Planetary nebula. Artwork of a planetary nebula dissipating at the end of its life. Planetary nebulae are unrelated to planets, instead being the outer layers of gas cast off by a dying star, which becomes a white dwarf. Radiation from the white dwarf causes the shell of gas to glow, and it appears as a disc or ring from Earth. Through early telescopes they had the appearance of planets, and were named due to this. As the shells of gas expand further, they dissipate until the gas is no longer visible. The white dwarf, a dense, hot star, cools and fades over billions of years. The Sun will form a planetary nebula in some 5 billion years time.