After a dark cloud of gas and dust collapses into dense globules that ignite into stars, the residual material is set aglow by the intense blue and ultraviolet light from newly formed stars. The glowing gas surrounding the stars is called a “diffuse nebula”, an “emission nebula”, or an HII (pronounced “H-two”) region.
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. Diffuse nebulae also have traces of ionized oxygen which also emit light at a characteristic wavelength near 500 nm (blue-green).
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These nebula 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 nebula occur in the same area of star formation (see the image of the Trifid Nebula, below). In a way, diffuse or emission nebula are much like the neon lights you see on buildings and billboards. The lights use electricity to make gases glow, while a diffuse nebula get its energy from the light of new stars embedded within.
Even a random search with a small telescope along the plane of the Milky Way reveals many diffuse nebula, which look to the unaided eye like hazy patches of silver-white light. The sword of the constellation Orion contains one of the brightest and most famous such 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 nebula in July through September such as the Swan, Lagoon, Trifid Nebulae in Sagittarius and Eagle Nebula in the constellation Serpens. All you need is 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 cone cells in your eye. But in dark sky, these objects are still quite striking.
Diffuse nebulae don’t last long, at least in astronomical time scales. After a few million years, the hot young stars burn off the remaining gas and dust, leaving a small open cluster of gravitationally-associated stars.