Planetary Nebulae

Planetary nebulae are formed at the end of life of low-mass stars, such as our Sun. In this phase the star changes as it inflates to a red giant and emits a part of its hull into interstellar space. A hot white dwarf is left behind, which excites the emitted gas layers to glow.
Many brighter of these nebulae are visible in the telescope as small, roundish disks that appear like faint images of the outer planets Uranus and Neptune. Since the first observers had never seen such objects before, they gave them the name "planetary nebulae", which, however, are not related to planets at all.