Stars can experience midlife crisis: Study by Indian, Nepalese astrophysicists explain why
Stars can experience a midlife crisis and transition into an inactive phase, according to a research paper by astrophysicists from Nepal and India
Stars can experience a midlife crisis and transition into an inactive phase, according to a research paper by astrophysicists from Nepal and India.
The study, published in the peer-reviewed Monthly Notices of the Royal Astronomical Society: Letters on Wednesday, provides a new theoretical explanation for the stellar mid-life crisis of stars that are approximately as old as the Sun in our solar system.
The study was authored by Bindesh Tripathi, Dibyendu Nandy and Soumitro Banerjee at the Center of Excellence in Space Sciences India (CESSI) and Department of Physical Sciences, Indian Institute of Science Education and Research (IISER), Kolkata. While Tripathi is from Nepal, Nandy and Banerjee are from India.
“Observations of sun-like stars show a sudden transition to an inactive phase. Stellar activity such as sunspots, flares, plasma wind output reduce for middle aged stars that are about 4.6 billion years old,” said Nandy, professor at CESSI.
In their work, the scientists used simulations of magnetic field generation in stars and found that around the age of the Sun, the magnetic field generation mechanism can suddenly become sub-critical or less efficient. This allows stars to exist in two distinct activity states – a low activity mode and an active mode. This is the first work that can explain a variety of peculiar behavior observed in middle aged stars.
Stars often experience magnetic braking due to a steady stream of magnetised plasma or charged particles, known as the solar wind, escaping from the star. This wind carries away angular momentum over time. Over billions of years, this outflow of plasma can slow down the rotation of the star.
The slower rotation leads to less efficient generation of magnetic fields and less stellar activity. This means a reduction in the numbers of stellar spots, magnetic storms, and similar phenomena in the atmospheres of stars, which are intrinsically linked to the strengths of their magnetic fields, and the breakdown of the widely used technique of stellar gyrochronology. Gyrochronology is used by astrophysicists across the world to estimate the age of a star based on a measurement of their rotational periods. However, since the relationship between the rotational period of a star and its age breaks down when the star becomes middle-aged, gyrochronology no longer works for stars older than the sun, computational models by the IISER Kolkata scientists indicate.
“Our dynamo simulations show that a middle aged star like the Sun can often switch to a low activity mode, resulting in drastically reduced angular momentum losses by magnetised stellar winds and the breakdown of gyrochronology relations,” said professor Nandy.
Divya Oberoi, associate professor at the National Centre for Radio Astrophysics (NCRA), Tata Institute of Fundamental Research, Pune, who was not a part of the study said, “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 shows its usual 11-year solar cycle of appearance and disappearance of sun (or rather star) spots, and a sub-critical state where for all outwardly appearances, the dynamo cycle seems to have switched off and there is an extended period of subdued activity on the star. For our Sun, the most famous of these extended minima goes by the name ‘Maunder Minimum’ which lasted from about 1645 to 1715. It also suggests a very interesting explanation for how observing other middle aged stars like the Sun during one of these periods of low activity can lead to inconsistent answers for a commonly used technique to estimate the age of a star. If true, this represents a significant advance in our understanding of the observed variations in solar cycles.”