Nearly all the objects presented in this series of articles appear on star atlases familiar to most beginners. So why bother to write about them? The answer is that most atlases show large numbers of deep sky objects, some bright and easy to find, others much fainter and more difficult. As a beginner I well remember trying to find many of the objects shown on my map without much success. Only later did I learn that most maps do not differentiate very clearly between the easy and difficult objects. They are all marked in much the same way. The symbols show the type of object - galaxy, cluster, globular, nebula, etc, but often not how bright (or faint) they are. I spent hours looking for objects on my map that seemed interesting, only to learn later that they were almost beyond the reach of my first seven inch aperture f/15 scope. I hope these articles will help beginners select targets that are easy and rewarding to find with fairly simple equipment.
One of the important lessons to learn early is that deep sky objects look their brightest and best when they are at their highest in the sky. This happens when they cross the meridian, an imaginary line that goes from north to south, passing through the zenith (the point directly overhead). I will present some of the brightest objects that will be on or close to the meridian at around 10pm during the month the article appears. Remember, the same objects will be on the meridian at 8 pm one month later, or at midnight a month earlier. Beginners might appreciate a few words about the types of objects we shall be looking at and the various maps I will suggest for reference.
Most deep sky objects are classified as globular clusters, open clusters, clusters with nebulosity, nebulae, planetary nebulae and galaxies. There are other objects, but they will not appear in these articles. All but galaxies are objects located within our own Milky Way galaxy. The oldest and, to my eye, the most spectacular of these are the globular clusters, bright collections of hundreds of thousands -even millions - of stars that formed very early in the life of our galaxy. They are gravitationally bound together and appear as hazy oval patches of light. The biggest and brightest can be resolved into individual stars. When observing them, look for the shape, usually round, but slightly flattened, then for the density of the central core and the width of the outer edges. A few of the brightest globulars show patterns of resolved stars, and in some, coloured stars can even be seen.
Open clusters consist of groups of stars formed relatively recently, having a common birthplace and still traveling together. They are usually less dense, have many fewer stars - perhaps hundreds or dozens of stars, and usually resolve into individual stars in the smallest scopes or binoculars. The individual stars will eventually separate from other cluster members over time. Look for the closeness and brightness of the stars in the cluster, for the number of stars, the colours, and the patterns they form. Some clusters still have the gas and dust from which they evolved around them, which is lit up by the stars and seen as nebulosity. They are known as clusters with nebulosity. Plain nebulae are usually clouds of dust or gas unconnected with a star cluster and lit up by light from a hot star nearby or by a new star forming deep within the nebula. Nebulae tend to be faint and need care to see. Rather than looking directly at them, shift your gaze to the edge of the eyepiece field, which allows your eye to use its more sensitive cones to detect the fainter light. This is known as using “averted vision”.
Planetary nebulae have been dealt with at length in a series of recent articles in Universe (Neat Southern Planetaries) by Andrew James. In short, they are stars in their death throes, shrugging off their outer layers and shrinking into their white dwarf stage. Planetary nebulae have several typical shapes, but often appear as small round disks similar to planets, hence their name. In a few cases we can see the central star shining through the disk.
Galaxies are huge collections of gas and stars existing as separate entities in the universe. Our Milky Way galaxy, which we see from the inside, is a rather large example of the genre. When observing external galaxies, we look for the general shape, the presence of a central core, or nucleus, and any evidence of spiral arms, knots of bright gas or other “structure” in the galaxy we are observing. Only a few galaxies are bright enough to make it on to these lists. Most are small and relatively faint as they are so very far away, usually tens of millions of light years.
OK, the brightest example of these are the objects we shall be looking for, but how do we find them? We will need a good star atlas. There are many on the market and most are adequate. I have selected a three of the popular atlases as examples, but you can use others just as well. Many beginners buy the Nortons Star Atlas, which is very useful for identifying constellations easily and for relating them to the whole sky. It is important to know the bigger and brighter constellations and the brightest stars as a means of basic navigation. Despite its smaller scale, Nortons is quite good for that. However, I find the way Nortons displays deep sky objects rather confusing, so I seldom use it for observing. Sky Atlas 2000 is a larger scale atlas that shows more deep sky objects and is very useful once you know the basic constellations and brightest stars. It is a good general purpose atlas for many users. Even more comprehensive is the Herald-Bobroff Astroatlas which contains a wealth of detail (far beyond the scope of these articles) but is printed on a scale that makes it easier to find your way around the sky. This atlas is particularly useful for beginners as it attempts to identify the brighter objects by using increasingly bold type to show them. I have provided Sky Atlas 2000 and Herald-Bobroff chart numbers for all objects mentioned in the articles. However, I am sure most objects will be shown on any other atlas you may be using. Even the monthly sky charts produced by most astronomy magazines will show many of them. You certainly won’t need a super-duper atlas for these!
