The sun, seen through the composite eyes of two space telescopes.
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The sun, seen through the composite eyes of two space telescopes.

NuSTAR Stares at a New Star

A space telescope designed to peer into the guts of distant dead stars has swiveled to gaze at one that’s very much alive: The sun.

This first image of the sun from NASA’s NuSTAR telescope, released today, suggests that high energy X-rays tend to cluster near active regions on the solar surface — just not all of them.

NuSTAR’s eyes are sensitive to these high-energy X-rays, which are emitted by the hottest, most energetic astrophysical processes.

In the image above, the bluish-green patches are X-rays detected by NuSTAR that occur near areas where gas has been heated to more than 5 million degrees Fahrenheit. The reddish sun is an ultraviolet image captured by NASA’s Solar Dynamics Observatory, which offers a glimpse of lower-energy, lower-temperature material. Taken together, the two views suggest those high-energy X-rays aren’t necessarily being produced by all of the sun’s active regions — visible as loops and swirls — but that they do cluster in the areas above some active regions.

“Although the structures can look similar in the extreme ultraviolet images, they in fact contain material at different temperatures,” writes astrophysicist Iain Hannah of the University of Glasgow. “We don’t know the exact mechanism(s) that do the heating.”

Years ago, scientists working on NASA’s NuSTAR mission realized the telescope could safely stare at the sun. It’s a feat that other X-ray observatories — NASA’s Chandra X-Ray Observatory, for example — can’t do. Like our eyes, those telescopes’ detectors would be damaged by a solar staring contest. NuSTAR, however, is blind to everything but the highest-energy X-rays, and the sun doesn’t produce enough of these to be harmful to the detectors.

So, scientists recently turned the telescope to stare deep into the sun’s atmosphere, and they expect to continue observing our closest star as it slips into a less active state. Maybe, they say, some of those observations will even solve some abiding mysteries. Among those is the question of whether a particular type of hypothetical dark matter particle exists. Called axions, these particles might be visible as a blob of X-rays in the center of the sun (which would be cool, but scientists admit that’s a longshot).

A more realistic observation might solve the puzzle of why the sun’s outer atmosphere is so dang hot. At an average of 1.8 million degrees Fahrenheit, it’s more than 100 times hotter than the solar surface. One explanation for this discrepancy is a hypothetical class of mini-solar flare, appropriately termed “nanoflares.”  Nanoflares are like the normal flares that erupt from the surface of the sun and sometimes fling blobs of charged particles at Earth — they’re just much, much smaller, and it would take a tremendously sensitive telescope like NuSTAR to see them.

“These regions are hot even though there doesn’t appear to be flares happening there at the time,” Hannah says. “The question is whether there is small flare-like activity doing the heating, or something else.”