Deep Sky Objects and Their Observation
To begin this article I will pose a couple of questions. Have you ever observed deep sky objects (DSOs) and perhaps been at a loss for words as to how you can adequately describe its appearance in your notes? Similarly, have you seen a feature that perhaps you didn’t fully comprehend its true nature? To be certain all of us at one time or another could answer yes to both. Some folks simply struggle more with the task of describing what they see through the eyepiece in a way that others may easily grasp. At the same time there can be a lack of understanding of precisely what we are seeing from time to time, which in turn makes it difficult to describe.
This article will hopefully clarify how we look at DSOs and what to look for while we study them with our optics. My goal is that it will benefit not only those who are beginning to observe and learn about deep sky objects, but also folks that have a great deal of experience in both observing and taking notes. We all at times may be uncertain of exactly what to look for and consequently how to describe what we do see. As with any set of broad guidelines, there may be some things that I have overlooked or simply not considered. Input is always appreciated as we all learn through the sharing our experiences and gained knowledge.
So with that in mind, let’s set forth some general ideas of exactly what we are looking at, looking for and how we may attempt to describe them. I hope the following paragraphs will prove useful and add to your enjoyment of the night sky. My intent here is that these are merely suggestions and not some rigid checklist for the field. Rather it is to give everyone some food for thought as they go about observing the various types of DSOs, and make them more aware of details that may be seen within them. With that said let’s take a look are the various types of DSOs and what they have to offer.
Open Clusters: These objects are some of the first things that many new observers encounter and pursue. They can range from bright and easily seen all the way to the very dim and tiny, challenging even the most experienced observer to ferret them out of a star field. Some of the most well known and considered easily observed open clusters would be Messier 45 (Pleiades) in Taurus, Messier 44 (Beehive) in Cancer and Messier 35 in Gemini to name but a smidgen. There are some that are quite distinctive in appearance, such as NGC 457 (Owl or E.T. Cluster) in Cassiopeia and NGC 869/884 (Double Cluster) in Perseus.
Locating and observing them is a lot of the fun and challenge, but how do we describe them in our notes and observing reports? What characteristics do we look for while we have our scopes turned their way? Here are some ideas for you.
How would you describe the cluster’s size within the field of view? Is it large, of medium size or small? If you are good at estimating sizes in arc minutes or arc seconds that can be useful. You can also add additional descriptors such as “somewhat large” or “slightly large” to further clarify. Though using such terminology is a bit subjective, it can still be useful/
Can you guesstimate the magnitude of the brightest stars and are the cluster’s stars mostly the same magnitude or widely disparate?
Are there areas where the stars are clustered more than the rest? For example are there knots or clumps of stars in a particular section or sections of the cluster field?
Are there any voids within the cluster? For example, when you notice clumps of stars in sections of the cluster, do you also notice areas of poor stellar density between them, creating voids between these concentrations?
Is it easy to distinguish the edge of the cluster, or does it blend gradually into the surrounding star field rendering edge definition difficult to discern?
What is its general shape of the cluster - round, oval, boxy, triangular, etc.?
Do you notice any specific patterns within the cluster’s stars? Such as, interesting arcs and lines of stars flowing in a certain direction or crossing one another. This can also include interesting doubles or triples that catch your attention. Perhaps a shape that conjures up thoughts of a known object, such as NGC 457 (Owl Cluster) mentioned previously.
Do you notice any stars whose color stands out particularly well, making them more prominent in the cluster field? The Jewel Box Cluster, NGC 4755 in Crux, is particularly notable for this attribute.
Do you see haziness behind the cluster indicative of unresolved members, or is the field cleanly resolved with no residual haziness?
Globular Clusters: These clusters are an interesting category of DSO. The majority lie in a halo around the Milky Way’s central hub, though some are farther afield as they orbit the galaxy center in quite large and long paths. As with most types of DSOs there are those that are bright and easy to see, even sometimes in the optical finder or binoculars.
