Keeping Tabs on the Dangerous, Unpredictable Sun
Using new technologies and centuries-old records, astronomers are trying to learn the habits of our ever-changing star.
Astronomy, a science born of darkness, owes an awful lot to our nearest source of light. After all, so much of what we know about other stars—their makeup, the way they form larger systems—is extrapolated from our study of the sun.
Yet it is all but impossible to sidle up to the sun and ask what makes it tick. Computer simulations can help, but they still can’t accurately predict things like solar flares, which can wreak havoc on Earth. To construct new views of the sun, scientists are using the most powerful computers ever built, combined with some of the oldest instruments in the history of astronomy.
“The sun is the most important astronomical object for humanity,” says Joe McMullin, project manager for the Daniel K. Inouye Solar Telescope (DKIST), a massive observatory being built on Hawaii’s Haleakala volcano. The new telescope will complement a fleet of observatories in space to paint an ever-clearer picture of the most important star in our sky.
Our scientific picture of the sun dates all the way back to Galileo. The father of modern astronomy is most famous for upending the cosmic order through his observations of the other planets. But he was also the first (or maybe second) European, in 1610, to observe the sun through a telescope. His student Benedetto Castelli figured out how to project a telescope image of the sun onto a wall in a dark room, enabling observations without frying the observer’s eyes. Galileo’s meticulous drawings based on these camera-obscura views are our earliest record of sunspots. Sunspots, in turn, were the first major evidence that the sun is hardly the passive disk most of us take for granted. Today we have this video, surely one of the most breathtaking livestreams on the internet, showing a churning and dynamic orb, captured by NASA’s Solar Dynamics Observatory.
Flares exploding from the sun’s surface pack a punch greater than a billion atomic bombs. Million-mph winds whistle from holes in the sun’s atmosphere. It experiences regular cycles of magnetism and activity every 11, 22, and maybe 100 years, yet it shoots out planet-sized fire-clouds, called coronal mass ejections, at unpredictable intervals. The radiation from these various conflagrations can gravely harm, or even destroy, life.
Earth’s magnetic field protects us from some of the radiation and deflects many of the sun’s hard-to-predict coronal mass ejections. This phenomenon, called space weather, is what produces auroras, or the northern and southern lights. But sometimes little Earth just cannot compete, and solar storms are powerful enough to knock out power grids and disable spacecraft. A particularly bad one in 1859, called the Carrington Event, produced auroras as far south as Cuba. Were a Carrington-sized coronal mass ejection to hit us today, the consequences would be catastrophic—imagine entire communications, navigation, and banking systems offline. This is one of the primary reasons scientists are trying to understand the physics of the sun.
This month, solar scientists described a new approach that uses data from the Solar Dynamics Observatory and a number-crunching method only possible with the most powerful supercomputers. Aaron Birch from the Max Planck Institute for Solar System Research says fiery magnetic loops driven by the sun’s dynamo are moving slower than scientists thought, so convection must be playing a bigger role.
New telescopes will make it possible to understand what’s happening in more detail; the DKIST will be the biggest piece of glass to ever train on the sun. But scientists are also still learning from the solar archives of the past.
In the century after Galileo, people began sunspot-tracking more assiduously, and one of the most prolific spot hunters was Johann Caspar Staudacher, an amateur observer from Nuremberg who used a three-foot-long telescope. Between February 15, 1749, and January 31, 1796, he produced more than 1,100 drawings of the solar disk and its sunspots. Using his records and a few others, scientists have developed several theories about sunspot frequencies and what causes them.
But comparing a centuries-old record with modern observations requires careful calibration, says Leif Svalgaard, an astronomer at Stanford University. Modern observations suggest a steadily rising number of sunspots since Staudacher’s records, but that’s not necessarily the case, Svalgaard says. It may just be that modern telescopes can see more detail, and there is nothing new on the sun.
“One way of checking that is to use telescopes with the same optical flaws that telescopes in the 18th century had,” he says.
Svalgaard is working with a group called the Antique Telescope Society to do these cross-checks, and make the modern record “backward compatible,” as he put it. Like Galileo four centuries ago, he makes careful drawings of his observations. “We make a drawing of the sun every day that the sun shines,” he says.
Svalgaard has an advantage Galileo did not, however: A host of much more powerful telescopes to compare them with. Svalgaard says his observations show modern scopes pick up roughly three times more sunspots than the antique ones. If he uses the same statistical assumptions in the entire Staudacher catalog, then modern solar activity is roughly on par with activity from Staudacher’s era, he says.
“We don’t think there has been any real increase in solar activity,” he says, which is in contrast to arguments that modern solar activity is higher than it has been in the past few centuries. “We believe the sun has not changed dramatically or significantly in its output over the past 400 years.”
Svalgaard shared his work earlier this summer at a meeting of solar scientists in Boulder, Colorado, the new home of the National Solar Observatory. His findings, while not published yet, are interesting for a few reasons. For one, climate models incorporate sunspot data; if they are less active than thought, it becomes harder to attribute the observed temperature increases to natural solar trends. And models of the sun itself also incorporate sunspot data. An edited historical record could change what we know about the sun, and its own atmosphere.
Sunspots appear dark because they are cooler than the rest of the sun, and they don’t do much on their own. They are accompanied by magnetic activity that can have dramatic changes in the star’s atmosphere, however. Computer models are helping scientists understand all this, but even today, computers aren’t powerful enough to reconstruct everything the sun is doing, explains Feng Chen, an astronomer at the High Altitude Observatory in Boulder, Colorado. He is combining different models to pick apart what happens in a sunspot, deep within the sun in areas that are not observable by us.
Or not observable yet. The DKIST will point its 13-foot mirror at the sun every clear day, and peer into its layers with a suite of instruments that can see in multiple wavelengths of light. Scientists will be able to directly measure the sun’s magnetic fields, which they hope will help them understand what the star is doing, and how we might predict its next moves.
“The sun feels like it’s constant; it doesn’t seem like it’s changing very much. But the reality is the sun is incredibly dynamic, and it affects us all on Earth,” McMullin says. Through telescopes, our modern view of the sun recalls the ancients, who saw the sun as a life-giving but capricious god—one whose prominence was unquestioned, and whose rhythms we would do well to understand.