High sodium content may keep stars perpetually young

The central region of the globular cluster NGC 6752, showing a few of the "blue straggler" stars. Those stars are representatives of a younger population that apparently fails to pass through an expected phase of the stellar life cycle.

ESA/Hubble, NASA

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.