These experts study the oscillations of earthquakes to build a picture of its core, mantle and crust as waves propagate through the menagerie of materials that our planet is comprised of.
However, inside a star there’s a whole different process at play. Stars can reach incredibly high temperatures of many millions of degrees and their cores are akin to gigantic furnaces that burn elements into new ones. It is this heat that drives waves through the star, causing it to pulsate and making it seem as though the stars are singing.
This means that the way to a star’s heart is to listen to the music made by these great industrious balls of gas. In order for asteroseismologists to be able to peer through the many layers, all of the way down to the core, these experts have to attend something of a stellar concert.
However, unlike the setting of an opera house, astronomers don’t use their ears to listen to, or more precisely monitor, the hum of stars – they’re not your standard performers and this isn’t your usual orchestra. The art of building an image of a star’s insides is seeing how the sound waves vibrating through its innards cause the surface to slightly vary its brightness. So, since the Kepler Space Telescope stares at the stars constantly it can monitor these slight flickers.
The way the stars vibrate and the time it takes for the vibrations to ripple through them depends on each body’s interior structure. In the same way seismologists are able to learn about the interior of Earth thanks to seismic waves, stellar waves tell us about the density, temperature and size of the layers inside stars.
In particular, we are able to probe into a star’s sweltering core, the radiative layer that envelopes it and where energy is radiated away, as well as a convective layer that comprises the star’s outer shell. Here energy takes the form of heat as convective waves, spitting out photons – particles of light – that race through the vacuum of space.
As we know, stars come in variety of types, so it follows that they sport differing insides. Varying stellar interiors – no matter how slight they are – means that their vibrations will each play a different tune at different musical notes.
Image Courtesy of Science/AAAS/ G. Perez, IAC, SMM