In this artist’s impression, a superflare erupts from a young sun-like star. Credit: CfA/Melissa Weiss
Solar flares are bright flashes from the Sun that release large amounts of electromagnetic radiation. And while normal flares can release up to 1025 the joules of energy, so-called superflares, observed on other stars are up to 10,000 times more powerful. Even normal solar flares and the charged particles that often accompany them can impact Earth’s upper atmosphere, impeding communications signals and disrupting the functioning of equipment, aircraft, energy infrastructure and more. To prepare and protect these systems, it is critical to understand how, when and why our Sun produces these violent bursts of energy.
In an article published today in the magazine Science, astronomers addressed this challenge by analyzing data from NASA’s Kepler space telescope from more than 50,000 Sun-like stars, discovering nearly 3,000 superflares. This suggests that every star like our Sun experiences a superflare about once a century.
In addition to appearing similar to the Sun in terms of temperature and variability, these stars also produce normal flares in the same general pattern as the Sun, so there is no reason to think that the Sun does not suffer from superflaring at the same rate. And since we haven’t seen one this big in recent history, we might be due.
The hunt for data
While our Sun emits small flares almost constantly, larger flares can swamp Earth’s upper atmosphere, dramatically altering how radio and other signals travel through it. Flares are often accompanied by coronal mass ejections (CMEs), explosions of charged particles that can more directly damage satellites and aircraft, as well as cause spectacular auroral displays.
Astronomers have never directly observed a large superflare on the Sun. Many have avoided the definition, hovering just above the 10-degree dividing line.25 joules, including (according to estimates) the famous Carrington event of 1859, as well as the Halloween solar storms of 2003. (For comparison, 1025 joule is approximately 16,000 times the total amount of energy produced globally in a year). And there is indirect evidence in rock records that suggests we may have received more powerful energy waves in the distant past.
This is because, fortunately, superflares are much rarer than their smaller brothers. But this also means that scientists have few examples to study to learn more. Astronomers have only been observing flares for a few centuries, leaving them with only a handful of truly powerful recorded events. To gather more data, they can look for indirect evidence (the small chemical fingerprints that strong CMEs leave on rocks) of solar activity before this period. Or they can observe the behavior of stars similar to our Sun.
Astronomers know that, with few exceptions, stars of the same size and temperature are essentially similar and will follow the same evolutionary life cycle. Because stars operate on celestial timescales of billions of years, astronomers can therefore learn about stars not by observing one star over eons, but by observing many stars at different stages of the same life cycle.
The Kepler space telescope is known for its exoplanet searching capabilities. He did this by staring at a patch of sky with half a million visible stars, waiting for the stars to blink. Its main mission was to look for small dips in light caused by an exoplanet crossing in front of its star, blocking some of the light. This meant that it collected data on all observed stars, planetary or otherwise, and how their brightness varied over time. This created a rich data set that informed astronomers about stellar variability of all kinds.
Twins!
That dataset is the same one mined in the new work, led by Valeriy Vasilyev of the Max Planck Institute for Solar System Research in Göttingen, Germany. The team identified 56,450 stars similar to our Sun in terms of temperature and intrinsic luminosity. (This sample excluded stars known to be particularly young or from binary systems.) In four years of Kepler observations, the researchers discovered 2,889 superflares out of 2,527 stars. Their energies ranged from 1026 at 1029 joules, or 10 to 10,000 times the maximum energy of a normal eruption.
The researchers point out that four years of data on their more than 50,000 stars is equivalent to being able to observe the Sun for 220,000 years – a true hat trick.
Overall, the team found that these stars exhibited flare behavior similar to that of the Sun: they produced many small flares, fewer large flares, and very few huge flares. So, even though the Sun has never shown a true superflare in history, it appears that stars do so just like our Sun, and at cosmically short frequencies of about once a century.
This is in line with previous research that had studied much fewer stars, yet had obtained a relatively similar interval of around 350 years between superflares.
So will our Sun emit a superflare in our lifetime? Only time will tell.
