Indian & Nepalese astrophysicists' study explains why some stars have 'midlife crises';

A research report by astrophysicists from Nepal and India suggested that stars can have a midlife crisis and enter an inactive phase. The study, which was published on Wednesday, July 29, in the peer-reviewed Monthly Notices of the Royal Astronomical Society: Letters, offers a novel theoretical explanation for the stellar mid-life crisis of stars in our solar system that are roughly as old as the Sun.

Astrophysicists explain why stars can face a midlife crisis

According to Dibyendu Nandy, professor at CESSI, Sun-like stars have a quick transition to an inactive period, as per observations. For stars that are roughly 4.6 billion years old, stellar activity such as sunspots, flares, and plasma wind emission decreases.

The researchers used simulations of magnetic field formation in stars to discover that around the Sun's age, the magnetic field generating mechanism can become sub-critical or inefficient. As a result, stars can exist in two separate activity states-- low activity and active. This is the first study to explain a variety of strange behaviour seen among middle-aged stars.

A constant stream of magnetised plasma or charged particles, known as the solar wind, escapes from the star, causing magnetic braking. Over time, the wind carries away angular momentum. This discharge of plasma can slow the star's rotation over billions of years.

Because of the slower rotation, magnetic fields are generated inefficiently, and star activity is reduced. This means fewer stellar spots, magnetic storms, and other phenomena in star atmospheres that are inextricably tied to the intensity of their magnetic fields, as well as the failure of the widely used technique of stellar gyrochronology. Astrophysicists all over the world utilise gyrochronology to estimate the age of a star by measuring its rotating periods. According to computational models developed by IISER Kolkata experts, gyrochronology no longer works for stars older than the sun because the link between a star's rotational period and its age breaks down as the star reaches middle age.

Stars can experience midlife crisis

Professor Nandy explained that dynamo simulations reveal that a middle-aged star like the Sun can frequently flip to a low activity mode, resulting in dramatically reduced angular momentum losses by magnetised stellar winds and the collapse of gyrochronology relations.

Divya Oberoi, an associate professor at the National Centre for Radio Astrophysics (NCRA), Tata Institute of Fundamental Research in Pune, who was not involved in the study, said that this study finds that the solar dynamo, which is responsible for continuously generating the magnetic field on the Sun, exists in two states: its normal state, where the Sun appears to follow its usual 11-year solar cycle, and its extreme state, where the Sun appears to follow its extreme 11-year solar cycle.

The most well-known of these extended minima for our Sun is known as the ‘Maunder Minimum,' which lasted from around 1645 to 1715. It also gives an intriguing explanation for why watching other middle-aged stars, such as the Sun, during one of these periods of low activity might result in inconsistencies in the results of a regularly used technique for estimating a star's age. If correct, this is a big step forward in our knowledge of solar cycle changes, she noted.

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