A planetary nebula, singular for nebulae, is a shell of illuminated gas surrounding certain dying stars. Planetary nebulae have central stars and these stars were once lower to medium-sized stars, like that of our sun, which eventually became giant stars, and will soon become white dwarfs. Anyways, other lessons discuss this in way more detail, but I'll give you a very quick and simple overview of what happens during the life of these stars to help understand the rest of our lesson better.
