Stars in their midlife have been known to go into a low-activity state suddenly for some time now. As a result of recent observations, the sun is now much less active than other stars of a similar age. According to a new study published on Wednesday in Monthly Notices of the Royal Astronomical Society: Letters, middle-aged stars can experience their own ‘midlife crisis’, experiencing dramatic breaks in their activity and rotation rates at about the same age as our sun.
Bindesh Tripathi, Prof Dibyendu Nandi, and Prof Soumitro Banerjee at IISER Kolkata have come up with a novel explanation for this disease. The study provides a theoretical explanation for the apparent breakdown of established methods of measuring the age of stars past their middle age, as well as the transition of solar-like stars into inactive states.
According to Dr. Nandi, astrophysicists are struggling to explain these puzzling observations of stars around the same age as the sun. ‘Together with an undergraduate student from Nepal (now pursuing his Ph.D. at the University of Wisconsin) and colleague Soumitro Banerjee at IISER Kolkata, we have developed a self-consistent explanation for these observations. We believe we have provided a solution to the stellar midlife crisis,’ Dr. Nandi said.
The magnetic braking of stars is well documented – a steady stream of charged particles, called the solar wind, escapes the star over time, carrying away a small amount of the star’s angular momentum. Over billions of years, this slow drain causes stars like our sun to slowly slow down their rotation. Consequently, a slower rotation produces 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 strength of their magnetic fields.
Due to the gradual loss of angular momentum, the decrease in activity and rotation rate over time is expected to be smooth and predictable. The idea led to the development of a tool known as ‘stellar gyrochronology’, which has been widely used in recent years to estimate the age of a star from its rotation period, according to a Wednesday press release.
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However, recent studies suggest that this intimate relationship breaks down around middle age. According to the researchers, magnetic field generation in stars becomes less efficient or subcritical at about the age of the sun, using dynamo models of magnetic field generation. This allows stars to exist in two distinct states of activity – low activity mode and active mode. As the sun ages, it often switches to low activity mode, resulting in dramatically reduced angular momentum losses from magnetized stellar winds.
The sub-critical magnetic dynamos of solar-like stars are consistent with diverse solar stellar phenomena, such as why stars beyond midlife do not spin down fast and why the sun may be transitioning to a magnetically inactive future. In the recent history of the sun, we have seen low activity episodes known as grand minima, in which hardly any sunspots were observed.
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