The Sun seen at the X-Rays by the NuSTAR space telescope

Combined image of Sun observations by NASA's NuSTAR and Solar Dynamics Observatory (SDO) satellites (Image NASA/JPL-Caltech/GSFC)
Combined image of Sun observations by NASA’s NuSTAR and Solar Dynamics Observatory (SDO) satellites (Image NASA/JPL-Caltech/GSFC)

NASA’s space telescope NuSTAR (Nuclear Spectroscopic Telescope Array), launched in June 2012, was built to study objects such as black holes and for this reason its sensitivity focuses on high-energy X-rays. For once, however, it was used in a way totally different from what was intended to observe the Sun in a new way.

The idea of using NuSTAR to study the Sun came to David Smith, a solar physicist and a member of the NuSTAR team, already seven years ago, during the construction of this space telescope. Fiona Harrison, the project’s principal investigator, initially thought it was a crazy idea but Smith convinced her that it made sense. The purpose of the proposal was to try to observe the faint flashes of X-rays that the Sun emits according to the theoretical predictions.

Generally, space telescopes are aimed well away from the Sun because the direct light of the star would damage their receptors. Instead, NuSTAR can look without problems at the Sun because it’s not bright at the high-energy X-rays detected by this telescope.

For a long time, the NuSTAR space telescope was booked to observe distant objects. All those researche were carried out and in the summer 2014 the NuSTAR’s primary mission ended and it received an extension of two more years. In the end it was possible to use it for this innovative observation of the sun.

The first observations showed that NuSTAR can actually help the research on the Sun. Its X-ray image has been combined with another taken by the Solar Dynamics Observatory (SDO) space probe. In the final image the NuSTAR data can be seen in the colors green and blue revealing high-energy emissions. In shades of red you can see the ultraviolets captured by SDO showing the lower temperatures of the materials in the solar atmosphere.

NuSTAR’s sensitivity to high-energy X-rays can for example help to understand the causes of the high temperatures above sunspots. Another possibility is to try to observe the hypothetical nanoeructions, smaller versions of the gigantic solar flares. Nanoeructions could explain why the solar corona, which is the outer part of the solar atmosphere, can reach a temperature of a million degrees Celsius when the surface of the Sun reaches about six thousand degrees Celsius.

The NuSTAR will space telescope keep on being used to observe blacks holes, supernova remnants and other objects very far away. However, it more observations of the Sun will be carried out, hoping to detect among other things traces of dark matter. Axions, hypothetical elementary particles associated with dark matter, may appear as spots of X-rays at the center of the Sun. The chances of finding them are low but whatever the result, the study of the Sun through NuSTAR has already proved very interesting.

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