Many scientific terms are very specific, but the term nebula is, well, nebulous. In a very generic sense, it simply means a fuzzy thing. But there are a wide variety of astronomical objects that can appear fuzzy. In the late 18th century Charles Messier made a list of “nebulae” that he didn’t want to confuse with comets. Many of the objects on his list were simply clusters of stars that were hard to resolve. But his list also included many “spiral nebulae,” which we now know are galaxies. I will quickly get in way over my head if I try to decipher English parts of speech, but the originally accompanying adjectives have evolved into necessary halves of compound nouns that help us differentiate, for example, between a reflection nebula and a planetary nebula.
Without any modifier, it is common to associate a nebula with clouds of interstellar gas in star-forming regions that emit their own light. These emission nebulae are mostly hydrogen gas that has been excited by ultraviolet radiation from nearby stars. While earthbound hydrogen is found exclusively in molecules, the pressures and temperatures found in many interstellar gas clouds allow individual atoms of hydrogen to exist.
Consisting of a single proton and a single electron, hydrogen is not only the simplest atom, but by far the most abundant in the universe. It takes a high-energy ultraviolet photon to excite an electron in a hydrogen atom, but then the atom will reradiate light as the electron falls back to lower energy levels. The simplicity of this atom results in very few spectral lines in the visible region, with the most prominent being the red, hydrogen-alpha line with a wavelength of 656 nanometers that gives these emission nebulae their distinctive color.
The color shows up easily in photographs, but unfortunately for visual observers, our eyes just aren’t sensitive enough to see anything but pale gray. An excellent example of an emission nebula is M42, the Orion Nebula, which is visible to the naked eye as part of Orion’s sword.
Reflection nebulae are also illuminated clouds of gas, but they are not glowing on their own. Just as our atmosphere preferentially scatters blue light from the sun, making the sky blue, reflection nebulae are scattering light from nearby stars. Because the scattering mechanism is the same as our atmosphere, the color of a reflection nebula is very similar to the color of the sky on a clear day. M42 has reflection components, but M78, another nebula in Orion is more noticeably blue.
A dark nebula is just what it sounds like. It is a cloud of interstellar gas and dust that is blocking light from more distant objects. A classic example of a dark nebula is the Horsehead nebula, also in Orion, near Alnitak, the left-most of the three belt stars.
The name planetary nebulae comes from their appearance as small, fuzzy disks being similar to that of planets as seen in early telescopes. Despite their name, they have nothing to do with planets but are a short-lived stage during the end-of-life events of a sun-like star. They are technically emission nebula, because they do emit their own light, and many are colorfully photogenic. NGC 2022 is a planetary nebula in Orion, but a more well-known one is M57, the Ring Nebula in Lyra.
Supernovae remnants are another type of nebula. Examples include the Crab Nebula, M1 or the Veil Nebula, both of which are visible through modest-sized telescopes.
The outer planets are still dominating the evening sky, but it is the time of year that our winter constellation favorites rise in the east. This includes Orion, where you can find many nebulae.
Mars made its closest approach to Earth on Dec. 8, so the next couple of months will be the best time to look at it for the next couple of years. It is the really bright object in the Eastern sky after sunset.
The evening of the winter solstice on Dec. 21 is a good time to look for Mercury in the Western sky. It will be 20 degrees away from the sun, which is its highest point above the horizon for this orbit.
The Geminid meteor shower is often one of the better ones during the year. Any clear evening next week should be a good time to look for meteors, but the peak is on Tuesday and Wednesday.
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Old Fort Lewis Observatory
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory.