Nebulae were once an astronomical mystery. The word nebula - plural nebulae - is Latin for cloud, because 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 more dense. 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. In comparison, the Solar System is only about two light years in diameter.
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 because it's energized by the light from bright stars. For example, you can see the glowing hydrogen in the nebula called IC 1396. The area shown in this picture by Nick Wright is about twenty light years across.
We can see a reflection nebula because the dust in it reflects starlight. The Witch Head Nebula, here imaged by George Greany, reflects light from the blue supergiant Rigel (Orion's left foot!). What makes it blue isn't the blue star, but the fact that dust absorbs red light and reflects blue 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 the familiar dark features of the Horsehead Nebula are seen against the lit background, but only in visible light. Infrared radiation can actually penetrate the dust and 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. A common one 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. It needs a disturbance to cause the gravitational collapse of the material. Since this occurs in different parts of the nebula, stars tend to form in clusters. 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, but the name has stuck since the eighteenth century when some of them showed a planet-like disk through a telescopes. They form when a star like the Sun runs out of hydrogen. It swells into a red giant, throwing off the outer layers of atmosphere. This often occurs fairly symmetrically, leaving something like the Little Ghost Nebula, here imaged by the Hubble Heritage Team. 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 these 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.
The Crab Nebula is the remnant of a supernova witnessed by Chinese astronomers in 1054. This stunning image is a mosaic of Hubble Space Telescope pictures processed by Davide De Martin. It shows an area about twelve light years across.
With the advent of infrared telescopes, nebulae became a particularly promising field of study. We know that they can tell us something about the chemical elements of which we and our world are made. In addition, 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 several others in a collection I've made on a Pinterest board. Just click here.
(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