An Introduction to Deep Sky Objects

M42Credit: Darren Wong Astrophotography

M42: The Great Orion Nebula

Introduction to the deep sky

This truly is the golden age for amateur astronomers. Never in history has there been so much information about our natural world on hand, or the availability of affordable astronomical equipment, such as computerized telescopes and image stabilized binoculars. But that is the subject for another article. My aim is to give you an overview of the observable universe from your backyard. Well almost. For the most of us, observing means having to travel a fair bit to get to darker pristine skies. Although we do live in a gilded age, advancements come with a price. With the advent of technology, our once dark night skies are not so dark anymore. As an example, go out tonight and look up. Depending on where you live, you might see a sky full of stars (not likely unless you live out in the sticks) or for the most of us just a handful of the brighter ones. Don't give up just yet! You will be amazed at what you can see, even from a not so perfect sky. And I am not just talking about our inner solar system (our moon, and the planets Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune...all are visible with small scopes with the exception of Pluto, which peaks at magnitude 13.65 at opposition), I am talking about deep sky objects. What are deep sky objects. Any object not within the realm of our solar system is classified under the broad umbrella term deep sky objects. These are the staple of amateur astronomers worldwide. We love looking at the deep sky, sketching deep sky objects and photographing the deep sky. So what is the deep sky and what does one need to begin a journey into observational deep sky astronomy? Read on!

Deep Sky Objects: General Classes and Definitions


Deep sky objects cover a wide range of subjects, that can be broken down into several classes. These classes can then be further broken down into sub classes. The backbone classes include nebulae (latin word for cloud since they literally looks like clouds with the naked eye and through the telescope), galaxies and star clusters. Nebulae themselves can be further subdivided into emission nebulae, reflection nebulae and planetary nebulae. With the exception of planetary nebulae, the other two consist of gas clouds that are collapsing under gravity to form infant stars. The only reason we are able to visualize these gas clouds is because they are illuminated by these young stars. As the name suggest, emission nebulae are visible because they are illuminated by the forming stars they contain within. By contrast reflection nebulae are illuminated by foreground or background stars that are nearby. Both emission and reflection nebulae represent stellar nurseries.

Planetary nebulae and supernova remnants, on the other end, represent the graveyard of stars. Depending on the mass of the star, their ultimate fate when they finally exhaust all their fuel is very different. Super-massive stars usually end their existence in a spectacular fashion, literally going out with a big bang. Its such stars that go supernova and glow brightly for weeks before blowing out its remnants and forming filamentous patterns in the sky. Less massive stars usually eject out their material slowly, forming all sorts of interesting shapes. I will cover examples of these in the following articles.

Star Clusters

Star clusters represent a class of object that is easiest to observe from light polluted skies. Since they are formed either from a chance grouping of stars, or stars that are actually gravitationally bound to each other, they have what is termed "high surface brightness" in astronomical terms. All this means is that they are point sources and their light is condensed in a small area. Again star clusters can be subdivided into two sub classes, open clusters and globular clusters. Open clusters form the observing staple of urban astronomers and are the easiest to observe as they are bright. Globular clusters are much tighter groupings of stars that are bound tightly together. These are usually found around the center of our milky way (in the direction of the constellations of Sagittarius and Scorpius in the summer months in the northern hemisphere) and most of them are relatively bright as well as their light is concentrated in one spot. Oftentimes though beginners find these class of objects less interesting as resolution is only possible in scopes 5 inches (127mm) and larger, and at high magnification.


The final class of deep sky objects, galaxies, are the hardest to observe from urban settings. Again there are several classes of galaxies depending on their morphology. Three broad groups include spirals, ellipticals and irregulars. As their names imply spirals usually present their spiral arms either face on or sideways. These are the objects people immediately think of when galaxies are mentioned as they are the poster-child of galaxies. Ironically they are also the hardest to observe as their light is spread out over a wide area, hence a very low surface brightness. There are sub categories within the spiral group. Ellipticals look, well like ovals with little of no detail, even in the largest of amateur scopes. This class of galaxies are the easiest to observe, even from light polluted sites since their light is concentrated in a smaller area. The final class of galaxies, the irregulars have no discernible shape to them. A good example of this class of galaxies is the Large and Small Magellenic clouds, satellite galaxies of our own milky way. Unfortunately these two galaxies are not visible form the northern hemisphere.

My next article will cover what one would require to start observing, as well as some tricks of the trade.