Astronomers Develop A Better “Clock for the Stars”

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Determining an accurate age for stars has been a key question for stellar astronomers and astrophysicists for some decades. Several methods for estimating the age of stars have been developed, but all to date are very time-consuming and have had significant limitations in accuracy.

The good news is the Harvard-Smithsonian Center for Astrophysics announced on Wednesday, January 7th that it had made further progress in developing a more accurate “clock for the stars”.

The research was first published on the website of the journal Nature earlier this week. The research is part of the bigger Kepler Cluster Study.

Clock for the Stars measuring “spin”

A star’s spin rate depends on its age as the spin rate slows with time. The spin of a star also depends on its mass. Larger, heavier stars spin faster than smaller, lighter stars. The new study confirms there is a close mathematical relationship between mass, spin and age, so that by measuring the mass and spin, astronomers are able to calculate age.

Barnes first proposed this method in 2003, building on prior work, and dubbed the method gyrochronology.

When measuring the spin of a star, astronomers look for dark spots on its surface. In effect, they watch for the star to dim slightly when a sunspot appears, and brighten again when the sunspot moves out of view.

Given the average star dims by much less than 1%, and it can take days for a sunspot to cross the star’s face, it’s difficult to measure these minute changes in brightness. The researchers accomplished the task by using data from NASA’s Kepler spacecraft, which provided continuous, exact measurements of the brightnesses of observed stars.

To confirm that gyrochronology ages for stars were accurate and precise, astronomers calibrated their new clock for the stars by measuring the spin periods of stars with known ages and masses.

Meibom and his team had already studied a cluster of one billion-year-old stars. This new study analyzed stars in the around 2.5-billion-year-old cluster known as NGC 6819, and confirmed that gyrochronology was also accurate for older stars.

Statements from researchers

“Our goal is to construct a clock that can measure accurate and precise ages of stars from their spins. We’ve taken another significant step forward in building that clock,” says Soren Meibom of the Harvard-Smithsonian Center for Astrophysics and principal investigator of the Kepler Cluster Study.

“Now we can derive precise ages for large numbers of cool field stars in our Galaxy by measuring their spin periods,” noted Meibom. “This is an important new tool for astronomers studying the evolution of stars and their companions, and one that can help identify planets old enough for complex life to have evolved.”

“We have found that the relationship between mass, rotation rate and age is now defined well enough by observations that we can obtain the ages of individual stars to within 10 percent,” commented co-author Sydney Barnes of the Leibniz Institute for Astrophysics in Germany.

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