March 22, 2004

Observing the Deep Sky

by Chuck Fuller

Introduction

People often ask, "What do you look at with that telescope?" Well, of course there are the moon and planets, not to mention all those stars. But it turns out that the sky is packed with interesting objects of many types - nebulae, clusters, galaxies, and more. We call them "Deep Sky Objects" because they physically reside far outside of our solar system, and even outside of our own Milky Way galaxy. There are thousands of such objects within reach of modest amateur telescopes, and many would argue they are the most interesting and beautiful objects to observe. Some observers become entirely engrossed in their pursuit.

This article will explain the many types of objects up there, the tools you need to find them, and techniques for observing them. We'll also take a tour of some of the most popular and beautiful objects in the night sky. It turns out that you don't need a Hubble Space Telescope to be awed.

Types of Deep Sky Objects

Where do these objects come from, and why are there so many types? Powerful physical processes occur in deep space that give rise to a wide variety of phenomena. Gas clouds are collapsing, exploding, and colliding. Stars are forming, evolving, and dying. They gather into galactic groups due to their mutual gravitational attraction. These processes take millions of years or more to occur, so no human lifetime is sufficient to observe the evolution of a star. But even our local patch of the universe is large enough that we can see all the stages of these processes occurring in one place or another. It is similar to walking down the street and seeing adults, children of all sizes, and pregnant women. Pretty soon the picture of how humans are born and grow up becomes apparent, without actually witnessing a person pass through every stage.

Over the past several centuries, astronomers have made observations of these objects, and have teased out the physical processes involved by scrutinizing their light. Using spectroscopy and other techniques, we now have a fairly clear idea of what we're looking at, even if our understanding of their formation is not yet perfect. Let's run through the major types of objects that you can observe with your telescope.

Emission Nebulae

Emission nebula are clouds of gas in interstellar space that are emitting light. Their energy comes from nearby stars or stars that are embedded in the nebula itself. The light from the stars excites the gas molecules in the cloud, and they emit more light, usually at specific sets of frequencies that depend on what the gas is made of. Typical suspects are oxygen, hydrogen, helium, carbon, and others. Excited emission is the same process that makes fluorescent light fixtures glow.

Generally, emission nebulae are actually star-forming regions. It turns out that a large fraction of the mass of our galaxy is actually in the form of interstellar gas, so there is plenty of material available. The cloud of gas is gravitationally collapsing, and as pockets of local density reach critical mass, a star is born. Newborn stars, depending on their mass, burn quite brightly, and their light shines out into the cloud, making the gas glow. The clouds are rather large - typically several light-years across. Some "nearby" emission nebulae cover a patch of sky as big as the moon. Prime examples are the Great Nebula in Orion (M42 - shown here), the Lagoon Nebula in Sagittarius (M8), and the Swan Nebula (M17).

Because emission nebulae emit light at specific frequencies, it presents the opportunity to use specialized filters to observe them, increasing contrast and bringing out structure and detail. We'll discuss the use of filters in the Tools section below.

Reflection Nebulae

Reflection nebulae are also clouds, but mainly of interstellar dust that doesn't emit light. In this case, the dust plays the important role - it reflects light from stars, making them visible. Typically, reflection nebulae are somewhat fainter and more difficult to see, and the use of filters is less effective because they are reflecting a broad spectrum of light, not emitting at specific frequencies. A prime example of a reflection nebula is the dusty glow surrounding the Pleiades star cluster in Taurus (shown). The photo is a time exposure to bring out the nebula. You can see it in a modest telescope under dark skies, but less distinctly.

Dark Nebulae

Dark nebulae are kinfolk of reflection nebulae, being clouds of interstellar dust. However, there are no nearby stars to make them shine. So how do we see them? We only see them when they lie in the plane of the Milky Way where they block the light from the dense star clouds behind them. They look like "holes" in the star cloud, but are actually much closer to us than the background stars.

You can see large patches of dark nebulae with the naked eye and binoculars just by looking at the Milky Way from a very dark sky location. The best clouds of this sort are in the summer Milky Way, in Cygnus, and in Sagittarius (shown here). There are also several dark nebulae that are best viewed in telescopes at low power (with a wide field of view).

