BASIC OBSERVING SERIES

by Greg Burnett (gburnett)

Nebula Observing

[This article was first published in Star Stuff, the newsletter of the Ford Amateur Astronomy Club, in April, 1994.]

Nebulae can be divided into three general classifications: bright nebulae, dark nebulae, and planetary nebulae. They are all what we would call "extended objects," and they vary in size from tiny planetaries that are easily mistaken for stars, to gigantic supernova remnants that span many degrees of sky. They are perhaps the most beautiful and certainly the most colorful deep-sky objects.

Bright nebulae comprise two very different types. "Emission" nebula are characterized by bright emission lines in their spectra. They are clouds of gas, mostly hydrogen, that is ionized by the ultra-violet radiation from stars embedded in the cloud. The ionized gas fluoresces as free electrons recombine with ionized atoms. The H-alpha spectral line at 6535 Angstroms predominates in the emitted light, giving emission nebulae their characteristic red color. The nebula gas is very tenuous, having only about 1000 atoms per cubic centimeter (as opposed to around 2.7x10^19 in normal air) and it is very hot, about 10,000 Kelvins. Well known examples of emission nebulae include M42, the Great Nebula in Orion, and M8, the Lagoon Nebula in Sagittarius.

"Reflection" nebulae are composed mostly of dust rather than gas. the dust particles reflect the light from nearby stars, so the spectrum of a reflection nebula is continuous, with dark absorption lines, characteristic of the illuminating stars. The dust particles are several ten-thousandths of a millimeter in size and therefore tend to scatter blue light more than red, causing the nebula to appear somewhat bluer than the illuminating stars. The dust is composed of various forms of silicon and carbon. Examples of reflection nebulae include M1, the Crab Nebula, which is a supernova remnant, and the nebulosity associated with the Pleaides, M45.

Contrary to what their classification would imply, most bright nebulae are somewhat faint, requiring moderate to large telescope apertures to observe them well. High resolution is not real important, even though many bright nebulae exhibit considerable detail. Several can be seen in binoculars, but most reveal their full extent only in long exposure photographs.

Dark nebulae, the second major class, are composed of the same type of dust as reflection nebulae, but have no nearby stars to illuminate them. They can only be seen in silhouette against a background of stars or bright nebulosity. Usually some stars can be seen through the nebula, and these stars will be reddened due to the blue scattering characteristics of the dust, as mentioned earlier in connection with reflection nebulae. The densest dark nebula will attenuate starlight by as much as five magnitudes. Well known dark nebulae include the Coalsack in Crux (southern hemisphere) and the famous Horsehead Nebula in Orion.

Dark nebulae are not very popular as amateur observing targets because they are usually difficult to see. In general, the same equipment requirements apply as for bright nebulae, but dark nebula are more challenging and reveal themselves clearly only in photographs.

Planetary nebulae are the third major class of nebulae. The name "planetary" was coined by William Herschel based on their visual similarity to the planet Uranus; planetary nebulae actually have nothing at all to do with planets. A planetary nebula is a spherical shell of gas surrounding a hot star, which provides the energy for its light emissions. They often have a circular appearance, and glow in the green light of doubly ionized oxygen, O-III. Perhaps the most famous planetary is the Ring Nebula in Lyra.

The following description of the origin of planetaries comes from Sky Catalog 2000.0, Alan Hirshfeld & Roger W. Sinnott eds., Sky Publishing, 1985.

"The progenitors of planetary nebulae ("Protoplanetaries") are bloated red-giant Mira variables. Although each starts with one to six times the Sun's mass, it continuously loses material in a slow stellar wind of typically 20 kilometers per second. When the bulk of the atmosphere is gone and the hotter, underlying layers are revealed, the surrounding gas is ionized and begins to glow. The wind increases sharply to perhaps 1,000 kilometers per second and plows the matter into a shell that is typically 0.15 parsec across. (This is about 30,000 times the distance from the Sun to the Earth.) The star then quickly evolves into a white dwarf, and the glowing nebula remains visible for less than 100,000 years. The entire process is described in more detail by Sun Kwok in Sky and Telescope, 62, May 1982, page 449."

Planetary observing has slightly different equipment requirements from other nebula observing. Most planetaries are compact but fairly bright (exceptions being the "Owl" and the "Eskimo" among a few others), so large apertures aren't generally necessary. Good resolution at high magnification is helpful in detecting structural detail. Some planetaries are very small and can be difficult to distinguish from nearby stars. Special filters are available that isolate the unique O-III light from the planetaries to aid in identifying them.

As you can see, there is a wide variety of nebulae available to the amateur astronomer. Most catalogs include lists of the most interesting objects, and