An Introduction to Nebulae
During a night under the stars, it's a common practice of amateur astronomers to include a bright nebula among the objects observed. Certainly, this is true especially on any night when the Great Nebula in Orion (M42) is visible. The range of nebulosity runs from such easy targets as M42 on down to wisps of light so faint that we are called-upon to summon all the observing prowess we possess.
Nebulae (the Latin plural of "nebula") are clouds in space composed primarily of gases. Although it's possible for us to detect nebulosity in other galaxies, the ones we commonly observe are located right within our own galaxy, the Milky Way. There are two types of nebulae - diffuse and planetary. The images of each type shown here were taken by various LCAS members.
Diffuse nebulae appear as irregularly-shaped clouds or as wisps of light. Many have been around since the formation of our galaxy and in some cases, the gas is condensing and forming new stars. Those with stars involved emit their own light as radiation from the stars causes the gas to glow like a fluorescent light. This type is known as an emission nebula. Typical examples are M42 and the Lagoon Nebula (M8) in Sagittarius. In each of these examples, we can see some of the stars that are causing the nebula to glow. M42 contains the most famous stellar nursery - the grouping of young stars referred to as the "Trapezium". Photographs show emission nebulae predominantly reddish in color due to the large amounts of hydrogen gas, which glows red when excited by radiation from stars.
M42 CCD image by Michael Purcell
Other nebulae are not powered by any stars involved, so we see them only because they are reflecting the light from nearby stars. These are reflection nebulae. One of the most famous of this type is the nebulosity surrounding the Pleiades star cluster (M45) in Taurus.
M45 photographed by Jay Bitterman
Planetary Nebulae got their name from the fact that they often appear circular in shape, and they looked to early astronomers like indistinct planets. This type of nebula results when a star explodes and blows off its outer envelope as a bubble of expanding gas. Planetaries are always emission type nebulae because the star that exploded is emitting sufficient radiation to cause the gases to glow. One of the most famous is the Ring Nebula (M57) in Lyra. In the following image, we can see the central star that gave birth to the nebula.
M57 CCD image by Michael Purcell
In not all cases do planetaries appear as elliptical or circular disks. It all depends on how the material was disbursed as the star exploded and how the glowing gases are oriented from our perspective here on Earth. The Dumbbell Nebula (M27) in Vulpecula exhibits a somewhat different shape.
M27 photographed by Kevin Bonges
Images of planetaries taken in the red-light emission of hydrogen atoms show an amazing amount of structure. The complexity of the images presents a challenge to astronomers trying to explain why they look the way they do. Planetary nebulae evolve from gas and dusty clouds surrounding red giant stars. Analysis strongly suggests outflow patterns that change direction could be shaping these nebulae. Some researchers theorize that the slowly expanding cloud of gas surrounding each dying star begins with a roughly circular shape. Then, jet-like outflows carve away at the inner part of the shell. Hot stellar winds continually push on the shell, eventually producing the delicate shapes we see in planetary nebulae. It's also possible that a binary companion to a dying star stirs up the glowing gases, further changing the shape of the nebula.
The famous Veil Nebula in Cygnus appears as a wispy system of widespread nebulosity. It may look like a type of diffuse nebula, but in fact is the result of a supernova; in effect, it's a planetary that has expanded so far that it no longer looks anything like its original shape.
One thing that endears emission-type nebulae to amateur astronomers is the fact that they emit most of their radiation in discrete wavelengths. This makes it possible to filter out all other wavelengths of light through the use of what have become known as "nebula filters". Even where light pollution is a problem, the background is darkened, while the light of the emission nebula is allowed to pass, making it more prominently visible. Some filters are so selective that they transmit only the light from certain elements; these are typically the Oxygen III and Hydrogen Beta line filters. Certain nebulae respond better to one or the other types of filters, depending on which gaseous element is predominant (i.e.: the frequency range of the light that is being emitted). Some nebulae will appear to slightly change shape when viewed with different types of nebula filters and without any filter. I've noted this especially with the Dumbbell Nebula (M27).
The smallest of the planetary nebulae are sometimes difficult to distinguish from stars in the field of view. But here again, the filter becomes useful to move between your eye and the telescope eyepiece to "blink" the nebula. What happens is that when the filter is in place, the planetary nebula stands out, but when the filter is moved out of the light path, the nebula is less bright and hides among the field stars.
As with any other deep sky object, the larger your telescope and the darker the sky, the more you'll see. A 4-inch telescope is about the minimum size that can be used profitably with a nebula filter. However, the brightest nebulae, such as M42, can easily be seen in a smaller scope without a filter, and even with binoculars, provided your sky is dark enough. Many diffuse nebulae are relatively large objects, so their light is spread over a wide area. When a magnitude is given for a nebula, it's an integrated figure; that is, for magnitude purposes, the nebula is treated as though it were a point source. So while the stated magnitude implies that it's pretty bright, it may actually appear surprisingly faint.
Whether they are glorious sights or elusive, you'll find nebulae always interesting.Published in the May 2001 issue of the NightTimes