Nebulae come in many sizes and shapes. From tiny planetary nebulae that look almost stellar or hardly larger than a planet to vast nebula complexes spanning many degrees of sky.
Some of these nebula are amongst the most beautiful sights known to astronomers. Like for instance the great andromeda nebula.
Nebulae can show themselves in different ways. Emission nebulae are nebulae that emit light by themselves. This can be due to radiation from nearby stars ionizing the gas or by the gas really coming to light due to pressure waves.
Other nebulae don't emit light by theselves but instead reflect the light of nearby stars. often these nebulae are blue. A great example are the bluish nebulae in the Pleiades
Some nebulae don't show themsves by light. but instead by darkness. Regions of dark dust and gas block light of stars behind them. A good example of such a dark nebula is the Horse head nebula. Which blocks the light of the cloud of glowing gas behind it. Or the coal sack that blocks the light of the stars behind it.
Nebulae often mark either the birth or the violent death of stars. Dust to dust on a larger scale
Planetary nebulae have nothing to do with planets. They got that name long before people understood their true nature.
Not all nebulae show their exstance by emitting light. Dark nebulae are clouds of dust that block the light of the stars behind them. Showing them as dark streaks in the sky.
© Wouter D'hoye 2016 - Last update February 1st 2015