Clusters such as Messier 13 in Hercules, Messier 5 in Serpens, NGC 5139 (Omega Centauri) and NGC 104 (47 Tucanae) are prime examples of the easiest members of this class. Others, like NGC 2419 in Lynx, NGC 6749 in Aquila and those of more recent discovery found in some obscure catalogues such as Palomar, Terzan, etc., can be very difficult to observe. So here are some things to look for in these curious objects.
- How large or small does the cluster appear visually within the field? Typically published angular sizes of globulars are larger than what we see in the scope as they are based on imagery rather than our less sensitive optical sensors (eyes).
- Are any individual stars resolved within the cluster? There are times when stars are resolved only at the edge of the cluster. Other times we may resolve stars across its face while the bulk of its interior remains a bright diffuse glow behind the resolved members. On the whole, few globulars yield deep core resolution to most backyard scopes, but many do allow the observer to resolve some if not many stars.
- Does the cluster appear to have a grainy or curdled appearance? This can be indicative that one is near to resolving some stars within the cluster, though some of them do defy any resolution at all, and may even remind the observer of a small and dim elliptical galaxy.
- What is the shape of the cluster as a whole? Some appear round while others may seem out of round if not slightly oval in shape.
- How densely packed does the globular core appear to be? Does the density of the central core seem small or large? Some exhibit a small tight core with a more diffuse outer section, while others may show a larger packed core with a smaller outer halo. Still others are more loosely structured lacking any significant core concentration if any at all. This all ties into the cluster’s position on the Shapley-Sawyer Concentration Class scale. The scale contains 12 levels (1 being most dense and 12 the least) and was developed in 1927-1929 by Harlow Shapley and Helen Sawyer Hogg in an attempt to classify globulars based on the visual concentration of their cores. It is a bit subjective and not of significant relevance, but an interesting attempt at classifying these objects. If one has observed all the globular clusters contained within the Messier catalogue, then they have covered levels 1 to 11 of the scale.
- There are also a few cases where the core may appear stellar in appearance, though that is not particularly common. This may occasionally happen with smaller apertures.
- Do you notice any prominent lines or arcs of stars emanating out from the core? For example, Messier 13 has five curving arms of stars that some may liken to a starfish pattern.
- Globulars too can seem to have pockets or lanes of less stellar density that may appear as a slightly darker void. Make note of this type of feature if seen. An example of this is also in Messier 13, where a small and challenging feature called “the Propeller” can be found. It consists of three small dark (relatively speaking) lanes, caused by dust, that emanate from a common point like a three bladed propeller.
- Does the cluster’s edge blend gradually into the surrounding star field or is it clearly defined? As with open clusters, you may notice some have clearly delineated edges in relation to the surrounding general star field, while others are more difficult to discern. The latter can be particularly true with clusters lying within the richest fields along the Milky Way plane.
Galaxies: We now come to my personal favorite type of DSO. These distant star islands come in a variety of shapes and sizes, and range in ease of observation from the fairly easy to dismally dim far beyond the reach of even the largest amateur scopes. These behemoths are far and away the most numerous class of deep sky objects to be found in the night sky. Just a few of the more prominent members of this category are Messier 31 in Andromeda, Messier 51 in Canes Venatici and Messier 81 in Ursa Major.
Some galaxies such as Messier 33 in Triangulum and Messier 101 in Ursa Major have bright visual magnitudes, but this is deceptive. Due to their quite large angular size, their light is spread over a larger area and thus effectively dimmed to our eyes. This gets into the realm of Surface Brightness, which is ultimately a very important factor in terms of galaxy observability. This is particularly important for those dealing with significant light pollution.
Galaxies can therefore be supremely frustrating to hunt and observe, while at the same time being infinitely rewarding. Because of their diffuse nature, their visibility is significantly impacted by sky quality, aperture and observing experience. They can often be elusive and devoid of any significant detail, especially to the untrained eye. So what are some of the details we can hopefully see and interpret? Let’s take a look at the following suggestions.
- One of the more obvious details that we can often make out when it comes to galaxies is the shape. Does it appear round, oval, as a thin sliver, lens shaped (lenticular) or perhaps oblong? Does it have a fat middle tapering out to thin needle-like tips, or are the ends blunter? Does it have more of an irregular shape that is not uniform along its major axis (length)?
