What's 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 NASA's 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 sun's 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 Sun's Coronal Heating Mystery?
Staring at the sun is as an unhealthy proposition for space telescopes as it is for the human eye. NASA's 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 sun's corona is so hot. On comparison with the sun's '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 doesn't make sense in our everyday experience; it would be like the air surrounding a light bulb being hotter than the bulb's glass, a situation that completely violates basic thermodynamic laws - normally it gets cooler the further you step away from a heat source, not hotter!
ANALYSIS: Magnetic 'Braids' May Cook the Sun's Corona
So in an effort to explain this mysterious coronal heating phenomenon, solar physicists have arrived at two key theories that have some observational evidence. Magnetohydrodynamic waves - basically waves that travel from the sun's interior and through the magnetized corona - are thought to resonate with the energetic coronal plasma, causing a heating effect. Another theory suggests that tiny 'reconnection' events in the magnetic field of the corona cause rapid heating of the coronal plasma, generating nanoflares. If these nanoflares occur throughout the corona, perhaps they act as sparks that maintain coronal heating to millions of degrees.
Nanoflares are predicted to generate high-energy X-rays, but they have so far proven illusive as we haven't had the instrumentation to filter out all the noise.
"NuSTAR will give us a unique look at the sun, from the deepest to the highest parts of its atmosphere," said Smith, who is also a member of the NuSTAR team. "NuSTAR will be exquisitely sensitive to the faintest X-ray activity happening in the solar atmosphere, and that includes possible nanoflares."
ANALYSIS: Epic Solar Flare Pops Sun's Magnetic Cork
As the above image shows, a weak signal of high-energy X-rays have been detected by NuSTAR, but much more observation time is needed before we can definitively say whether or not nanoflares are causing it.
In addition to studying the emissions from possible nanoflares, the NuSTAR team are also taking a punt at the possibility of detecting a dark matter candidate deep inside the sun. Known as axions, these hypothetical dark matter candidates may be generating anomalous high-energy X-ray emissions in the solar core. But that's a long shot.