The Sun Appears To Be Less Active Than Similar Stars

The sun caught releasing a solar flare in October 2015. NASA/SDO

The light from the Sun is responsible for supporting almost all life on Earth, and despite occasional flares that put technology at risk, it is a quiet, well-behaved star. Now, a comparison with similar stars reveals that the Sun is possibly unique or just going through a long inactive phase. 

As reported in Science, an international team investigated stars like the Sun to see how they stack against their galactic twin. The astronomers looked for stars that were at the same evolutionary stage and of similar age, surface temperature, composition, and rotation period. The rotational speed is key because it is linked to the magnetic field of the star, the main driving force in producing fluctuations in activity.

The team used data from NASA’s Kepler Observatory to estimate the rotation period of the stars. This was possible only for stars that were forming stellar spots so that their motion across the stellar disk could be tracked. The team picked stars that rotate every 20 to 30 days, and then further refined the sample using stellar properties observed by the European Space Agency’s Gaia space telescope.

The final selection was 369 stars whose stellar variations were measured between 2009 and 2013. The difference between active and inactive phases for the Sun is about 0.07 percent. A few stars in the sample were similar to the Sun, but for most of them, the difference in activity was typically five times as strong as the difference in activity seen for the Sun. This average value is also 1.8 times higher than the maximum solar variability value. 

Brightness variations of the Sun in comparison with the star KIC 7849521. MPS / hormesdesign.de

Why do the Sun and a few other stars appear to be different?

The scientists are not certain. To investigate further, they looked at a wider sample of Sun-like stars that were excluded from the analysis because they could not measure their rotations. For these 2,529 stars, the changes in activity were less pronounced and more in line with the Sun. Based on this, the team has two interpretations: The first is that there are yet-to-be-discovered factors that separate Sun-like stars into either an active or less active group. The second is that a single group goes through phases of high activity and low activity.

"It is just as conceivable that stars with known and Sun-like rotation periods show us the fundamental fluctuations in activity the Sun is capable of," co-author Dr Alexander Shapiro, of the Max Planck Institute For Solar System Research (MPS), said in a statement.

Based on indirect analysis using tree rings and ice cores, scientists believe the Sun has kept the same level of activity for the last 9,000 years with no wild peaks or lulls of activity.

"However, compared to the entire lifespan of the Sun, 9,000 years is like the blink of an eye," lead author Dr Timo Reinhold, also at MPS, added. "It is conceivable that the Sun has been going through a quiet phase for thousands of years and that we therefore have a distorted picture of our star."

The team stated that the Sun might “alternate between epochs of low and high activity on time scales longer than 9,000 years,” although given the current data, we can’t be sure if it does.

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