False-colour X-ray image of the Sun's corona, made by a telescope payload of a sounding rocket on 23 October 1987. This image was made in wavelengths of 171-175 Angstroms (1A = 0.1 nanometres). Most of the X-rays were emitted by excited iron atoms in the Fe-IX and Fe-X emission lines, at a temperature of around 1 million Kelvin. The corona is the outermost part of the solar atmosphere, rising up to 75,000km above the Sun's surface, and is mainly made of slow-moving particles.