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Among the loveliest of sights in the heavens are the bright emission nebula. For northern observers, my choice for the finest is M42, the Great Orion Nebula, with M8, the Lagoon Nebula in Sagittarius, my choice for second. One could choose others for their preferences. Perhaps the North American Nebula in Cygnus, or the Veil in Cygnus (also called the Great Looped Nebula, the Filamentary Nebula, the Network Nebula and probably other descriptive names.) Some may find the fainter and more difficult objects to be their favorites.

But what is an emission nebula? Why is it different than any other nebula?

The answer is in the light we see, and how that light originated.

All nebula are clouds of gas and dust, at various distances from us, illuminated by the stars around and within them. The word nebula is from the Latin for 'cloud', and is as good a descriptive as one can find.

Years ago, before the external galaxies were recognized as being stellar swarms at vast distances from our galaxy, they were also referred to as nebula. Of course, they are now classified separately with various sub descriptions. But the term "Great Nebula in Andromeda' will persist til the end of time..... for us amateurs anyway. It is, after all, easily visible to the naked eye as a "nebulous" patch, as are some other bright objects which also look 'nebulous', even though they may be star clusters or external galaxies.

Back to the subject.

As we know, matter is composed of various elements. The commonest element in the universe (let's not go into alternate universes or theoretical nuclear physics here) is hydrogen. Much of the mass of interstellar gas is hydrogen just floating merrily around in it's elemental or molecular state. As elemental hydrogen, it consist of one proton, one electron and may contain a neutron or two if it's either the isotope deuterium or tritium.. Regardless, we're going to call it a simple hydrogen atom.

It happens that energy of all wavelengths is madly zinging around. The longest wavelengths are far beyond our visible red, in the 'radio wave', microwave and infrared range, and contain very little energy. Short of the visible blue, we have the ultraviolet, X-rays, gamma rays, cosmic rays etc. The shorter the wavelength the more energetic. I believe the formula is the 4th power of the ratio of the wavelengths, but don't quote me on that one. Suffice it to say that anything in the visible blue and shorter contains lots of energy and the amount goes up dramatically as the wavelength shortens further.

So, to relate all this. The electrons in an atom can be in a number of energy states. The higher the number of the energy state, the more energy it contains. Eventually, these electrons will settle back to their lowest state. It may go in one jump, or it may move a level or two at a time ('cascading') until it reaches home. It happens that this electron will emit light as it moves to a lower state. This is a natural consequence of atomic level life. It obviously required energy input to move that electron to a high energy state, so it must release that energy to move to a lower one. This is related to the 'conservation of matter and energy',

When the electron jumps down and emits light, the light is at a specific wavelength for that jump. If the jump is from level two to zero, the light will always be a specific wavelength (you can think of the wavelength as a color). In other words, a jump from level two to zero may always emit blue, while a jump from level one to zero will emit red. Remember that the longer the jump, the shorter the wavelength. Blue is shorter than red and contains more energy.

It also happens that the energy absorbed in moving this electron can only be at a number of discrete levels. Suffice it to say, if the energy is too high or low that particular packet of energy will not be absorbed by the atom. The energy absorbed in an electron jump will match the energy released. This is 'quantum mechanics'. Energy levels are referred to as 'quanta' from the Greek for 'amount'

Enough theorizing.

So here sits this cloud of hydrogen and other elements, say about 10,000 suns worth of mass. If there are nearby stars, one of two things will happen to this mass. If the stars are very hot, say over 20000C, the light from this star will be energetic enough to knock the electrons into a higher orbit. Energy is absorbed. Well, since nature wants everything to be at it's lowest energy state, eventually this electron will go back to ground, emitting a packet of light as it does so. If this light is in the visible band, we can see the nebula glowing. Some of the common wavelengths generated in this process, which are visible to us, are the red of hydrogen alpha and the green of oxygen.

Many of us have seen this particular nebula. It is the Great Orion Nebula, M42. Actually, the entire constellation of Orion is immersed in a vast cloud of glowing hydrogen, but the illumination is too low for our eyes to detect. The enormous arc called Barnards Loop, which circles much of the constellation, the Horsehead and most of the other patches in Orion are all part of the same immense cloud. The energy is provided by the young blue stars emitting copious ultraviolet.

The electron is kicked way up when UV light hits it, but it may come back to the lowest level in steps, so we can see some of the energy it emits in the visible band.

This whole process is called fluorescence, and is the same thing as a mineral glowing in response to an ultraviolet light source. Or the light of a neon tube, or a fluorescent lamp.

Why, you may ask, does the Orion Nebula look white or slightly greenish to me, if it also emits huge amounts of red, and looks red in photos? Good question. The eye is relatively insensitive to hydrogen alpha at 6563 Angstroms (656.3 nanometers for metric freaks), but is very sensitive to the blue green light emitted. Photographic film composed of three layers sensitive to different colors happens to have a weak spot in the blue green where the layer sensitivities overlap. So most photos look reddish. This is also why some films show my aurora shots as redder than they were to the eye (note: the film processing can also affect this). Aurora also are fluorescing gases, usually oxygen (the red and the greens are mostly form oxygen) with some nitrogen, argon and others thrown in for the off the wall colors sometimes seen.

Clouds of gas can also simply reflect light from stars, in which case it is referred to as a 'reflection nebula'. This light is usually white to the naked eye, and somewhat bluish to the photographic film.

Enough for now. This not a course in atomic physics. There is much info to look into if one is interested. I'm sure I've made some mis-statements in here, but it's the best I can do.

So next time you look through a scope at one of these marvels, contemplate the enormous energies emitted by the blue stars which light up the emission nebula. For a thought.......the North American Nebula is partially illuminated by Deneb, a blue supergiant star about 100 light years from the cloud. That's a lot of distance to cover and still provide one of the finest sights in the sky.

This article was provided by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.