Old at heart: Solution to red giants' age paradox

[Judi Lynn]

June 27, 2019

Main sequence star, red giant, white dwarf - in the course of their lifespan covering millions or even billions of years, stars pass through different stages of stellar evolution - all differing greatly in appearance. Yet, stars do not reveal their ages easily, at least not at first glance. The duration of each phase differs too greatly from star to star. With deeper look, however, researchers can reconstruct the star's life story. Various methods now make it possible to reliably determine the age of a star.

But there are tricky cases: Four years ago, two groups of researchers led by the Leibniz Institute for Astrophysics and the Max Planck Institute for Astronomy discovered confusing red giant stars. The results of different age measurements diverged by a full four billion years. "The stars seemed to be old and young at the same time," Dr. Saskia Hekker from MPS and the University of Aarhus in Denmark, who was part of both discovery teams at the time and is now the first author of the new study, recalls. "This apparent paradox has intrigued me ever since", she adds. Together with her colleague Dr. Jennifer A. Johnson from Ohio State University, she has now solved the mystery of some of these stars. Both researchers are convinced that the strange stars only feign youthfulness.

The red giants' building material points to an ancient age of more than 10 billion years. The stars contain comparatively little iron, an element that in the course of galactic evolution was produced only slowly. Old stars therefore contain little iron compared to other substances such as magnesium, silicon, and calcium, while young stars contain more. In order to determine these elements' ratios scientists split the light from the respective star into its individual wavelengths. In this so-called spectrum, each element found within the star leaves a characteristic fingerprint. Another method of age determination looks at the oscillations of a star. With methods of asteroseismology it is possible to then deduce the star's mass.

Since particularly high temperatures prevail inside heavy stars, their fuel burns comparatively quickly. Heavy stars therefore have a much shorter life expectancy than low-mass ones. The red giants in question proved to be true heavyweights. The asteroseismic method therefore yields ages of less than 6 billion years.

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