The amateur astronomers here on the Zone often talk about observing “deep-sky objects”, DSOs, or “objects” for short. The term “deep sky” refers to objects external to the solar system, but stars in general, (variable stars, flare stars, multiple stars, carbon stars, novae and supernovae, etc) are not included in this list either. I have also left external galaxies off this list. It is restricted to nonstellar objects inside our Galaxy. For those of you not familiar with the lingo, here’s a quick guide to what we’re talking about. If you want photographs, just do a Google Image on any of these categories. Keep in mind their appearance in astronomical imagery is always more dramatic and spectacular than they appear through the eyepiece.

Open clusters (OC)

These are groupings of physically related (common birth and gravitationally connected) stars. Formerly known by the obsolete and misleading term “galactic clusters”, they are groups of from several dozen to several thousand stars, often fitting neatly into the field of a medium power eyepiece. There are several hundred of them within easy reach of amateur telescopes, and in my opinion, provide the most varied and beautiful DSOs for visual contemplation. Their visual telescopic appearance is of clumps of stars, standing out easily from the surrounding field. “Diamonds scattered on black velvet” is a phrase often used to describe them.

Open clusters are where stars are born, the young suns condense out of nebulae like raindrops from a thundercloud. They are often found embedded in nebulae, and we can watch the process of star formation taking place right before our eyes, frozen like an instantaneous photograph of an explosion. Others are a bit older, and the surrounding nebulosity has been blown away by the stellar winds of the hot young stars. All stars are born in open clusters, but the clusters are only loosely bound gravitationally and tend to disperse in a few million years, ripped apart by the gravitational tides of the Galaxy. With a few exceptions, most tend to be relatively new objects, several hundred million years old. Several thousand are accessible to amateur telescopes. For the most part, open clusters are concentrated in the galactic plane, embedded in the starfields of the Milky Way, where the gas and dust tends to concentrate. They are typically several hundred to several thousand light years away. Further than that, obscuration by dust in the plane of the Milky Way makes them hard to see.

Globular Clusters (GC)

These are also clusters of stars, but very different from the open clusters described above. Globulars are huge (although typically much further away than OCs), containing tens or hundreds of thousands, even millions, of stars, packed in globular masses where the star density is high near the center, and rapidly diminishes towards their ragged edges. Globulars are scattered randomly about the galactic nucleus and travel in long, narrow elliptical orbits around it. They are very old, probably as old as the Galaxy itself, and typically are much further away than their open cousins. Several hundred are known. Their telescopic appearance is of misty balls of starlight, bright towards the center, rapidly dimming towards the edges. If the globular is close enough, and your scope big enough, sometimes you can see partial resolution of the cluster’s brightest members, giving the ball of light a grainy appearance. GCs are beautiful, but not as varied as OCs. Except for size and brightness, and whether or not they resolve into individual stars, they all pretty much look the same.

Other galaxies also have their accompanying clouds of GCs. The ones orbiting the nearby Andromeda Galaxy are within reach of 8″ scopes and larger, although they just look like very faint stars. They are hard to identify without detailed charts.

Dark Nebulae (DN)

These are the most challenging DSOs for visual observation: they are invisible! Dark nebulae are clouds of dust and gas that emit no light, so they can only be seen in silhouette, when they obscure star fields or a bright nebula located behind them. They show up easily on long-exposure images, and several hundred are known, all clustered along the galactic equator. Curiously, a few are visible to the naked eye,and quite spectacular, as “holes” in the Milky Way starfields. Among these are the Great Rift in Cygnus, and the Coal Sack next to to the Southern Cross.

Diffuse Nebulae (DF)

These nebulosities are easily visible, and are among the most beautiful and varied of all DSOs. Physically, they are identical to the Dark Nebulae, and often found in association with them, although they are easily visible because they shine with their own light . There are two types, although they too are often found associated with each other and with Open Clusters. Emission Nebulae glow red in photography in the Hydrogen Alpha spectral line, as the gas flouresces due to excitement from UV radiation from nearby hot stars. Reflection Nebulae appear blue in color photography, and shine by starlight reflected from dust grains. Very often, the same nebula and its associated OC will have dark, emission and relection components. Except for the nearest and brightest seen in the largest telescopes, they always appear white to the eye. The eye’s sensitivity to color at low light levels is very poor. Most nebulae cluster along the plane of the Galaxy, although many of the very nearest ones appear to be quite a distance from it due to our being embedded ourselves in the Galactic Plane. Hundreds are known, thousands more are visible in large telescopes or in long exposure photographs.

Planetary Nebulae (PN)

“Planetaries”, have nothing to do with planets. They got the name because they sometimes resemble distant planets, little, round, relatively bright, blueish spheres or whitish blobs of nebulosity. Some are so small they look like stars, and can only be identified by passing a nebular filter over the eyepiece. The filter partially obscures continuum starlight and sky glow, but easily passes the narrow spectral lines of the planetary’s glow, causing it to appear to wink on and off.

Planetaries are the shells of gas thrown off by stars near the end of their evolution. Some day our own sun will have its own planetary nebula surrounding it. The planetary stage of stellar evolution is relatively short, and the nebulae themselves disperse quickly (in astronomical terms) so most of them are relatively young and temporary objects, less than a few thousand years old. In large telescope imagery, planetaries display fantastic shapes as the stellar winds of the dying star interact with the surrounding interstellar medium, and are twisted by its magnetic fields. Some of this variety is preserved for the visual observer, although planetaries are small objects, only about a light-year across. The more compact and distant ones appear starlike, and glow fiercely blue, the light of the oxygen III lines in the spectrum of the rarefied gas.. For older amateurs, with aging, yellowing lenses in their eyes, they will appear green. In a few, the faint white dwarf at the center of the planetary causing the gas to flouresce can also be spotted.

Supernova Remnants (SNR)

The shattered remains of massive stars that exploded millennia ago, these are few and far between. Most, like the Veil Nebula in Cygnus, are faint filamentary structures glowing feebly as the gas from the exploded star collides with the interstellar medium in an ever-expanding spherical shock front. Probably the best known SNR is the Crab Nebula, what’s left of a supernova recorded by Chinese astronomers almost a thousand years ago. The Crab is easily visible in small telescopes, and it glows in synchrotron radiation, the glow of electrons accelerated by the intense magnetic fields generated by the neutron pulsar at its center.