- What of the galaxy’s light distribution across its disk? Some can have brighter centers (core area), that may display a broad brightness, or even seem to have a stellar point at its core. Sometimes a so-called stellar core may be nothing more than a foreground field star superimposed upon the center of the galaxy. However, this difference can be difficult to discern. Some galaxies are uniform in light distribution with no increased brightness across its body. I often use the term “homogenous” for this characteristic. But something like “evenly illuminated” also effectively imparts that notion.
- At times you may see a bright knot off to the side of the galaxy center, particularly in spiral galaxies in the outer portions of the structure. These can many times be star forming or HII regions in the galaxy’s spiral arms. Again, one has to be careful that they are not picking up a foreground star in front of the galaxy. Studying images of the particular galaxy after an observing session should be able to confirm whether or not you’ve spotted an HII region.
- Sometimes more than one galaxy may be overlapping either because they are truly an interacting pair or two at different distances that simply lie in the same line of sight from our perspective. Sometimes this overlapping can be obvious, though in some cases it may be quite subtle when the secondary galaxy is dim and its light simply melds into the light of the primary.
- When and if there are multiple galaxies in the larger field of view make note of their proximity to the main target. Are they widely separated, close or overlapping? You can also add descriptions of those to the extent possible.
- There are times when foreground stars are very obvious, and visually involved with the disk of the galaxy. These should be noted as such and sometimes can give the galaxy an interesting appearance.
- Sometimes we are able to see internal structure within a galaxy. Particularly with face-on spirals that may include some of the spiral structure. Depending upon the galaxy in question, the aperture in use, the quality of the skies and the observer’s experience level, this structure may be somewhat obvious (such as Messier 51) or more subtle (such as Messier 83). It may only be detected as a very subtle arc of light emanating from the core, made more apparent by the dust lane along its edge. Sometimes we can trace out the curves of the arms by following that arc of subtle contrast between the light in the arms and darkness of the dust lanes lining the arms.
- Make note if you see a grainy or mottled texture across part or the galaxy’s entire disk. NGC 253 in Sculptor, a tilted (to our view) barred spiral, is a good example of a galaxy that can show heavy mottling. This is indicative of variations in brightness levels caused by the contrast between the brighter arms and the dusty dark lanes along their edges.
- Tied into the earlier paragraph about core brightening, barred spiral galaxies bear mention. Sometimes you may pick up an elongated bit of brightness within the central region of the galaxy. This is attributable to the central bar structure of the host galaxy. In some cases the bar itself may be glimpsed as a separate entity, but often we are merely seeing the diffuse glow of its light without resolving the actual bar.
- When it comes to edge-on or thin galaxies, do you notice a dark lane bisecting it? You may not actually see the dark lane itself as much as inferring its presence by seeing a central bulge split into two distinct knots of light above and below an apparent void. On tilted spirals you might see a knife edge cutoff in the light along the edge of its major axis. This can be indicative of a dust lane at the edge of the nearside arm.
- Sometimes brighter field stars can cause visual detection problems. This comes from the glare of the star within the field of view overwhelming the more feeble light from the galaxy. Moving the offending star out of the field of view can be helpful. Also one can increase magnification which increases visual separation between the star and the galaxy. Messier 109 in Ursa Major and NGC 404 in Andromeda are both good examples of this type of interference from brighter stars lying nearby.
- Another thing you may notice from time to time is that a field star is found right off the tip of a galaxy. This can give the false impression of added length to the galaxy’s major axis because the eye is drawn to the star. It takes concentration to ignore the influence of the star’s presence. However, one can simply increase magnification to better separate the star and galaxy and this illusory appearance dissipates.
- Typically filters don’t enter into the discussion about observing galaxies. That said there are some filters on the market that are sold as galaxy filters. However, their impact is negligible, and they are not a magic bullet for pulling in galaxies. However, there are certain applications where a narrow-band nebula filter can be useful. Not in terms of the galaxy as a whole, but rather to boost the contrast of any visible HII star forming regions within the galaxy. Two wonderful examples are Messier 33 and Messier 101. While the galaxy is dimmed with the filter in place, its HII regions can take on new prominence within the view, making them much easier to identify.
