On a dark night, as you gaze along the arc of the Milky Way, you will see small patches of diffuse light and other patches where there appear to be few stars. These are diffuse nebulae and dark nebulae, respectively. These patches are made of gas and dust, and the are places where new stars form.
Many of these patches are so-called dark nebulae where cool gas and interstellar dust block the view of the background stars. These nebulae are laced across the spiral arms of the Milky Way in irregular patches and rivulets without definite shape or boundaries.
Dark nebulae are part of cool, giant molecular clouds where dust and gas from old stars and gas from the earliest days of the universe slowly pull themselves together by gravity. The icy dust grains are less than 1/1,000 of a millimeter across, but they have an interesting chemistry, consisting of frozen nitrogen, carbon monoxide, ammonia, formaldehyde, and more complex organic molecules (even ethyl alcohol).
Because dark nebulae are the birthplace of stars and planets, astronomers find them intensely interesting. Computer modeling shows that although the nebulae are tenuous, with only a few particles per cubic centimeter, passing stars push and pull on the particles, causing them to coalesce into denser patches that begin to fall in on themselves and heat up. Hundreds of tiny globules in a dark nebula may eventually become hot enough to start the process of nuclear fusion, where hydrogen in the center of the globule begins to burn into helium, releasing huge amounts of energy. When this happens, dense globules of gas and dust turn into clusters of new stars which light up the remaining dust and gas into what astronomers call diffuse nebulae.
After a dark cloud of gas and dust collapses into dense globules that ignite into stars, the leftover material is set aglow by the intense blue and ultraviolet light from newly formed stars. The glowing hydrogen gas surrounding the stars is called an “emission nebula”.
These nebulae usually glow a reddish-pink color. That’s because the new stars excite the atoms of hydrogen gas that remain in the cloud, and the atoms relax again by emitting red light at 656 nm, a wavelength set by the structure of the hydrogen atom. Emission nebulae also have traces of ionized oxygen which also emit light at a characteristic wavelength near 500 nm (blue-green). In a way, emission nebulae are much like the neon lights you see on buildings and billboards. The lights use electricity to make gases glow, while an emission nebula gets its energy from the light of new stars embedded within.
These nebulae also contain a fair bit of dust that reflects the blue light of the new stars. The reflective dust is called a “reflection nebula”; in many cases, the two nebulae occur in the same area of star formation (see the image of the Trifid Nebula, below).
Both emission nebulae and reflection nebulae are sometimes called diffuse nebulae.
The Trifid Nebula in the constellation Sagittarius. Red light comes from hydrogen gas atoms excited by the light from stars within the nebula; blue light from these stars is reflected off fine dust particles back into our line of sight.
Even a random search with a small telescope along the plane of the Milky Way reveals many diffuse nebulae which look like hazy patches of silver-white light. The sword of the constellation Orion contains one of the brightest and most famous such nebulae. It’s often just called the Orion Nebula. A telescope gives you an astoundingly beautiful view of the Orion Nebula; no amount of observation is enough to reveal all its detail. You can see many more such nebulae such as the Swan, Lagoon, Trifid, and eta Carina nebulae with a small telescope or pair of binoculars. Just remember, you won’t see color when you observe such nebulae visually; there isn’t enough light to stimulate the color-sensing cells in your eye. But in dark sky, these objects are still quite striking.
Diffuse nebulae don’t last long, at least on astronomical time scales. After a few million years, the hot young stars burn off the remaining gas and dust, leaving a small loose cluster of gravitationally-associated stars.