Main sequence stars fuse hydrogen in their cores. This is how they produce the energy they need to glow and prevent them from collapsing under their own weight. Since hydrogen is fused to form helium, there is less hydrogen available in the core. This can be a challenge for large stars. They have to fuse a tremendous amount of hydrogen to keep shining, and they can’t when the hydrogen in the core is depleted. Fortunately, they can solve this problem by mixing more hydrogen into their core. A new study in natural astronomy shows us how this blending takes place.
The inside of our sun. Photo credit: Kelvin Ma, via Wikipedia
In stars like the sun, the core is surrounded by a layer of radiation. This layer is so dense that photons take tens of thousands of years to move through it. The atoms in this layer don’t spin much, so there’s not much mixing. Above the radiation layer there is a convective layer that mixes. Hydrogen in the sun’s core is not replenishing because it has fused into helium, but there is still enough nuclear hydrogen to power the sun for billions of years.
If larger stars had an internal structure similar to that of our sun, they would burn through the core’s hydrogen fairly quickly, filling the core with “helium ash,” which would limit the star’s ability to melt hydrogen. Hence, astronomers have assumed that large stars have a convective core that would allow hydrogen from higher layers to mix into the core. But how do you prove it?
The inside of big stars. Photo credit: May Gade Pedersen
This new study used a method known as asteroseismology, which studies how the surface of a star moves and how its brightness changes. While some of it can be caused by things like star flares, a lot of it is caused by sound waves inside the star. The process is similar to how you can study the vibrations of a bell by listening to its ringing tone. Because a star’s internal vibrations are affected by the density and movement of its interior, asteroseismology is a powerful way to study stars.
The team studied 26 type B stars that are known to pulsate in brightness. These light blue stars have 3 to 20 times the mass of our sun and pulse at a speed of 12 hours to 5 days. Using data from NASA’s Kepler mission, the team was able to show that many of these stars have a convective core so that hydrogen can mix.
One interesting finding was that the amount of mixing does not correlate with the age of the star. It’s not that the mixing increases and gets hotter as the star ages. Instead, the mixing speed is quite variable. Some stars have very little core mixing, while others mix a million times higher. Instead of age, the intermingling appears to be related to the amount of internal rotation of a star.
There’s a lot more to learn here. The degree of intermingling in the core of a star can affect the life and evolution of the star. While large stars typically have much shorter lifetimes than our Sun, their lifespan may not simply depend on their mass. If we apply asteroseismology to more stars in the future, we will likely find more factors that are in the mix.
Reference: Pedersen, May G. et al. “Internal mixing of rotating stars, derived from dipole gravity modes.” Natural Astronomy (2021): 1-8.