planetary nebula; NGC 6720, M 57
A large cloud of dust and gas released from and surrounding a massive hot star - typically associated with a Wolf-Rayet, O or very occasionally, a hot B star.
A ring nebula is a huge cloud of dust and gas that is shaped like a ring. Pictured above is M-57, a ring nebula in Lyra which is 2,000 light-years from Earth and was generated by a dying star (this nebula was discovered by Ch. Messier).