Planetary nebula PK164+31.1, optical image. A planetary nebula is a series of shells of gas cast off by a Sun-like star near the end of its life. The shells comprise the upper atmosphere of the star, which is cast off due to instabilities in its core. Radiation from the newly exposed core then ionises the gas in the shells, causing them to emit light. A planetary nebula is a transient object: as the gases becoming too dispersed and the central star cools, the shells are no longer ionised and the nebula fades. This nebula lies in the constellation Lynx.