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The sun, like all stars, is a fiery ball of plasma fueled by fusion. Magnetic field lines emerge from its surface that can cause dark spots known as sunspots. Increase the activity of these magnetic spirals, and you get more solar storms that spew deadly charged particles and radiation throughout our solar system. If enough of these punishing waves hit a rocky planet, that planet could microwave up in a sad condition where nothing could live.
So how come we are alive? A study published Thursday in the journal Science suggests that our sun is fairly tame compared to its star siblings, and that hundreds of other sun-like stars in our galaxy have on average five times more magnetic activity than our parent star. In other words, the sun is a little monotonous, which could be good for life here on Earth.
Astronomers have been tracking the appearance of sunspots since Galileo’s time, providing a proxy for solar activity that dates back four centuries. Some previous studies also implied that the sun was calmer than other similar stars. But competing evidence has also found that the sun’s activity level is normal for stars of its size.
“This prompted the question:” Is the sun a true star similar to the sun? Said Timo Reinhold, an astrophysicist at the Max Planck Institute for Solar System Research in Göttingen, Germany, and co-author of the article.
Dr. Reinhold and his colleagues looked at the data collected by NASA’s retired Kepler Space Telescope, which continuously monitored approximately 150,000 stars in the Milky Way over four years to find exoplanets, and was able to observe brightness variations in activity, such as the appearance and disappearance of star dots.
The researchers selected stars with masses, temperatures, ages, chemical compositions, and rotation periods comparable to those of our sun. They finally found 369 stars to compare, the largest sample to date.
Stars like the sun go through regular cycles during which spots cross their surfaces more or less frequently. During peak magnetic activity, when dots appear across the surface, a star will dim. The cycle of our sun lasts about 11 Earth years.
For the sun, this attenuation is negligible. Data from the past 140 years indicates that its brightness changes by less than a tenth percent over the course of its cycle. But for the stars Kepler studied, the variability could be up to 12 times that amount.
The team has come up with two quite different potential explanations for what this means.
The first is that the sun is in an unusual period of torpor, and one day it will wake up and be more like its relatives. Evidence for this idea comes from significant changes in sun activity levels during recorded history. Between 1645 and 1715, an era known as the Maunder minimum, astronomers observed few or no sunspots. More than a century later, in 1859, the sun launched one of the largest electromagnetic storms ever recorded, the Carrington Event, which knocked out telegraph lines and generated auroras in the southern Caribbean.
But Natalie Krivova, co-author and also astrophysicist at Max Planck, said the data from ice cores, which contain chemical indicators of solar activity dating back 9,000 years, do not suggest that the sun has been more strident in the geologically recent past. . . On the other hand, nine millennia is a bump compared to the sun’s 4 billion years of life.
The second idea, Dr. Krivova said, is that the magnetic dynamo within the sun, whose power is its colossal magnetic field, is nearing the end of its high-powered stage, and is currently in transition to a period of reduced activity. Stars older than the sun show marked decreases in magnetic activity, and the sun is about to reach the age when this change should occur.
Some star scientists believe the sun’s magnetic dynamo could be “reaching its final state, or near death,” said Ricky Egeland, a solar physicist at the National Center for Atmospheric Research in Boulder, Colorado.
The 369 sun-like stars observed by Kepler could simply be at an earlier stage of evolution than the sun, these scientists say. Or perhaps something particular about the sun is causing an early transition. Dr. Reinhold’s team does not favor one explanation over the other.
In any case, a calm sun has benefited our species. When the sun comes on, its energy emissions harm astronauts and orbiting satellites, and particularly powerful bursts can affect power grids on the ground. Radiation from such events is not particularly conducive to the existence of living organisms.
Models indicate that when the sun was younger, perhaps half a billion or a billion years old, it had more magnetic activity than today, Dr. Egeland said.
“I always wonder what effect this variability had on the development of life,” he said. “It may not be a coincidence that we live around a very dormant star.”
