Nebulae were once an astronomical mystery. The word nebula - plural nebulae - is Latin for cloud. Through seventeenth and eighteenth-century telescopes they just looked like cloudy bits of sky.
Most astronomers simply ignored nebulae. William and Caroline Herschel were the first to take them seriously, but even William Herschel could only speculate about what they were. We now know that some of the fuzzy patches are large star clusters and others are galaxies. The rest are true nebulae - giant clouds of gas and dust in the spaces between the stars.
The Herschels cataloged over two thousand nebulae, but nebulae couldn't be studied until the late eighteenth century when the techniques of photography and spectroscopy became available.
Diffuse nebulae are the largest ones, but the material in them is so tenuous that an industrial vacuum on Earth is denser. There is nonetheless a lot of matter in them because nebulae can be spread out over many light years. The Orion Nebula, for example, is about 150 light years across. Compare this to the Solar System - its diameter is only two light years out to the Oort Cloud.
Since gas and dust don't emit light, nebulae continue to be difficult to study. However two kinds are made visible through illumination by nearby stars: emission nebulae and reflection nebulae.
The gas in an emission nebula glows red when it's energized by the light from bright stars. On the other hand, you can see reflection nebulae because the dust in it reflects starlight. They tend to appear blue in color like the Witch Head Nebula. Although this nebula is reflecting light from the blue supergiant Rigel (Orion's left foot!), it isn't the blue star the makes it look blue, but the fact that the dust scatters the blue light more effeciently than the redder light.
Other nebulae are only seen because their dark shapes stand out against visible backgrounds. They are called dark nebulae. In this pair of pictures of the Horsehead Nebula the nebula is clear against the background, but only in visible light. Infrared radiation can actually penetrate the dust. In the Herschel Space Observatory picture on the right, a bright star-forming region shows up.
Dark nebulae haven't been a historic feature of western astronomy. The Herschel catalogs didn't include them, though William Herschel did note the existence of "holes in the sky." They do often look like strange, starless regions. Yet the constellations of Australian aboriginal astronomy do include dark nebulae. One of the best known is the Flying Emu, here imaged by Barnaby Norris. It's near Crux (the Southern Cross) and Scorpius (the Scorpion), and has the dark Coalsack Nebula as its head. Terrestrial emus, of course, don't fly, being large flightless birds which are cousins to the ostrich.
Stars can form from the vast accumulation of matter in these nebulae. Usually, the process begins with a disturbance that starts the gravitational collapse of the material. Since this occurs in different parts of the nebula, stars form in groups. Such an area of developing stars is often called a stellar nursery.
Most of the matter in nebulae is primordial hydrogen, which means that it formed shortly after the Big Bang. Heavier elements are made in stars, so nebulae are now enriched with elements from previous generations of stars. In fact, two further types of nebula are actually formed from dying stars: planetary nebulae and supernova remnants.
Planetary nebulae aren't connected with planets. However the name has stuck since the eighteenth century when Herschel observed some of them showing a planet-like disk through a telescope. They form when a sun-star runs out of hydrogen fuel. It swells into a red giant, throwing off the outer layers of its atmosphere. This often occurs fairly symmetrically, leaving something like the Little Ghost Nebula. This will happen to the Sun several billion years from now.
If a star is many times more massive than the Sun, when it finally runs out of nuclear fuel, it explodes as a supernova, releasing vast amounts of energy. For a time, a supernova shines as brightly as an entire galaxy. In such extreme conditions, the heaviest chemical elements are forged. Then, although the core of the star collapses into a neutron star or black hole, the outer layers form a nebula called a supernova remnant. Probably the most famous one is the Crab Nebula (Messier 1), which is the remnant of a supernova witnessed by Chinese astronomers in 1054.
With the advent of infrared telescopes, nebulae became a particularly promising field of study. For example, they can tell us something about the chemical elements of which we and our world are made. Astronomers are finding complex organic molecules in nebulae, suggesting that they might also have something to tell us about the origins of life in the Galaxy.
You can see all of the nebulae in this article and many others on my Pinterest board Nebulae.
(1) H. Frommert & C. Kronberg, "Nebulae" http://seds.org/messier/nebula.html
(2) Ray Norris, "In Search of Aboriginal Astronomy" http://www.atnf.csiro.au/people/rnorris/papers/_n217.pdf