This talk by Dr Stephen B Foulkes and Martin Stubbs will introduce astronomical nebulae.

Many nebulae are created from the gravitational collapse of large gas regions in the interstellar medium. As the material collapses under gravity massive stars may form in the centre and radiate ultraviolet radiation that ionises the surrounding gas, making it visible at optical wavelengths. The size of these nebulae varies depending on the size of the original cloud of gas. Other nebulae may form the final stage of a low-mass star, such as our Sun.

Stars with a mass up to 8-10 solar masses evolve into red giants and slowly lose their outer layers during pulsations in their atmospheres. When such a star has lost enough material, its temperature increases and the ultraviolet radiation it emits can ionise the surrounding nebula that it has thrown off.