NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) is the most sensitive high-energy X-ray telescope ever invented. It recently took its first picture of the Sun, and it looks absolutely amazing. It's like the prettiest Christmas tree ornament.
NuSTAR's image is laid over another picture by Solar Dynamics Observatory. According to NASA:
Green shows energies between 2 and 3 kiloelectron volts, and blue shows energies between 3 and 5 kiloelectron volts. The high-energy X-rays come from gas heated to above 3 million degrees.
The red channel represents ultraviolet light captured by SDO at wavelengths of 171 angstroms, and shows the presence of lower-temperature material in the solar atmosphere at 1 million degrees.