Ring nebula (M57), optical image. The Ring nebula is a planetary nebula lying around 2300 light years from Earth in the constellation Lyra. Planetary nebulae are glowing shells of gas and plasma ejected by medium and low-mass stars at the end of their lives. As the shells collide with interstellar material, the energy of the collision ionises the gas at the interface, causing it to glow. The Ring nebula is actually barrel-shaped, appearing round because we see it end-on. It is illuminated by a central white dwarf star.