Stellar mid-life crisis: Why the middle-aged Sun may sink into a period of low activity

At about the age of the Sun, the magnetic field generation mechanism of stars becomes sub-critical or less efficient

Stars like our Sun can go through a mid-life crisis, according to new research carried out by scientists from IISER Kolkata. This can lead to dramatic changes in their activity and rotation rates. The study also provides an explanation for the breakdown of the long-established relation between rotation rate and age in middle-aged sun-like stars. The work has been published in the Monthly Notices of the Royal Astronomical Society: Letters. At about 4.6 billion years of age, the sun is middle aged, that is, it will continue to live for roughly the same period. There are accurate methods for estimating the age of the Sun, such as by using radioactive dating of very old meteorites that have fallen on the Earth. However, for more distant stars which are similar in mass and age to the Sun, such methods are not possible. One of the methods used is called gyrochronology. There is a relationship between rotation rate and age, that is the rotation rate of a star slows down with age. When the stellar wind escapes from the star, it carries away with it a part of the angular momentum of the star, which results in its slowing down. The stellar wind has two drivers: one is the high temperature of the outer atmosphere of stars – the corona – which results in an outward expansion and hence plasma winds that emanate out. The other is the magnetic field. “The magnetic field actually heats the corona and so when magnetic activity is strong the winds are strong and since wind carries away the internal (rotational) angular momentum of the star, it slows down its rotation,” explains Dibyendu Nandi, one of the authors of the paper. This is called magnetic braking. As the star ages, due to this mechanism, its rotation slows down and this relationship is used in gyrochronology to estimate the age of the star.