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NASA Uses X-Rays to Find Out Why the Sun Is So Hot

The image will help scientists decide whether mini-flares make the sun’s atmosphere hotter than the surface itself

January 5, 2015

An image of the sun captured by NuSTAR for NASA — This image, created by a telescope called NuSTAR, is the most detailed of the sun using high-energy rays to date, according to NASA. NASA

NASA recently turned a high-energy telescope designed to gaze into the far reaches of space for black holes and supernova onto the sun. One result was the spectacular picture above, which was created by representations of data from the Nuclear Spectroscoptic Telescope Array, overlaid onto a photo of the sun. But NASA researchers are also hoping that by using NuSTAR to examine our own star, they might find the answer to a long-standing mystery: how could the sun's atmosphere be so much hotter than the surface below?

The sun's atmosphere (called the corona) pops at a relatively hot 1.8 million degrees Fahrenheit, while the surface remains a comparatively cool 10,800 degrees. In the above composite image, high-energy emissions from gases heated to as many as three million degrees appear in green and blue. The red shows ultraviolet light and atmospheric material that is roughly one million degrees.

Mission leaders for NuSTAR believe that the answer to the atmosphere-surface heat riddle may lie in closer examination of so-called nanoflares, which are analogous to (but smaller than) these larger emissions charged particles and radiation. As Amina Khan of the Los Angeles Times writes:

Scientists think that there may be tiny flares constantly popping off the sun’s surface, and that the magnetic field dynamics powering these so-called nanoflares might be what’s responsible for what’s super-heating the thin, tenuous corona.

Nanoflares have never before been glimpsed, but NuSTAR should be able to detect them: the telescope array is actually calibrated to detect rays even brighter than those in the atmosphere of the sun. That's an important advantage, as scientists have otherwise been unable to stare at the sun through telescopes for fear of permanently damaging sensitive detectors.