Our sun may be special to us, but among all the stars in the galaxy, it’s not that unique.
According to a study published Thursday in the journal Science, our beloved star can be classified as an ordinary “solar-type” star, meaning that the internal processes that control its activity are similar to those seen in many other nearby stars.
The sun goes through an 11-year cycle where its magnetic poles flip — imagine the north and south poles on Earth changing place — and during this time the sun’s activity changes between subdued and tumultuous. When activity is low, it is known as solar minimum, and when activity is high, it is known as solar maximum.
As the sun nears solar maximum and its activity cycle ramps up, its surface gets covered in sunspots, which are ephemeral dark marks created by strong magnetic activity.
“Above sunspots you have complex structures that trigger dynamic phenomenons, eruptions that are like volcanoes,” said Antoine Strugarek, a solar physicist at the French Alternative Energies and Atomic Energy Commission and at the University of Montreal. “Those eruptions can impact our Earth.”
The sun’s emissions can interact with satellites and even influence power grids on Earth, according to Dr. Strugarek. So to better predict the sun’s activity, scientists need to better understand the 11-year cycle and how it generates magnetic fields.
Additionally, some scientists have argued that our sun’s 11-year cycle is fundamentally different from those of other stars, so Dr. Strugarek and his colleagues designed a model to investigate what controls a star’s activity cycle. They used the model to study how the hot, turbulent plasma that flows inside a star can generate magnetic fields that affect activity cycles.
Using data from more than 25 stars, Dr. Strugarek said they found that a star’s activity cycle depended on two factors: luminosity and rotation.
Luminosity is simply the star’s brightness, but it also gives insight into how much energy it emits, which is affected by the star’s plasma flow. Rotation refers to how long it takes a point on the star to completely circle around it. Together, these two factors create what is known as the star’s Rossby number.
They found that Rossby numbers and solar cycles have an inverse relationship, so as Rossby numbers increase, solar cycles decrease. When they plotted that information they found that our sun also follows that trend, which helps support the idea that it is similar to other solar-type stars.
According to Dr. Strugarek, their work could help scientists create future models that would help better predict the ferocity of the sun’s activity cycle.
Huw Morgan, a solar physicist at Aberystwyth University in Wales, also studies the sun’s solar activity cycle, with a particular interest in the corona, its extremely hot outer layer.
The corona, which burns at more than a million degrees, is hundreds of times hotter than the sun’s surface, has long been shrouded in mystery. Dr. Morgan wanted to investigate how the sun’s activity cycle affected the heat of the corona, and overcome limitations in existing research.
“For a long time, people have been estimating coronal temperature over small regions over small time scales,” he said.
Using a supercomputer, he collected hundreds of thousands of images of the sun taken by NASA’s Solar Dynamics Observatory between 2010 and 2017. From about 22,000 miles above Earth, the satellite snaps a photo of the sun about every 10 seconds.
Those images allowed him to study the temperature of the sun’s entire outer atmosphere as its activity changed over the course of seven years.
As the sun reaches solar maximum, more sunspots pop up on its surface. Scientists have already known that sunspots make the areas of the corona immediately above them hotter. But what scientists did not know was how the areas of the corona that are not above sunspots, the so-called quiet corona, heat up or cool down during the sun’s activity cycle.
In a study published Friday in the journal Science Advances, Dr. Morgan found that when the sun is at solar minimum, the quiet corona measures around 1.4 million degrees Celsius. But at solar maximum it jumps to around 1.8 million degrees.
Dr. Morgan said he was not sure why the entire corona, including the areas not above a sunspot, heat up as the sun’s activity increases.
“The solar corona remains a mystery,” he said. “But we are getting far better at measuring what it’s doing and that’s enabling us to start to understand it.”
