Stars like our Sun can go through a mid-life crisis, according to new research carried out by scientists from IISER Kolkata.

Stellar Middle Age

• 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.

How does it occur?

• 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.
• 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.

Impact

• 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 sunlike stars.
• However, there is a breakdown of the gyrochronology relationship, because, after midlife, a star’s rate of spin does not slow down with age as fast as it was slowing down earlier.
• Another intriguing fact is that the Sun’s activity level has been observed to be much lower than other stars of similar age.

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