NEBULAE...

Emission nebulae are clouds of gas that are being made to glow by a nearby source of radiation - usually a star or cluster of stars. Quite often emission nebulae are star-forming regions, such as the vast Lagoon Nebula, where clusters of emerging and young stars sit nestled inside the vast glowing expanse of the nebula. As the maternal cloud of gas is buffeted by harsh radiation from the hot young stars within it, it begins to glow. It is this emitted light that we see on Earth. Their mass generally ranges from 100 to 10,000 solar masses and this material can be spread over a volume of less than light year to several hundred light years. For this reason, their densities are highly varied.

Against the backdrop of a bright nebula or a rich star field, a dark nebula appears as a silhouetted swirling mass. Dark nebulae are cold clouds of gas and dust that absorb light from stars or a bright nebula behind them. The iconic Horsehead Nebula in the constellation of Orion is perhaps the most famous dark nebula of all. It has a total mass of about 300 times that of the Sun. Within the dark cloud from which it "rears" is a scattering of young stars that are in the process of forming. The Coalsack Nebula, in the constellation of Crux in the southern hemisphere, is another excellent example. It blots out the light from a dense Milky Way star field that lies behind it.

Most nebulae have tiny particles of dust inside them, which can reflect light from a star nearby. The density of the dust surrounding young stars is sufficient to produce a noticeable optical effect, and nebulae that appear to shine due to this reflected light are therefore called "reflection nebulae". Because the dust inside these nebulae is more effective at scattering the shorter-wavelength blue light, these nebulae usually appear pale blue in colour. The stunning wispy streaks around the bright star Merope, in the Pleiades star cluster show the characteristic blue colour. While Merope is easy to see through a small telescope, the nebula around the cluster is much harder to observe.

Planetary nebulae are heated halos of material shed by dying low-mass stars in the red-giant phase of their lives. They were named planetary nebulae by William Herschel in 1785, who thought they resembled planets. As a red giant dies, it expels the layers of gas in its outer atmosphere to expose the very hot core - a dense ball known as a white dwarf. Planetary nebulae are subject to contortion by magnetic fields and occasionally the influence of other stars. Their incredible shapes - such as the Dumbbell Nebula  makes them some of the most intriguing sights in the night sky. This stage of a star's life is relatively short, and eventually the nebula will disperse back into the interstellar medium.