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Astronomers have found that the core of a red giant, the type of star that our Sun will eventually become, spins ten times as fast as its surface. And it happens because of a phenomenon we can see here on Earth, too.
You have probably seen a figure skater perform a so-called 'scratch spin', where she starts out with arms and free leg extended, before pulling them in – and spinning faster as a result. This happens because of a property known as angular momentum, a measure of how much an object is spinning. More specifically, it happens because the angular momentum of an object – in this case the figure skater – must stay the same before and after the manoeuvre. But angular momentum is not a property confined to figure skaters, people in general, or even things on Earth. Every spinning object in the universe has angular momentum, and each must obey the same physical law as the figure skater. In fact right at this very moment, across the universe, stars are performing scratch spins of their own.
Stars like our Sun run on hydrogen. When a star runs out of hydrogen, it is forced to burn other fuels. This switch triggers a change in the star. The core of the star collapses as the outer region expands and cools, creating a type of star known as a red giant.
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We know that the angular momentum of the star must be conserved, so we also know that the core of the star that collapses must be spinning faster than the surface of the red giant. So far, though, our understanding of exactly how a star's angular momentum changes as the star evolves is not especially good.
This is partly because we cannot directly observe how fast the core of a star is spinning. Now, though, an international collaboration of astronomers led by Paul Beck at the Institute of Astronomy at Leuven University in Belgium have found a way to measure the rotation of the core by probing the star's interior using techniques from astroseismology. Astroseismology is a bit like the normal seismology that we use to study earthquakes, but instead of looking at waves traveling through Earth it looks at waves traveling through stars — starquakes. Their research was published in the latest issue of Nature, but is also available on arXiv.
Beck and his colleagues looked at small, regular variations in the light coming from several red giants observed by the Kepler spacecraft. Kepler's main job is searching for planets outside of our solar system, so it is well suited to detecting extremely small changes in the brightness of stars, as this is a major way to spot that a star has a planet orbiting it.
The variations in light are caused by different waves traveling to different depths inside the star. Once Beck and his colleages had collected nearly two years worth of data on these variations, they compared what they had with theoretical predictions, and found that the core of the stars must be rotating at least ten times as fast as the surface.
This study advances astronomers' knowledge of how the angular momentum of parts of a star change as it evolves, but there are still many questions left unanswered. The next step will be to study a larger sample of red giants at different stages in their lifetimes to learn more about how these stars change as they grow old, and what fate is in store for our Sun.
Reference
Beck, P., Montalban, J., Kallinger, T., De Ridder, J., Aerts, C., García, R., Hekker, S., Dupret, M., Mosser, B., Eggenberger, P., Stello, D., Elsworth, Y., Frandsen, S., Carrier, F., Hillen, M., Gruberbauer, M., Christensen-Dalsgaard, J., Miglio, A., Valentini, M., Bedding, T., Kjeldsen, H., Girouard, F., Hall, J., & Ibrahim, K. (2011). Fast core rotation in red-giant stars as revealed by gravity-dominated mixed modes Nature, 481 (7379), 55-57 DOI: 10.1038/nature10612