Now a word about how objects are classified. On most charts, different symbols are used for each type of object. Alongside the symbol is a catalogue number. Many objects in our lists will be shown with the letter “M” followed by up to three digits. These appear in the Messier Catalogue, compiled by Charles Messier in the eighteenth century. Messier was a comet hunter and made up his catalogue of just over 100 objects which he thought could be confused as comets by the unwary. The optics of the telescopes he used were equivalent to, or less than, that provided by a good pair of binoculars today. Despite that, he found so many of the brightest deep sky objects that his catalogue numbers are still in use today. During the nineteenth century, a much more comprehensive catalogue of deep sky objects was published under the title “New General Catalogue”. The brighter objects on this list appear on most charts marked as NGC XXXX (up to four digits). However, the letters NGC are often deleted to avoid clutter. Most charts show objects from many other catalogues as well, but the NGC and Messier catalogues contain virtually all the objects we shall be looking for. Note that the NGC catalogue also contains all the Messier objects, giving each a four digit NGC number. In these articles I will identify all Messier objects by their Messier number.
Finally, a word about eyepieces. If you have more than one eyepiece, use the one with the larger number printed on it - meaning it has the wider field of view and less magnification - while you are looking for the object. When you find it, you may care to change to an eyepiece with a smaller number, meaning that it has greater magnification but a smaller field of view. Many of the objects we are seeking may well look their best at lower magnification, depending on the telescope optics and the steadiness of the sky at the time.
For the sake of better navigation, we should start out tour at the northern end of the meridian. If we face due north, we will see the bright star Vega (Alpha Lyrae) about 15 degrees above the horison and almost on the meridian. About 6 degrees south-east (above and to the right) is the 3rd magnitude star Gamma Lyrae, alongside which, to the west (left), lies our first target. M57 is a planetary nebula which is quite bright and appears in modest scopes as a smoky ring. My observing notes say “Superb bright annulus, flattened on the north and south sides, with a bump on the outer south-west edge of the ring. There are slight irregularities on the inner ring edge and the central disk is much fainter. There is no central star.” Some observers claim to see grey or green tinges in the ring, but I was not so favoured. It is undoubtedly a great object and exciting to find.
On the east side of the star Gamma Lyrae and some three degrees to the south (a little up and slightly right) lies M56, a fairly bright globular cluster. My observing notes say “Irregularly round-shaped, resolved to the core, condensed to the centre, with ragged-looking but resolved outer edges. It is set in a field of bright stars.”
Our next target, in the constellation of Vulpecula, can be seen easily from a dark sky site without optical aid. Look straight up from M56 and you will see a brightish group of closely-placed 5th and 6th magnitude stars. Best seen through binoculars or the finder of a telescope, this is one of the few objects on our list that does not have a Messier or NGC catalogue number. In fact it is a rather large open cluster of stars known as Collinder 399 which, when seen at low magnification, looks remarkably like a “coat hanger”, by which name it is known to most amateurs. It is quite a remarkable aggregation of stars. They are marked on Sky Atlas 2000 close to the border of Sagitta, beside the faint open cluster NGC 6802, but not identified as a cluster. The Herald-Bobroff Astroatlas identifies them as “Col399”.
A few degrees east and slightly north we find the bright planetary nebula M27, known as the “Dumbell Nebula”. It lies just south of star 14 Vupeculae, which completes the eastern apex of an equilateral triangle formed with the 4th and 5th magnitude stars 12 and 13 Vulpeculae as a base. My observing notes say: “A superb, bright and luscious object. The huge, bi-polar shape is very clear. The central “dumbell” handle appears brightest, with the two lobes somewhat fainter, but showing very complex structure. There are several stars embedded. It is hard to pick which is the progenitor star (or planetary nebula nucleus). An O 3 filter extends the lobes but blurs them slightly. An excellent object.”
About 4 degrees south and slightly west, lying almost centrally between the 4th magnitude stars Gamma and Delta Sagittae lies our next target, M71, a fairly bright globular cluster. My observing notes say: “Small, well resolved, slightly condensed to the centre, the shape appears almost triangular with the peak of the triangle facing to the east. A most unusual globular.”
Skimming south-west along the band of the Milky Way, we move into the oddly shaped constellation of Serpens to find our next target, the very bright open cluster with nebulosity, M16, known as the “Eagle Nebula”. The central cluster of stars which form the body of the eagle are very bright in smaller scopes, but the spectacular nebulosity which forms the wings may prove somewhat more elusive. Larger apertures, especially if enhanced by OIII or UHC filters, show this clearly. My observing notes refer to the complexity of the nebulosity, especially the winding dark lanes that thread throughout the whole complex. It is a wonderful object. Most observers will be familiar with the extraordinary Hubble Space Telescope image of star forming areas in this nebula.