Perhaps the most famous dark nebula is the "Horsehead" in Orion. It is a dark cloud shaped like a horsehead superimposed on a background emission nebula.

Planetary Nebulae

The term planetary nebula is somewhat of a misnomer. Early astronomers observed these curious non-stellar objects with the limited equipment and knowledge of the era, and dubbed them planetary nebula because they appeared round(ish), and were similar in apparent size to planets like Jupiter. Actually, Planetary Nebula are a shell of glowing gas closely surrounding a star. They emit light by the same mechanism as emission nebula, but their formation is entirely different.

As a star nears the end of its life, strange things begin to happen in its core. It begins life fusing hydrogen into helium (as our Sun is doing now), and in the process converts matter into energy, giving the star its power source. However, over time, it uses up the hydrogen in its core. When this happens, the force balance in the star's center becomes unstable - the force of gravity is no longer balanced by the radiation pressure from the fusion process. At some point, gravity wins and the star begins to collapse. This increases the pressure and temperature in the core dramatically, until in a flash, the star begins to burn helium, fusing it into heavier elements. The helium flash results in a sudden outburst of radiation, causing the star to "burp" off a large fraction of its outer layers of gas. The outer layers expand outward rapidly, escaping the star's gravity. A roughly spherical shell of expanding gas then continues to grow in diameter, for millions of years, and is excited into emission by the central star, now burning hotly. By the time we see them, the shell of gas is perhaps 1-5 light years across.

Planetary nebula are perhaps some of the strangest looking and most widely varied objects in the sky. The precise conditions of their creation give rise to various asymmetries in their structure. For example, a nice, solitary star that undergoes a Helium flash might create a nice egg-shell nebula such as the Ring Nebula in Lyra (M57). It appears as a ring because when we look along the sides of the shell, the line-of-sight through the gas is thicker than when we look through the center.

If, however, the star is a double star with a very close companion, or has a very massive planet in orbit, the expanding shell of gas is gravitationally distorted, often resulting in bizarre and wonderful shapes. Some of the most spectacular Hubble shots are close-ups of these kinds of planetaries. However, even in modest telescopes, you can clearly see some of these structures. Prime examples are the Eskimo Nebula, the Saturn Nebula (named for its dual lobes), The Dumbell Nebula (M27 - shown here), and the Cat's Eye.

Because planetaries glow by emission processes, they are good candidates for the use of filters to enhance detail and contrast.

Supernova Remnants

When a star has about 5 times the mass of our Sun, it ends its life abruptly in a massive explosion called a supernova. These are among the most violent and energetic events in the Universe. The explosion blows off a large fraction of the star's mass into an expanding shell of gas at extremely high velocities. The gas expands into the interstellar medium to form interesting nebulae.

One of the most famous supernova remnants is the Crab Nebula (M1 - shown here) in Taurus. This is the remnant from a supernova that blew up in 1054 A. D. that was observed in many parts of the world, with the most detailed observations coming out of China. Today, we know the nebula in Taurus is the aftermath of that explosion almost 1000 years ago. Since then, it has expanded into a bizarre object with various tendrils and features reminiscent of a crab.

The sky is full of many of these, far older than the Crab, some being millions of years old. A prime example of an old (and hence very large) remnant is the Veil Nebula in Cygnus. The Veil has been expanding for so long that it covers a region of the sky several degrees across. It has dissipated so much that all that is left are wispy veils of nebula that are rather faint. However, because supernova remnants glow by emission processes, they respond very well to the use of filters. The Veil in particular is spectacular in a large telescope using an Oxygen-III filter.

Open Clusters

Open clusters are also known as "galactic clusters" because they lie in the plane of our Milky Way. These are groupings of 10 to 200 stars that all formed at about the same time. Recall that emission nebula are star-forming regions - open clusters are the result of that process, when most of the gas has collapsed to form stars, and the remaining gas has blown away due to the stellar winds of the newborns.

There are hundreds of these in the sky of varying sizes and densities. The Beehive Cluster in Cancer and the Pleiades in Taurus are two of the largest and closest clusters, easily visible with the naked eye and binoculars. One notable example is the Double Cluster in Perseus (shown here). There are numerous smaller, fainter, but richly populated clusters all over the sky.

