A cloud of gas and dust in space, nebulae are the birthplaces of stars.An emission nebula, such as the Orion nebula, glows brightly because its gas is energized by stars that have formed within it. In a reflection nebula, starlight reflects off grains of dust in the nebula, such as surrounds the stars of the Pleiades cluster. A dark nebula is a dense cloud, composed of molecular hydrogen, which partially or completely absorbs light behind it. Examples include the Coalsack nebula in Crux and the Horsehead nebula in Orion. Some nebulae are produced by gas thrown off from dying stars.
A shell of gas thrown off by a star at the end of its life.
Planetary
nebulae have nothing to do with planets. They were named by William
Herschel, who thought their rounded shape resembled the disc of a planet.
After a star such as the Sun has expanded to become a red giant, its outer
layers are ejected into space to form a planetary nebula, leaving the core
as a white dwarf at the centre.
The explosive death of a star.
Supernovae are thought to be the main source of
elements heavier than hydrogen and helium. The last supernova occurring in our Galaxy was in 1604, but have occurred since in other galaxies. In 1987 a supernova visible to the unaided eye occurred in the Large Magellanic Cloud.