Kepler supernova remnant. Chandra X-ray Observatory image of the Kepler supernova remnant, an expanding cloud of glowing gas that formed following a supernova, the explosive death of a large star. Also known as SN 1604, this object is around 13,000 light years from Earth in the constellation Ophiuchus. The supernova occurred in 1604, and was observed by the famous astronomer Johannes Kepler. The energy of the expanding gases is high enough to generate X-rays, colour-coded here as low energy (red), medium energy (yellow/green) and high energy shockwaves on the outer edges (blue).