- I will leave the galaxy category with one final, personal thought. When gazing upon these far away entities, I encourage you to consider the enormity of time and distance you are observing across. Every time we look at one through our optics we are seeing a snapshot of what it was long ago. Depending upon which galaxy you have in your view at that moment, the light you see may have left there long before, during or after the dinosaurs roamed our planet. I like to ponder upon the changes that have since occurred in that galaxy. My mind often drifts to the thought of whether there could be an advanced life form there who may be looking at our own Milky Way across time and distance wondering the exact same things.
Emission Nebulae: Their name alone gives us a clue as to the nature of these objects. They are clouds of ionized gas stimulated by stars buried within and emit light in various optical wavelengths. As with many other types of DSOs they can come in a wide range of angular sizes, shapes and visual magnitudes.
One of the most famous of this group is of course Messier 42 in Orion, otherwise known as The Orion Nebula. Even more prominent is NGC 3372 in Carina known as the Eta Carinae Nebula. These objects are associated with recently born stars (relatively speaking) and star nurseries, whose energy stimulates the surrounding gas fields causing them to glow. Here are some of the details to you may notice about them visually.
- Make note of the nebula’s general apparent visual size and brightness.
- Are there any visible stars seemingly associated with the nebulosity seen?
- Describe the general appearance of the nebula. Is it somewhat a solid mass, or is it more delicate, exhibiting a diaphanous appearance? Does it have tendrils or fingers of nebulosity that seem to flow outward from the main mass of the nebula? In reality, it may be a combination of all those characteristics. They tend to be very ill-defined in terms of shape as the gas clouds are blown about by the solar winds of their stellar companions.
- What variations in brightness within the nebula do you notice? Sometimes interstellar dust within the nebula complex will block light causing decreased contrast and giving the appearance of darker areas.
- Do you notice any color within the nebula? This does not occur often with emission nebulae visually. But in the case of Messier 42, many people report seeing a greenish tint, something I have seen personally. With larger apertures some have noted a pinkish tint within the interior portions.
- When talking about emission nebulae, the subject of filters often enters the discussion. A good general purpose filter for such objects is a narrow-band nebula filter, sometimes called an Ultra High Contrast (UHC) filter. However, one must be careful not to confuse these with some UHC filters that are in fact broad-banded in nature and marketed as Light Pollution Reduction (LPR) filters.
- Another filter of use for these objects is one designed to pass the lines of doubly ionized oxygen, or O-III. For emission nebulae I would recommend the narrow-band nebula filter as a first purchase with the O-III a second choice. If you have both it can be fun to use them and compare the views between them with an unfiltered view. As an example, I find the view of Messier 42 noticeably different using the narrow- band nebula filter versus the O-III line filter.
- Finally, there are some objects that respond more favorably to an even tighter and more aggressive filter, the Hydrogen-Beta. Though the number of objects that perform better with this filter is significantly lower than with the other two, it is still nice to have in the case should you choose to pursue them. It is highly recommended for such objects as Messier 43 in Orion, the main portion of Messier 20 (Trifid Nebula) in Sagittarius, and is the filter of choice for detecting the Horsehead Nebula (B33) embedded in the IC 434 emission nebula in Orion and NGC 1499 (California Nebula) in Perseus to name some of the more notable objects.
- Bottom line here is that when you do make use of a filter for emission nebulae, what is the impact to its appearance? Plus if you have more than one type of filter, how do they compare and which works best?
Reflection Nebulae: Unlike emission nebulae, this type does not emit light, though they can be visual objects. Their name clearly refers to the fact that we see them only because they reflect the light of a nearby star, and are not energized by its emissions. When seen in images they are typically blue in color due to their proximity to young hot stars and how their dust reflects the light. The easiest to see is Messier 78 in Orion. However, the extensive reflection nebula around Messier 45 (Pleiades) is the most famous example. Reflection nebulae are more challenging visually since they only glow by reflected light rather than being energized and emitting their own light.