Globular Clusters

Globular Clusters are one of the most spectacular types of deep sky objects, especially when viewed in larger telescopes that can resolve some of the individual stars. Globulars contain hundreds of thousands of stars, all packed into a spherical ball only a few light years across. They reside in a "halo" around our Milky Way, not in the galactic plane. The details of their origin and formation are still hotly debated among astrophysicists. They are fairly ancient objects, coming together shortly after our galaxy was formed. Some theorize that the center of each globular cluster contains a black hole that attracts the matter that in turn forms the stars in the cluster. Globular clusters are observed in similar halos around other galaxies also.

Prime examples of globulars include the great Hercules Cluster M13 (shown here), the cluster M22 in Sagittarius, and Omega Centauri (the largest and closest) visible from southern latitudes.

Galaxies

Galaxies, galaxies, galaxies. They're everywhere, and the deeper we look, the more of them we see. Each galaxy is akin to our own Milky Way, consisting of literally trillions of stars each. They come in a variety of shapes and sizes, and many of them have "active" cores - massive explosions, cosmic jets, and other little understood phenomena. Galaxies are typically 40,000 to 150,000 light years across. Our Milky Way is around 70,000 light years across and is thought to be rather average. The general consensus in the past few years is that galaxies each have a super-massive black hole in their center.

Galaxies tend to be grouped into clusters, and the clusters are grouped into super-clusters. In between are vast voids of space. The distances between galaxies are enormous; our nearest neighbor, Andromeda, is about 2 million light years away. In spite of these distances, galaxies do have a tendency to collide because of their mutual gravitational attraction. There are many examples of collisions in progress, collisions nearly finished, and collisions about to happen that we can see with modest telescopes.

Let's look at some of the major types of galaxies:

Spiral Galaxies: these are what folks typically think of as a galaxy - a large flattened pinwheel with spiral arms rotating about a central bulge. Our Milky Way and Andromeda are both spirals, and are fairly similar. Spirals come in a variety of sub-classes also:

• Regular spirals typified by Andromeda, and the Whirlpool (M51 - shown)
• Barred spirals with "bars" coming out of the central bulge, then swirling into arms

Elliptical Galaxies:these are roughly spherical or ellipsoidal in shape, with no features such as spiral arms. It is thought that when spirals collide, the result (after a billion years or so of settling) is an elliptical.

Lenticular Galaxies:these are disk galaxies without any obvious structure in their disks. Astronomers think the lack of structure is because they have used up most of their interstellar matter and they therefore consist of old stars only which have found a smooth and even distribution in the disk. Alternatively, it may be because the galaxy has not closely encountered any neighbor in the past few hundred million (or few billion) years.

Irregular Galaxies:these are strangely shaped, probably due to a recent collision or some incredible activity in their nucleus that has distorted their shape.

All in all, galaxies are probably the most numerous type of object in the sky accessible to amateur telescopes.

Galaxy Clusters

Galaxy clusters occur on a variety of distance scales. Our Milky Way is a member of a small handful of nearby galaxies called "The Local Group". The local group is in turn part of the Virgo cluster, which is in turn part of the Virgo super cluster. The Virgo cluster is a rich hunting ground for telescopes (in, you guessed it, the constellation Virgo), covering about 10x10 degrees, where a 10" telescope can bring in 20 or 30 galaxies easily. Another cluster is in Coma Berenices, called the "Coma Cluster".

In these clusters, you will only see one, or sometimes 2 or 3 galaxies in a single telescopic field at a time. More distant clusters are much fainter, but you get to see 4, 5, or 6 together in a field of view. The Abell galaxy cluster catalog lists some 100 galaxy clusters, but you'll need a serious instrument (16" or larger) to see these wonders.

Tools for the Deep Sky

The tools for deep sky observing are not all that different than for general astronomy, with a couple of important differences:

• Light gathering power is critical. You want the biggest aperture you can afford and transport
• Dark skies are critical. You want to make the effort to get as far away from light pollution as possible
• It's important to get above the haze, so head for the hills.
• Filters deserve some special considerations.
• Finder charts and high-quality star atlases are critical since most of these objects are far too faint to see with the naked eye.

