NuStar portrait of sun
X-rays stream off the sun in this image showing observations from by NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, overlaid on a picture taken by NASA's Solar Dynamics Observatory. The NuSTAR data, seen in green and blue, reveal solar high-energy emission. The high-energy X-rays come from gas heated to above 3 million degrees. (NASA/JPL-Caltech/GSFC)

The "most sensitive" portrait of the sun has been taken with Nasa's NuStar telescope, originally designed to peer into black holes and search out Dark Matter.

Fiona Harrison, of the California Institute of Technology in Pasadena, said: "At first I thought the whole idea was crazy. 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?"

However, solar physicist David Smith, who works on the NuStar project, said the telescope can provide a "unique look at the sun", as it is able to look into the deepest and highest parts of the atmosphere without damaging its detectors.

The sun is too bright for other telescopes to look at safely. However, because the sun is not as bright in the higher-energy X-rays of NuStar, the telescope can be used to gather information about the sun, including the extremely high temperatures around sunspots, its winds and its solar cycle.

Scientists say the telescope could be used to search for dark matter particles known as axions: "NuStar may be able spot axions, one of the leading candidates for dark matter, should they exist. The axions would appear as a spot of X-rays in the centre of the sun," Nasa said.

Experts also believe NuStar can be used to capture nanoflares – unconfirmed smaller versions of the giant flares that erupt from the Sun. If they do exist, nanoflares might explain why the sun's outer atmosphere is "sizzling hot" – reaching one million degrees Celsius.

"NuSTAR will be exquisitely sensitive to the faintest X-ray activity happening in the solar atmosphere, and that includes possible nanoflares," said Smith.

Until more observations are made, NuStar will continue to be used for "its galactic pursuits", examining black holes, supernovas and other galactic events outside our solar system.