- Visual detection of a reflection nebula can often be very challenging and they are normally quite dim in comparison to emission nebulae. Typically they do not reveal much detail unless one is under dark skies with good transparency so they can be seen with maximum contrast. Taking Messier 78 as an example, observers often state it appears as two headlights glowing through fog. Two 10th magnitude stars, whose light are being reflected by the nebula are the headlights.
- When observed under dark and transparent conditions reflection nebulae appear fragile and delicate. They are beautiful, but certainly not as robust as many of their emission counterparts.
- Because reflection nebulae are not energized and emitting their own light, filters are of little use overall. There can be a very subtle boost in contrast using a broad-band LPR filter, particularly at darker locations. But again, that improvement is very slight. Observing from the darkest skies possible is actually the best tool when it comes to reflection nebulae. Using a narrow-band nebula filter will only dim the nebula as it does the stars, unless of course there is also an emission component within the nebula complex. Frequently nebulae will have both types within their structure, so it never hurts to try a narrow filter.
Planetary Nebulae: This type of nebula is so named because of its vague similarity to the disk of a planet, and the term is generally attributed to William Herschel. Basically they are a type of emission nebula formed late in the life of medium massed star as it throws off ionized gas.
Several planetary nebulae (PNe) are famous for exhibiting color, which can typically be seen in the green to blue range, depending upon the individual. However, the vast majority simply appear whitish-gray in nature. So what can we look for in these very intriguing objects?
- When observing PNe, do you perceive a disk or does it remain stellar? Sometimes it may appear as nothing more than a slightly bloated star that will not quite reach focus as do other stars within the field of view. If a disk is it large, medium or small? Some can be quite diminutive at just a few arc seconds in diameter. What is its general shape – round, out of round to oval, boxy?
- Does the nebula appear as a smooth disk or are there variations of light and dark within? Messier 97 in Ursa Major has two darker areas within its disk that appear like eyes, hence its nickname the “Owl Nebula.” Does it appear annular or doughnut-like? Messier 57 in Lyra is a good example and nicknamed the “Ring Nebula.” Does it appear lobed in structure? Messier 27 in Vulpecula is a fine example of a dual lobed planetary viewed from the side, with its nickname being the “Dumbbell Nebula.” As an aside, if Messier 27 were oriented to our line of sight in the same manner as Messier 57, it would also appear annular. Conversely, if Messier 57 were oriented to us as is Messier 27, then it would appear similar to its dumbbell or apple-core shape. With annular PNe we are looking down the pipe so to speak.
- Is there any color other than white to gray associated with the nebula? Good examples of those displaying color are NGC 3242 in Hydra, NGC 6210 in Hercules, NGC 7009 in Aquarius and NGC 7662 in Andromeda.
- Is the progenitor or “central star” of the nebula visible?
- Is the planetary a so-called blinking type, such as NGC 6826 in Cygnus? This occurs when we look directly at the planetary and its bright central star overwhelms the nebula around it and somewhat disappears. But when we look at the object with averted vision the central star appears to dim and the nebula become more pronounced visually. I have seen this with NGC 2392 in Gemini with its magnitude 10.5 central star.
- Try applying a narrow-band nebula filter, or typically even better, an O-III filter. The majorities of PNe emit heavily in the doubly ionized oxygen lines (O-III) and thus tend to respond better to those filters. Though as with anything there are some exceptions. Overall how does the filter impact the nebula’s general visibility? Does it help enhance any internal structure? What differences do you perceive between the filter(s) used in terms of appearance versus a non-filtered view? If you have difficulty identifying the PN within the field of view, try the simple “blinking” method with your filter. Holding it between thumb and index finger, move it in and out between the eye and eyepiece. The PN should gain contrast with the filter in place whereas the stars around it will dim.
Dark Nebulae: This unusual category of DSO is nothing more than dense patches of interstellar dust blocking the light from background stars and emission nebulae. They become visible by their resulting starless or mostly starless void in an otherwise bright field, and are sometimes called “absorption nebulae.” Because they block light rather than emit it, they can among the most challenging objects to discern. That said, many beginning observers have seen examples of dark nebulae but may not have really grasped that they had.