Light Buckets

The most commonly used telescope for the deep sky hunter is the "Big Dob", a large-aperture Dobsonian telescope, affectionately referred to as a "light bucket." An 8" Dob is a great starter scope for the deep sky enthusiast, and will bring in many hundreds of objects under good observing conditions. A 10" or 12.5" is even better, and will really start resolving globulars, galaxy arms, features of planetary nebula, and the like. When you get up to the 16" to 25" range, the views become truly spectacular. Even if you can't afford or manage a scope that large, be sure to attend a star party and get some views through larger instruments. An 18" Dob is shown at the left.

Filters

Because many of the deep sky objects glow by emission, narrow-band filters play a large role in accentuating their features. Additionally, wide-band filters can be very effective for reducing light-pollution and sky glow, increasing contrast on globulars, galaxies, and reflection nebula.

Light-Pollution Filters:light pollution is a pervasive problem, but there are ways to mitigate its effect on your observing enjoyment. Some communities mandate Mercury-Sodium vapor streetlights (especially near professional observatories) because these types of lights emit light at only one or two discreet wavelengths of light. Thus, it is easy to manufacture a filter that eliminates only those wavelengths, and allows the rest of the light to pass through to your retina. More generally, both wide-band and narrow-band light-pollution filters are available from major vendors that help substantially in the general case of a light-polluted metro area.

Nebula Filters:nebula filters enhance the specific emission lines of emission nebula, planetaries, and supernova remnants. Most famous is the OIII (Oxygen-3) filter available from Lumicon. This filter eliminates almost all the light at other wavelengths other than the Oxygen emission lines generated by many interstellar nebulae. The Great Nebula in Orion (M42) and the Veil Nebula in Cygnus take on an entirely new aspect when viewed through an OIII filter. Other filters in this category include the H-beta filter (ideal for the Horsehead nebula), and various other more general-purpose "Deep Sky" filters that enhance contrast and bring out faint detail in many objects, including globular clusters, planetary nebula, and galaxies.

Pointing Devices

Since most of the objects you'll be locating are too faint to see, you'll need some aides to help point your telescope close to the neighborhood of the object of interest. While most telescopes come equipped with a finder scope (typically 6x30 or 8x50), the preferred pointing device these days is an LED pointer. Some models use a red dot projected onto a transparent screen, allowing you to aim the scope by pointing the red dot. Others use a projected reticle (concentric circles and cross hairs). The now famous "Telerad" was the first, and still highly popular, device of this sort. Many finder charts (including printed charts, on-line catalogs, and PC software) display the stars in the object's neighborhood with the reticle pattern drawn in.

Additionally, a pair of binoculars can come in handy for scouting the stars near the object that you can use to "star hop" to the object.

Deep Sky Catalogs

The Messier Catalog: In the 1700s and 1800s, a comet hunter named Charles Messier spent night after night searching the skies for new comets. He kept running into faint smudges that did not move from night to night, and so they could not have been comets. For convenience, and to avoid confusion, he constructed a catalog of these faint smudges. While he did discover a handful of comets during his life, he is now famous and best remembered for his catalog of over 100 deep sky objects visible from the northern hemisphere. These objects now bear their most-used designation stemming from the Messier catalog. "M1" is the Crab Nebula, "M42" is the great Orion nebula, "M31" is the Andromeda galaxy, etc. Finder cards and books on the Messier objects are available from many publishers and on the Web, and are highly recommended if you have a modest telescope and a very dark sky.

The Caldwell Catalog: Additionally, a new "Caldwell" catalog gathers another 109 objects that are of similar brightness to the M-objects, but were overlooked by Messier. These are ideal starting places for the beginning deep-sky observer. The Caldwell catalog was created by Patrick Moore, this list doubles the number of "must see" objects for amateur astronomers.

The NGC Catalog: The New Galactic Catalog, or "NGC" was compiled by J. L. E. Dreyer in 1888 and replaced all previous lists and catalogs. It is a superset of the Messiers', and covers the entire sky. There are approximately 10,000 objects in this catalog, the vast majority of which are accessible by modest amateur telescopes in dark skies. There are several observing guides emphasizing the most spectacular of these, and a high-quality star chart will show thousands of NGC objects.

The IC Catalog: The Supplementary Index Catalogs was compiled after the NGC list. The first IC contains 1529 objects discovered between 1888 and 1894. The second IC contains an additional 3856 objects found through 1907.

