High sodium content may keep stars looking perpetually young

Matthew Francis - arstechnica.com - 05/31/2013
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Oh, my! When I first read the title of this article, I thought movie stars, pop stars, etc. keep their youth by eating a lot of salt.


The story is familiar to many: stars approaching the end of their life pass through several stages of swelling and shrinking before expiring. These stages—the red giants and asymptotic giants—create some of the brightest stars in the Universe. As a result, they are useful tracers of stellar age and evolution in galaxies and globular clusters (roughly spherical collections of stars).

However, a new observation of one of the Milky Way's globular clusters turned up a problem: the younger generation of stars in the cluster didn't seem to be passing through the asymptotic giant phase. Simon W. Campbell and colleagues found that while the red giant star population included stars from both older and younger populations, the asymptotic giant stars only represented the older generation. That's in strong contradiction to theory: the era of a star's formation shouldn't affect its life cycle. The reason for this deviation is mysterious.

While the hot aftermath of the Big Bang produced most of the hydrogen, helium, and a smattering of other light elements, most of the heavier elements—carbon, oxygen, and so forth—were produced by stars. As a result, stars forming earlier in the Universe will contain fewer of the heavier elements, while later stars have higher abundances. Astronomers, being weird, call those elements "metals," and their relative abundance is called metallicity. In that way, metallicity is a good way to determine the era in which a star formed.

Globular clusters contain hundreds of thousands of stars and are found orbiting many galaxies, including the Milky Way. Many globular clusters are very old, containing some of the oldest known stars. However, those associated with the Milky Way harbor at least two generations of stars, as determined by metallicity.

NGC 6752 is one of the Milky Way's globular clusters, and it's interesting in part because it has a number of "blue straggler" stars. These are too young to have formed when the cluster was born, since this type of star burns through its life cycle relatively quickly. As a result, astronomers are interested in studying NGC 6752 and similar clusters in hopes of understanding stellar life cycles better.

According to theory, when a Sun-like star exhausts its available hydrogen fuel in its core, it rapidly expands into a red giant. Subsequent nuclear processes in the star lead to a later evolutionary stage in which the star grows to an even larger size: the asymptotic giant branch (AGB) phase. Stellar models predict that any star in NGC 6752 with a sufficiently short lifetime for us to see it will pass through the AGB phase. The rate at which stars enter and pass through these phases is determined by mass, so two red giants need not have formed at the same time: one may be lower mass and older than the other.

The new observation involved taking the spectrum of 20 AGB and 24 red giant stars in NGC 6752, specifically focusing on a pair of bright emission lines generated by sodium in stars' atmospheres. The older generation of stars in the cluster have little sodium compared to the younger population, making sodium a reasonable tracer of which star belongs to what group.

The researchers found that none of the AGB stars had the expected sodium emission marking them as part of the younger generation, even though the red giants showed the expected split between newer and older stars. The implication is either that younger stars somehow got rid of their sodium before entering the AGB phase—a highly unlikely option based on what we know about stars—or that for some reason the younger stars in NGC 6752 weren't entering the AGB phase of evolution. (We could even insert a joke here about high sodium wrecking health at the end of life.)

While the authors tentatively extend a hypothesis of accelerated mass loss for younger stars to explain this phenomenon, they rather bleakly state, "There is currently no clear explanation for such a high proportion of AGB stars." The potential implications are staggering: globular clusters are used to constrain models of stellar life cycles and star populations. If NGC 6752 is representative, astronomers may have to rethink certain aspects of how stars behave when they reach the ends of their lives.

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