A planetary nebula is a shell of low-density ("thin") gas that was ejected by the central star as it neared the end of its life and in
which the atoms are excited by absorbing high-energy ultraviolet radiation from the hot central star. The nebula emits radiation as the excited atoms
return to lower energy levels. The line patterns depend on which atoms in the nebula are most efficiently excited - in this case the lines are
due to Hydrogen (red & violet), Helium (red) and ionized Oxygen (green).
Spectrum of Planetary Nebula M57