New model developed to determine age of stars

Researchers from the University of Rochester in the US described how they have corroborated known, observable data for the activity of Sun-like stars with fundamental astrophysics theory. To understand how stars similar to our Sun evolve, scientists have developed a new model which may help determine the age of stars more precisely than current methods. The framework helps explain how the rotation of stars, their emission of X-rays, and the intensity of their stellar winds vary with time.

By looking at the physics behind the speeding up or slowing down of a star’s rotation, its X-ray activity, and magnetic field generation, the research is a “first attempt to build a comprehensive model for the activity evolution of these stars,” said Eric Blackman, professor at the University of Rochester.

Using our Sun as the calibration point, the model most accurately describes the likely behaviour of the Sun in the past, and how it would be expected to behave in the future.

“Our model shows that stars younger than our Sun can vary quite significantly in the intensity of their X-ray emission and mass loss,” said Blackman.

“But there is a convergence in the activity of the stars after a certain age, so you could say that our Sun is very typical for stars of its mass, radius, and its age. They get more predictable as they age,” he said.

“We’re not yet at the point where we can accurately predict a star’s precise age, because there are simplifying assumptions that go into the model,” said Blackman.

“But in principle, by extending the work to relax some of these assumptions we could predict the age for a wide range of stars based on their X-ray luminosity,” he said.

Empirically determining the age of stars is most easily accomplished if a star is among a cluster of stars, from whose mutual properties astronomers can estimate the age.

Blackman said that its age can then be estimated “to an accuracy not better than a factor of 25 per cent of its actual age, which is typically billions of years.”

The problem is worse for “field stars,” alone in space for which the cluster method of dating cannot be used.

For these stars, astronomers have turned to gyrochronology and activity ageing - empirically ageing the stars based on the fact that older stars of known age rotate more slowly and have lower X-ray luminosities than younger stars.

Over the past few decades astronomers have been able to empirically measure these trends in rotation and magnetic activity for stars like the Sun, but now we are trying to devise a comprehensive theoretical interpretation, said Eric Mamajek, professor at the University of Rochester.

“Ultimately this should lead to improved constraints on the evolution of rotation and activity in Sun-like stars, and better constraints on how the magnetic properties of our Sun have changed over the course of its main sequence life,” Mamajek said. The study was published in the journal Monthly Notices of the Royal Astronomical Society.