July 29, 2021 1:50:45 am
For some time now, it has been known that stars in their midlife can suddenly switch to a low activity state. Recent observations show that the sun now is much less active than other similarly aged stars.
According to a new research published on Wednesday in Monthly Notices of the Royal Astronomical Society: Letters, middle-aged stars can experience their own kind of “midlife crisis”, experiencing dramatic breaks in their activity and rotation rates at about the same age as our sun.
The new work, carried out by Bindesh Tripathi, Prof Dibyendu Nandi, and Prof Soumitro Banerjee at IISER Kolkata provides a novel explanation for this mysterious ailment.
The study provides a new theoretical underpinning for the unexplained breakdown of established techniques for measuring the age of stars past their middle age, and the transition of solar-like stars to a magnetically inactive future.
Dr Nandi told The Indian Express that astrophysicists have struggled to explain these puzzling observations in stars that are of a similar age to the sun. “Working with an undergraduate student from Nepal (now pursuing his PhD at University of Wisconsin) and colleague Soumitro Banerjee at IISER Kolkata, we have come up with a self-consistent explanation of these intriguing observations. We believe we have provided a solution to stellar midlife crisis,” Dr Nandi said.
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 our sun to gradually slow down their rotation over billions of years. In turn, slower rotation leads to altered magnetic fields and less stellar activity — the number of sunspots, flares, outbursts, and similar phenomena in the atmosphere of stars, which are intrinsically linked to the strengths of their magnetic fields.
This 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 a 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, read an official press release issued on Wednesday.
Recent observations, however, indicate that this intimate relationship breaks down around middle age. Using dynamo models of magnetic field generation in stars, the team shows that at about the age of the sun, the magnetic field generation mechanism of stars suddenly becomes sub-critical or less efficient. This allows stars to exist in two distinct activity states – low activity mode and active mode. A middle-aged star like the sun can often switch to low activity mode resulting in drastically reduced angular momentum losses by magnetised stellar winds.
This hypothesis of sub-critical magnetic dynamos of solar-like stars provides a self-consistent basis for a diversity of solar stellar phenomena, such as why stars beyond midlife do not spin down fast and recent findings that the sun may be transitioning to a magnetically inactive future.
The new work provides key insights into the existence of low activity episodes in the recent history of the sun, known as grand minima, where hardly any sunspots are seen.
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