Thanks to gyrochronology, astronomers can now estimate the age of a star by studying the speed at which they are spinning and comparing it to their mass. A team of astronomers belonging to various countries led by Dr. Soren Meibom from Harvard-Smithsonian Center of Astrophysics have staked this claim. Their ‘cool star clock’, they say, can improve the accuracy of determination of stellar age.

Using the method devised by this team, astronomers can now guess the age of ‘cool stars’, meaning stars almost the size of our own sun or smaller than that. Such stars are the most common in The Milky Way and also last for a long time. They also host many planets similar to our own.
“They act as lamp posts, lighting up even the oldest parts of our galaxy,” said senior author Dr Soren Meibom from the Harvard-Smithsonian Center for Astrophysics.
Since most properties of cool stars- like their mass, size, temperature and brightness- remain unchanged for almost their entire life, estimating their age has been a tricky job. That was till 1970 when the solution of studying the rate at which the star is spinning was proposed. The approach was dubbed ‘Gyrochronology’ in 2003.
“A cool star spins very fast when it’s young, but just like a top on a table it gets slower and slower as the star grows older,” Dr Meibom said.
Having noticed that stars start spinning slower as they grow older, this team of astronomers observed and monitored 30 “cool stars” in the NGC 6819 cluster using NASA’s Kepler space telescope. They then determined the rotational speed of these cool stars using gyrochronology and determined an age of 2.5 billion years for all of the stars in the NGC 6819 cluster.
Using the same approach, it is now believed that our own sun which rotates around its axis once every 26 hours is probably 4.6 billion years old. And since the planets are likely to have been born at the same time as their star, our planet is also 4.6 billion years of age.

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