They are scattered in profusion across the sky, but most notably along the primary plane of the Milky Way galaxy. A prime example is the so-called “Great Rift” which stretches from Deneb to Alpha Centauri, and includes the Northern Coalsack between Deneb and Sadr in Cygnus. Another very famous example is the Southern Coalsack or simply the Coalsack in Crux. Prolific cataloguers of these dark denizens were E.E. Barnard and Beverly Lynds. Here are some ideas of what to look for.
- How large does the dark nebula seem to be? When you observe dark nebulae, judging their extent can be challenging. It can sometimes be difficult to know where their blockage of light ends or if a field is simply of lower stellar density. Using charts, atlases and/or software we can get a better sense of their true size.
- What is your sense of the nebula’s opacity, or darkness? This can be tricky for observers in light polluted areas. Contrast is supreme is getting the most out of dark nebulae, and sky glow reduces contrast. Typically a 1 to 6 scale is utilized, with 6 being the darkest or most opaque.
- How many stars do you detect in the void, if any?
- You do not need large aperture. In fact smaller apertures with a wide field can yield excellent results, provided you can observe from a dark location that has good transparency.
- Experience is another critical factor. By having your eyes trained to detect subtle variations of light and dark, as mentioned in the galaxy section, you are more attuned to the subtleties that dark nebulae can sometimes present. Of course as with all observing of DSOs, dark adaptation is very important.
Supernova Remnants: These frequently elusive objects are the remains of a star’s violent end. The most famous of this category is Messier 1 (Crab Nebula) in Taurus and the haunting Veil Nebula in Cygnus. The former is the remains of Supernova 1054 which was first recorded on July 4th of that year. The Veil is all that remains of a star that gave up the ghost about 6,000 BC. It is so fragmented and intricate that its parts carry five designations in the NGC and one in the IC, namely NGCs 6960, 6992, 6995, 6974, 6979 and IC 1340.
These objects are considered a diffuse expanding nebula that over massive periods of time will finally dissipate. Though those generally visible to the average backyard scope are not large in number, you can look at them similarly to emission nebulae as far as details. Given their typically diffuse natures, dark skies and excellent transparency are quite beneficial. So what might we notice about them?.
- How much of the overall structure are you able to see? In the case of Messier 1, it remains in one piece visually. Does it appear homogenous or can you see variation in brightness within its envelope? Can you detect any filamentary structure within? Does its edge seem clearly defined or does it dissolve gradually into the surround sky? Further, is its edge ragged or smooth?
- With fragmented remnants like the Veil Nebula, how many of its pieces can you identify? Which portions are brightest and which are more difficult to see? Can you discern any background stars through the filaments and tendrils of its feathery structure?
- Many SNR are quite challenging. For example Cassiopeia A, which is the result of a stellar demise approximately 11,000 years ago. It is dispersing and visually can be difficult, requiring larger apertures. Often only the brightest arcs of material can be seen, if at all. Under very dark skies with excellent transparency, larger scopes may show a great deal of its diffuse circular structure.
- Because many of the SNRs are still strongly emitting in the oxygen lines, an O-III filter can be quite beneficial. For instance, the Veil Nebula responds remarkably to such a filter, with a tremendous difference when compared to an unfiltered view, even under dark skies. While a narrow-band nebula filter can be helpful, they typically do not match the contrast gain of the O-III. However, as older SNRs deplete their oxygen supplies, a Hydrogen-Beta filter may be beneficial for observation.
So there we are. If I have forgotten or overlooked some bit of information I apologize for the oversight. I do welcome feedback and comments, as I consider this a living document that can always be improved upon. There is a lot of stuff in there to digest, but of course it is up to each of us just how much of it we wish to focus on. Above all else, I hope that I have given you some good ideas of what to look for and perhaps how to describe what you see.
Again, I don’t intend this as a field checklist in the strictest sense, but rather as a general guide to what details are possible when observing each type of object. Of course what is seen is based on many variables, such as aperture, experience and sky quality. What I have set forth is gleaned from my many decades of observing experience. But even so, I am still learning something each and every time I step out there. Engaging the mind keeps us fresh and wards mental stagnation. So I hope you all find it useful and can apply it to some degree to your nightly journeys through our wonderful universe.