Arp Galaxies: These are a special subset of the NGC list. Compiled by Dr. Halton C. Arp. They are a collection of the more peculiar and irregular galaxies. These are, generally speaking, quite faint. If you're into the Arp galaxies, you've probably already made the decision to acquire a larger aperture telescope - perhaps something in the 12-16" range. These are, however, within the reach of more modest telescopes if you can use imaging. These are some very interesting targets.

The Abell Planetary Catalog: It's particularly exciting to find a deep sky objects that have only been discovered in the last half century, because very few people have ever seen them. To join this exclusive club, check out George Abell's Catalog of Planetary nebula containing objects discovered on plates from the Palomar Sky Survey in the mid-1950s.

The Abell Galaxy Cluster Catalog: This catalog is a homogeneous all-sky catalog of rich galaxy clusters with populations of 30 or more galaxies. The catalog combines a northern survey, originally published by George Abell in 1958, and a southern survey begun by Abell and Harold Corwin in 1975, which (after Abell's untimely 1983 death) was completed by Corwin and Ronald Olowin in 1987. Most of these clusters require significant aperture (12.5" minimum) to locate.

Observing Techniques

There are several important considerations and observing techniques that will help maximize the success of your deep sky observing efforts.

Dark Sky Site

Not enough emphasis can be given to the benefits of a truly dark-sky observing site. Light pollution, even from a modest city of, say, 50,000 people, can create sky glow 20 miles away, and reduce contrast. The darker the sky, the more these faint fuzzies will reveal themselves. In addition to improving the views of deep sky objects, a dark sky is majestic and beautiful to the naked eye, especially with a rich Milky Way high in the sky. Spending time with binoculars in a lounge chair under dark skies is another wonderful reason to make the trek out there.

The two primary considerations for a good observing site are darkness and elevation. If you live near mountains, try to get as much elevation as you can to get above haze and water vapor (and dress warmly - it can get quite cold at 7,000 feet, even in August!). Really amazing dark skies are found a good 50 miles from the nearest city, and have minimal local lighting. Many campgrounds are good candidates, if they have open spaces without too many trees.

Dark Adaptation

One of the most important considerations for seeing detail in dark sky objects is the dark-adaptation of your own eyes. Visual purple, a chemical responsible for increasing the acuity of your eyes in low-light conditions, takes 15-30 minutes to develop, but can be eliminated immediately by one good dose of bright light. That means another 15-30 minutes of adaptation time. Besides avoiding bright lights, astronomers use flashlights with deep red filters to help navigate their surroundings, view start charts, check their mount, change eyepieces, and so on. Red light does not destroy visual purple like white light does. Many vendors sell red-light flashlights for observing, but a simple piece of red cellophane over a small flashlight works just fine.

Averted Vision

To improve the amount of detail seen, and to glimpse faint objects at the limit of your telescope's grasp, use the technique of "averted vision". The human retina is composed of differing sensors called "cones" and "rods". The center of your vision, the fovea, is mainly composed of cones that are most sensitive to bright, colored light. The periphery of your vision is dominated by rods, which are more sensitive to low light levels, with less color discrimination. Averted vision concentrates the light from the eyepiece onto the more sensitive part of your retina, and results in an ability to discern fainter objects and greater detail.

Star Hopping

Star hopping is a technique for locating objects that can't be seen in a finder scope or binoculars. The idea is to plan a set of stars, starting with a fairly bright one, near the object. You first get the initial star in your field of view, and then move gingerly to a fainter star. Typically, you estimate the direction and distance from one star to the next in terms of "clock" directions and field widths. For example, move in the 2-o'clock direction 3 fields. Continue this until you get to a known star fairly close to the object. By sweeping around the neighborhood of the star, you'll find your target.

It is important to know the actual field of view of your eyepiece/telescope combination. To calculate the actual field, take the apparent field of view of the eyepiece, and divide it by the magnification. Star hopping, and pointing/locating in general, is best done with the lowest magnification and widest field eyepiece you have. Once you locate the object, you can then increase the magnification if desired. The only caveat to this rule of thumb is that some planetaries are fairly small, and at low magnification, will appear star-like. These are a bit more challenging, and star hopping at higher magnification will help you find them.

Deep Sky Tour

Let's take a quick look at some additional deep-sky objects that are must-see.

