Planetary nebula. Hubble Space Telescope (HST) image of the planetary nebula NGC 2440. These clouds of gas are the outer layers of an old star which have been ejected near the end of its life. The ejecta here has a more chaotic shape than in most planetary nebulae, which are usually ring- shaped. This is thought to be due to the ejections being periodic and random in direction. The central star is one of the hottest known, with a surface at 200,000 degrees Celsius. Radiation from this star causes some of the surrounding gas to glow. NGC 2440 lies around 4000 light years from Earth in the constellation Puppis.