Ottawa Centre Monthly Observing Challenges

Beginning in 2007, Ottawa Centre has introduced a new item to its monthly meetings: an observing challenge object. Each month the centre will post a description of an interesting object. Members are invited to observe, sketch, photograph, or otherwise learn about this object. At the subsequent monthly meeting, an agenda item will provide an opportunity to share your thoughts, photos, sketches, or comments.

April 2007:

Deep Sky

by Glenn LeDrew

Beginner: Melotte 111, the Coma Berenices cluster.

To the ancients, the faint swarm of stars located above the hind end of Leo was identified as the tuft of the Lion's tail. In Classical mythology it was transformed into its own constellation, Queen Berenices' hair. Today we know it as the third nearest of all known star clusters. (The nearest is the 75 l-y distant Ursa Major cluster at the top of the finder chart, followed by the famous Hyades at 145 l-y.) While the unaided eye can fairly well resolve Melotte 111 in a dark country sky, humble binoculars are the ideal instrument with which to observe this charming cluster. And because the spring evening sky has the milky way poorly placed near the horizon, there are all too few bright star clusters to be seen, making Mel 111 an atypical object for the season. And a refreshing change from galaxy hunting for the avid deep-sky observer!

The large apparent size of Mel 111 is due to its comparative nearness, lying as it does some 280 l-y distant. Catalogues of clusters list its diameter as 5 degrees, so its true diameter is therefore about 24 l-y (nearly six times the distance between us and the Alpha Centauri star system). Containing only about 80 members, the space density of the cluster is quite low, being only 0.1 stars per cubic parsec, equivalent to the average star density in the Solar neighbourhood. Clearly, at best this group is only weakly bound gravitationally.

The low density is almost certainly due to its rather advanced age. After nearly 500 million years since its formation--2 full trips around the Galaxy--Galactic tidal forces, and perhaps the odd close passage of a large molecular cloud, have seemingly stripped away all the low mass K- and M-type stars. The more massive stars have long since faded away, either in supernova explosions or as evolved white dwarfs. The remaining stars occupy a rather small range of masses in the middle of the main-sequence, from early-A to early-G. As a result no strongly coloured stars are present; indeed the only non-white you're likely to see could be qualified as "cool-white" or warm-white". The handful of yellow and orange-ish stars in the field are all non-members, although they do contribute a nice touch to the scene.

The visually brightest star is 4.8-magnitude 12 Com, which is also an easy binocular double, its magnitude 8.3 companion lying just over one arcminute to the south. The other double in this cluster is 17 Com, its 5.3m and 6.6m stars being separated by a very generous 2.4 arcminutes. These two doubles are labelled in the close-up chart; 12 Com is partly resolved, while 17 Com is fully resolved. Also identified (with vertical bars) are several of the "outliers" which are well removed from the more concentrated and obvious central core.

An interesting fact regarding this cluster has to do with its location on the sky. Open clusters are found almost exclusively in or near the band of the milky way; only a very small fraction of their number can be found more than 20 degrees removed, and most of those are comparatively nearby. Mel 111 happens to lie about as far from the milky way band as it's possible to get--practically at the north Galactic pole (NGP)! So it's positioned "right over our heads" with respect to the plane of the Galaxy's disk plane, in which we reside. The Galactic disk can be considered to be some 2,000 l-y thick, so this cluster is certainly well within it--in our back yard, so-to-speak.

Advanced: M53 and NGC5053--a study in contrasts.

Like Melotte 111, these two globular clusters are located in Coma Berenices, and hence are seen in the direction of the north Galactic pole. But where the former is well within the thin Galactic disk, the latter pair lie far out into the halo, some 200 times farther than Mel 111, and twice as distant from us as the Galactic centre. In spite of its great distance, M53 can be easily spotted as a fuzzy star in most binoculars. NGC5053, on the other hand, will present something of a challenge for small telescopes, or for larger 'scopes when sky conditions are mediocre.

For the star hopper, finding these objects is simplified by the fact that they lie quite close to the easily-identified alpha Com. M53 is almost exactly one degree north-east of alpha, and NGC5053 is in turn almost exactly one degree south-east of M53.

Both of these clusters lie at fairly similar distances, and furthermore lie almost exactly along a common line of sight, so their differences in appearance must be due primarily to their intrinsic qualities.

M53 is a typical globular cluster having a moderate degree of central concentration (class 5 on a scale of 12, where 1 is the most concentrated). Its total light output equals that of 250,000 Suns, which makes it intrinsically a bit brighter than the average.

NGC5053 is among the least concentrated (class 11) of all known globulars. It's also among the least populated, with a total light output of 35,000 suns, or 1/7th that of M53. This brightness difference is in itself not large. What makes NGC5053 so much harder to see is its very slight degree of central concentration, combined with its large diameter. In other words, the light is spread out almost evenly and over a large area, so that even at the centre it is just barely as bright as the darkest night sky! Such low contrast requires higher magnification for detection, as compared to the much more condensed M53.

The differences in the appearance of globular clusters can be appreciated by the comparison of three parameters; core diameter, apparent diameter and tidal diameter. The core diameter is usually defined as the circumference where the surface brightness (SB) has fallen to half the peak SB at the centre. The apparent diameter is somewhat arbitrarily defined, but in most cases can be taken as the circumference where the SB is 25 magnitudes per square arcsecond (three magnitudes fainter than a dark sky). The tidal diameter is the ultimate limit of a cluster's boundary, outside of which the Galaxy's gravitational field dominates. While a calculated concentration class (as opposed to the arbitrary 12-step scale still in use by visual observers) uses the ratio of the core and tidal diameters, for visual observation the apparent diameter is better than the tidal diameter.

M53 has a central SB nearly 5 magnitudes brighter than that of NGC5053, or a factor of about 100! Its core is a very small 0.5 arcminute in diameter, which is only 1/25 of the apparent diameter. That's quite a central "spike" in surface brightness! In contrast, NGC5053 has a much larger core 4.5 arcminutes in diameter, which is almost half the apparent diameter. No surface brightness "spike" here.

The similar distances to both clusters would imply similar brightnesses of stars of a given type.To be more precise, stars in the nearer NGC5053 will be 0.28 magnitudes brighter. At any rate the brightest red giants in each cluster are around magnitude 14, requiring a 7-inch telescope to glimpse. The dense haze of M53 will make it easily seen under most sky conditions as a well concentrated glow with numerous superimposed stars. In the case of NGC5053, if the sky is not sufficiently dark only a handful of individual stellar specks will mark its location. In a truly dark sky, however, a large binocular can reveal the subtle glow from the combined light from the faint, but more numerous stars which would be invisible in even large telescopes from the suburbs.

Also included as an even harder challenge is the tiny field galaxy IC858. The computed mean surface brightness is a fairly low 23.8 magnitudes per arcsecond, although the central SB is certainly brighter. A dark sky and a large telescope are required!

Header Image Credit

The icon leading to this page and image in the header are images of globular cluster M3 produced by Ottawa Centre member Albert Saikaley. This was chosen as the icon of the "Challenge Objects" section because the globulars are within the range of beginners with modest telescopes, yet advanced amateurs are still challenged by teasing out the spectacular colours and details such as those in Albert's image.

Modified: June 11, 2007