Main sequence stars fuse hydrogen in their cores. It’s how they produce the energy they need to shine and keeps them from collapsing under their own weight. As hydrogen is fused into helium, there is less hydrogen available in the core. This can pose a challenge for large stars. They need to fuse a tremendous amount of hydrogen to keep shining, and they can’t do that when core hydrogen is depleted. Fortunately, they can solve this problem by mixing more hydrogen into their core. A new study in Nature Astronomy shows us how this mixing happens.


The interior of our Sun. Credit: Kelvin Ma, via Wikipedia

With stars like the Sun, the core is surrounded by a radiative layer. This layer is so dense that it takes photons tens of thousands of years to move through it. The atoms in this layer don’t churn much, so there isn’t much mixing. Above the radiative layer is a convective layer, which does mix. Hydrogen within the Sun’s core isn’t replenished as it’s fused into helium, but there is still plenty of core hydrogen to power the Sun for billions of years.

If larger stars had a similar internal structure as our Sun, they would burn through core hydrogen fairly quickly, filling the core with “helium ash” that would limit the star’s ability to fuse hydrogen. So astronomers have thought that large stars have a convective core, which would allow hydrogen from higher layers to be mixed into the core. But how do you prove that?


The interior of large stars. Credit: May Gade Pedersen

This new study used a method known as asteroseismology, which looks at how the surface of a star moves and changes in brightness. While some of this can be caused by things like stellar flares, much of it is caused by sound waves within the star. The process is similar to the way you might study the vibrations of a bell by
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