Salty stars die young

While the health benefits of a low sodium diet in humans may be unclear, it appears that the amount of sodium in older stars has a strong effect on their physical condition.

Globular star cluster NGC 6752 (image: ESO VLT)

Sun-like stars live for billions of years, with greatly expanding waistlines in old age, a final burst of nuclear burning, and the puffing away of most of their atmospheres. Expelled gas and stardust goes on to form new generations of stars, and provides material for the formation of planets.

The life cycle of stars is well understood, or at least it was according to models of stellar evolution that have stood the test of time and critical evaluation. Using images taken from the European Southern Observatory’s Very Large Telescope, together with results from previous studies, astrophysicist Simon Campbell and colleagues have found evidence that some Sun-like stars bypass the girth expanding and final fling stages of their lives. It all depends on the amount of sodium they contain.

Campbell’s surprising finding is that all of the elderly giant stars studied have low levels of sodium, with none of the higher-sodium stars reaching the Asymptotic Giant Branch stage of existence. As many as 70% of the stars have not gone through the final nuclear burning and mass-loss phase.

“It seems stars need to have a low-sodium ‘diet’ to reach the AGB phase in their old age,” says Campbell. “This observation is important for several reasons. These stars are the brightest stars in globular clusters – so there will be 70% fewer of the brightest stars than theory predicts. It also means our computer models of stars are incomplete and must be fixed!”

By “fixed” Campbell means revised rather than overturned. Still, the discovery has at least one major ramification for astrophysics: the inclusion of Asymptotic Giant Branch stars will reduce the reliability of globular cluster star counts used to test timescales of stellar evolution.