Just two degrees due south of M 16, and just over the border of the constellation Sagittarius is the equally spectacular, slightly larger cluster with nebulosity comprising M17, popularly known as “The Omega Nebula”. In this object, the situation appears reversed: the nebulosity is as bright as, if not brighter than, the cluster stars. The appearance is unmistakable. The nebulosity is very bright and the characteristic shape of the figure 2 is seen in scopes of quite modest aperture. Larger apertures reveal much more extensive nebulosity in the main object, supplemented by fainter, detached clumps of nebula around it and extensive rivers of darkness threading throughout, most noticeably around the throat of the number 2. This must rate as one of the brighter nebulae in the sky and lies in a very busy area of the Milky Way.
To prove the point, move almost due south by another three degrees and you will come to one of the greatest spectacles in the Milky Way - M24, also known as “the star cloud in Sagittarius”. Turning to this remarkable area always makes my pulse race; the sight of hundreds of thousands of tiny stars strewn across the field of any eyepiece is a spectacle not repeated often. Here we are looking towards the centre of our galaxy where most of its stars reside. Unfortunately for us, the richness of the galaxy centre is obscured by vast clouds of dust. Only rarely do they part, allowing us a glimpse of the delights they hide. M24 is one of the few places where they do and, to my mind, provides the best viewing. My observing notes wax lyrical with such phrases as “superb area, billions of bright and colourful stars massed together…one of the truly great sights in astronomy…” and similar. Form your own opinion! There is even a small individual star cluster, NGC 6603 within and lying at the northern end of the cloud, which will challenge smaller instruments. See if you can pick it out.
Moving south-west about four degrees along the line of the Milky Way we come to one of the favourites in Sagittarius, M20, the beautiful Triffid Nebula. This is a large, almost round nebula that is crossed by dark lanes that divide it into three prominent sectors, very bright in the centre and fading gradually away to the edges. Careful observation shows many embedded stars, as well as a faint reflection nebula and many smaller patches of nebulosity on the perimeter. A very fine object.
But so, too, is its companion just one degree to the south, M8, the Lagoon Nebula, another even larger mass of nebulosity, once again crossed by complex dark lanes and illuminated by many stars embedded in the nebulosity. A wonderful object that repays detailed study. Almost seven degrees almost due east lies M22, one of the finest globulars in this incredibly rich area of the sky. Large, almost round, very bright, dense, resolved to the core, with extended edges that blend into the starry background, this is a marvellous example of the genre, and not to be missed. There are many other globular clusters in this locality, far too many for the scope of this article. Moving south west again along the milky way we come to two large, very obvious naked eye objects M6 and M7. Both are bright and rich clusters, best seen with binoculars or a finder scope but too large to impress when seen through a scope.
We have strayed a little west of the meridian and must leave the seductions of the Milky Way to get back on course. Our next objects are right back on the meridian, the globular cluster NGC 6723 and the adjacent area of nebulosity, NGC 6726/27/29 found on the border of Sagittarius and Corona Australis. My observing notes of NGC 6723 state “Well resolved with extensive spiral sprays of small stars across the face. The core is condensed and surrounded by wide, resolved edges containing patterns of tiny stars. A most interesting object.” Less than one degree to the south east lie three nebulae. NGC 6726 and 6729 are reflection nebulae, each comprising a bright haze surrounding a pair of stars, while NGC 6727 is a rather comet-shaped area of bright and dark nebulosity, also lying between a pair of stars of magnitude 7 and 8. These objects are set in interesting field of rather dark and obscured sky with few stars on display.
Further south lie the rich constellations of Ara and Pavo, both containing excellent globulars and rich clusters of galaxies. NGC 6397 in Ara is a fully resolved globular cluster, well suited to smaller apertures. It lies in a busy, bright star field and my observing notes say: “Highly resolved, strongly flattened on the south side, with bright stars in spiral patterns across the whole face. Condensed to a small, dense, hazy core, the edges blend into the star field. Good object.” Moving south east into Pavo we find NGC 6352, a very large and bright globular cluster that can be glimpsed with the naked eye from a dark sky site. My observing notes say “Superb globular. Oval shape, with bat-like wings on the south side. The core is small, intense but resolved, fading out to wide irregular edges. There is a mag 7 star close to the southern edge. Pretty object.”
CRAB NEBULA - The Most Conspicuous known supernova remnant. This object caused Messier to begin his catalog
Finally, NGC 6744, a fine spiral galaxy in Pavo rates mention, even though it is somewhat fainter than most other objects in this article. However, quite modest apertures will show the oval, barred core, and the fainter, tightly wound arms fading into a general halo effect. This is a most interesting example of a spiral galaxy.
Here our tour concludes for this month. We have traversed all too quickly over the richest span of the Milky Way, picking the eyes out of an unbelievably rich offering of astronomical treasures. In numbers, we have missed far more than we have seen, but hopefully, we saw the brightest and best. Next month we move on two hours to a new area of the sky. See you then.