Did you know? Stars too have their 'mid-life crisis'

Like many of us, the stars too have their “mid-life crisis”, which has now been decoded by Indian astrophysicists.

Middle-aged stars experience their own kind of crisis, undergoing dramatic breaks in their activity and rotation rates at about the same age as the Sun.

For some time now it has been known that stars at their midlife can suddenly switch to a low activity state with a sharp drop in releasing radiation and high-energy particles. Recent observations show that the current Sun is much less active than other similar aged stars.

Astrophysicists have struggled to explain such a mystery, which has now been cracked by Dibyendu Nandy, Soumitra Banerjee and Bindesh Tripathi at the Indian Institute of Science Education and Research (IISER) Kolkata.

Their work, published in the Monthly Notices of the Royal Astronomical Society Letters, also paves the way to solve a related puzzle – how a widely used tool in astrophysics, known as stellar gyrochoronology, breaks down at about the age of a Sun-like star.

Astronomers have long known that stars experience a process known as ‘magnetic braking’: a steady stream of charged particles, known as the solar wind, escapes from the star over time, carrying away small amounts of the star’s angular momentum. This slow drain causes stars like the Sun to slow down their rotation over billions of years.

In turn, the slower rotation leads to altered magnetic fields and less stellar activity - the numbers of sunspots, flares, outbursts, and similar phenomena that are intrinsically linked to the strengths of their magnetic fields.

Such a decrease in activity and rotation rate over time is expected to be smooth and predictable because of the gradual loss of angular momentum. The idea gave birth to the tool known as ‘stellar gyrochronology’, which has been widely used over the past two decades to estimate the age of a star from its rotation period.

However recent observations indicate that this intimate relationship breaks down around middle age. The new work by the IISER trio provides a novel explanation for this mysterious ailment.

The Sun, for instance, is going through a weakened braking phase at about its mid-life of 4.5 billion years. But what causes such weakened magnetic braking remains an intriguing question.

Using dynamo models of magnetic field generation in stars, the team show that at about the age of the Sun the magnetic field generation mechanism of stars suddenly becomes less efficient, allowing the stars to exist in two distinct activity states – a low activity mode and an active mode.

“Our work provides a consistent theoretical explanation unifying diverse puzzling observations in the field of stellar astronomy, such as why does the rotation period age relationship for stars breaks down at about stellar midlife and why the Sun is less active compared to similar stars,” Nandy told DH.

A middle-aged star like the Sun can often switch to the low activity mode resulting in drastically reduced angular momentum losses by magnetized stellar winds.

The work provides key insights into the existence of low activity episodes in the recent history of the Sun known as grand minima – when hardly any sunspots are seen.