planetary nebula
Shell of gas thrown off by a star at the end of its life. Planetary nebulae have nothing to do with planets. They were named by German-born English astronomer William Herschel, who thought their rounded shape resembled the disc of a planet. After a star such as the Sun has expanded to become a red giant, its outer layers are ejected into space to form a planetary nebula, leaving the core as a white dwarf at the centre.
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