Whats the sun got in common with distant black holes? Well, at first glance, not a lot. But as this psychedelic solar portrait shows, there is one trait that the sun and black holes do have in common the emission of high-energy X-rays.

Now NASAs Nuclear Spectroscopic Telescope Array, or NuSTAR, has turned its gaze from distant black holes and focused on our sun, producing the most sensitive measurement of high-energy solar X-rays ever achieved.

GALLERY: NuSTAR Probes a Spinning Black Hole

Long before NuSTAR was even launched in 2012, solar physicist David Smith, of the University of California, Santa Cruz, approached the NASA NuSTAR mission team to request that the space telescope spend some of its observing time looking toward our nearest star.

Shifting focus from the high-energy X-rays generated by supermassive black holes in the centers of galaxies millions of light-years away to the sun may seem strange, but only NuSTAR has the capability of sensing the faint high-energy X-ray flashes generated by small-scale solar flares known as nanoflares deep inside the suns atmosphere, or corona.

At first I thought the whole idea was crazy, said NuSTAR principal investigator Fiona Harrison of the California Institute of Technology in Pasadena, Calif. Why would we have the most sensitive high energy X-ray telescope ever built, designed to peer deep into the universe, look at something in our own back yard?

ANALYSIS: Monster Waves Behind Suns Coronal Heating Mystery?

Staring at the sun is as an unhealthy proposition for space telescopes as it is for the human eye. NASAs Chandra X-ray space telescope, for example, would be blinded if it turned its gaze toward the sun as our nearest star generates a broad spectrum of lower-energy X-rays. But NuSTAR is unique in that it only detects the highest energy X-rays (and doesn't see the low-energy X-rays Chandra is sensitive to) that are generated by powerful relativistic processes surrounding black holes.

And it is high-energy X-rays, which the sun very weakly radiates, that Smith is interested in. But why?

Solar physicists and space weather forecasters have been puzzled for decades as to why the suns corona is so hot. On comparison with the suns surface the photosphere which has a temperature of a few thousand degrees Fahrenheit, the corona is (on average) 1.8 million degrees Fahrenheit (1 million Kelvin). That doesnt make sense in our everyday experience; it would be like the air surrounding a light bulb being hotter than the bulbs glass, a situation that completely violates basic thermodynamic laws normally it gets cooler the further you step away from a heat source, not hotter!

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NASA's Black Hole X-Ray Hunter Could Solve Solar Mystery