Andromeda Galaxy (M31):The great Andromeda Galaxy is our nearest full-size galaxy neighbor. It spans almost 4 degrees of sky (that's 8 moon diameters!), but the outer stretches of it are fairly faint. In dark skies, you can easily see it with the naked eye - being the farthest object one can see without visual aid (over 2 million light-years). It is best observed with a wide-field, low-power eyepiece. It has a bright central core, and two satellite galaxies. Larger instruments will clearly show structure in the spiral arms.

The Ring Nebula (M57):The Ring is a classic planetary nebula in the constellation Lyra. This is a good object to experiment with different kinds of filters to reveal various aspects. The central star of the Ring is a rather faint magnitude 14 or so, and is quite difficult to see generally because of the nebula's glow.

The Helix (NGC 7293):The Helix is a large, close planetary in the constellation Aquarius. It has very low surface brightness, making it challenging to find. A nebula or OIII filter makes a huge difference on this object.

The Lagoon Nebula (M8):The Lagoon in Sagittarius is one of the largest, brightest diffuse nebula out there. It has beautiful lane structures and a rich grouping of stars embedded in it. Again, a nebula filter does wonders on this object.

Deep Sky Resources

There are thousands of books on astronomy. Below are some recommendations suitable for folks new to the field of deep sky gazing. Click on the links to read the reviews at Amazon:

	Observing the Caldwell Objects by David Ratledge. This is a guide to 110 famous non-stellar objects, including nebulae, clusters, galaxies, x-ray objects and more. Each object gets its own double page spread along with complete technical details, including position and NGC number. A large fold-out map locates all of the Caldwell objects in the sky.
	Deep-Sky Wonders by Walter Scott Houston and Stephen James O'Meara. This is an easy-to-read monthly guide to star clusters, galaxies and nebulae that will keep the dedicated stargazer busy all year. It is a compilation of almost 50 years of wonderful articles by "Scotty" Houston from Sky & Telescope.
	Seeing the Deep Sky by Fred Schaaf. This is a project-oriented guide to stars, nebulae, galaxies and clusters. Goes beyond mere observation to discuss the physics and chemistry of this diverse selection of deep-sky objects.
	Deep Sky Companions by Stephen James O'Meara. In the 1700s, Charles Messier cataloged all the fuzzy spots in the sky that might be confused with his favorite objects: comets. These were soon found to encompass some wonderful deep-sky objects like nebulae and galaxies, and they have become a favorite set of objects for generations of astronomers.
	Field Guide to Deep-Sky Objects by Mike Inglis. This is a great reference guide with table after table of data on deep-sky objects. The guide makes it easy to figure out what to look for based on the time of the year. A great way to plan your star parties!
	Deep-Sky Observing: The Astronomical Tourist by Steven R. Coe. This is a great introduction to deep-sky observing, including tips on choosing a location, what to bring and what to do once you're set up. From modest 8-inchers to monster 36-inch scopes, he details what you might be likely to see. An enthusiatic introduction to the subject.
	Steve O'Meara's Herschel 400 Observing Guidea> by Steven O'Meara. This is a guide to 400 relatively difficult objects compiled by Sir William Herschel and his sister Caroline. It includes galaxies, clusters and nebulae. O'Meara has organized them by optimal viewing dates. Great viewing for big 'scopes.
	Sky Atlas for Small Telescopes and Binoculars: The Beginners Guide to Successful Deep Sky Observing by David S. Chandler and Billie E. Chandler. A great guide for beginners, this book lists constellation and other objects that can be seen with smaller power scopes, binoculars and the unaided eye.

Links

SEDS Messier Catalog: http://www.seds.org/messier/ (Students for the Exploration and Development of Space)

Deep Sky catalogs, charts, and observing: http://www.utahskies.org/deepsky

Deep Sky Database, observing list generator: http://www.virtualcolony.com/sac/

Deep Sky Observing Forum at Astronomy.com: http://www.astronomy.com/community/forum/forum.asp?FORUM_ID=3

The Abell Catalog of Planetary Nebulae: http://www.angelfire.com/id/jsredshift/abellpn.htm

CCD Images of Deep Sky Objects: http://www.arizonausa.com/sky/deepsky.htm