Identification of Star Clusters

The task envolved in identifing faint open clusters is sometimes not as easy as first assumed. The number of stars and the angular size from a catalogue can give a rough assessment of the appearance in the eyepiece. This, however, assumes that you know the size of the apparent field of view, the magnification and the limiting magnitude of the telescope. Yet the geometric parameters of clusters and the magnitude range or quantity of observable stars become the major unpredictable variation. Only a few star clusters are large in size and resolvable to the naked-eye. The most famous include than, say, the Pleiades, the Hyades or the best soutern cluster, NGC 2516. Visible as naked-eye objects, these clusters are readily seen in any clear and dark nighttime sky. All, too,are partially resolvable assuming reasonably good eyesight! Stellar numbers increase substantially with optical aid (like binoculars). However, the angular resolution will not improve as much, unless medium-sized telescopes are used. To do justice to any tight open cluster, or even some globular star clusters, either 20cm or 25cm telescope will improve the resolution tremendously, especially if the observing location has good seeing.It is certain 25cm telescope could study all clusters contained within the NGC and IC catalogues. In fact the basis of both these catalogues is 30cm telescope, so the description will meet the standards of advanced amateurs.

Amateurs should also be aware that other catalogues of star clusters can be identified in medium-sized telescopes. With the advent of the larger Dobsonian telescopes, the clusters far out weigh those written in the non-NGC and non-IC catalogues.

Identification of the majority of clusters are mainly limited to the confines of the Milky Way. The fainter stars that lie within the cluster are normally difficult to identify because they blend with the foreground stars. Most of the outlying stars will normally be misidentified, as they merge with these same stars. If the cluster happens to lie beyond the band of the Milky Way, then identification of the outlying stars become far more easier to distinguish.

Some clusters can also be seen associated with bright nebulosities. In crisp skies, the reflection nebulae can also be sometimes be glimpsed in medium to large telescopes using low magnification and averted vision. Most of these are much easier to photograph, especially using blue sensitive film or the CCD (possibly with an addditional blue filter.) Newer aged clusters may also be associated with the bright emission nebula. In these cases, it would be best to use a red filter. Photographers using film or CCDs have to show some caution because the brightness of the nebula may eliminate the stars if the exposure is extended too long. Nebulae regions like M8 / NGC 6523 The Lagoon Nebula, and M16 / NGC 6611 The Eagle Nebula — both in the constellation of Sagittarius - can be even be telescopically seen with both stars and nebulosity. Other clusters like NGC 2174-2175 The Monkey Nebula in Orion, NGC 2467 in Puppis and NGC 6960 / Veil Nebula in the northern constellation of Cygnus. All these nebulosities that can be easily photographed, and are common targeys for amateurs.

A few clusters show distinct visual colours. Most of the newer clusters display a distinct blue or greenish colouration. Some of these clusters, like M7 / NGC 6475 in Scorpius and NGC 2156 in southern Carina have one solitary bright red star among many distinctly blue stars, that can appear quite attractive. A few shows some coloured range, like that of the Jewel Box / NGC 4755 is probably the most stunning of this type.

Older star clusters are normally not favoured by amateurs. These can show distinct yellowish or orangish colouration due to their distance and the amount of interstellar absorption. Most are uninteresting because they do not have much variation in the apparent magnitudes of the components. (23) For most observers, these specific clusters are not usually high on the to do observing lists. Once seen most are not often reexamined by amateurs. Southern observers can find many examples. Among the best are some of these open clusters that appear in southern Centaurus and Lupus, in places just beyond the northern edge of the Milky Way.

Some of these visually older cluster also appear redder because of the reddening of the starlight due to significant interstellar absorption. Examples of these include both NGC 5617 and NGC 5236 and NGC 5281, all in Centaurus and are each suitably placed near the southern pointers of α and β Centauri.

Among the best very examples of all of these clusters types mentioned
throughout this Webpage can be seen on the Open Cluster Tables.

Descriptions of Some Bright Southern Open Clusters

Table 1. Selected List of Bright Open Star Clusters for Small Telescopes

h   m
 o  m
Pleiades/ M45 03 47.5 +24 07 1.2 125 100 η Tau
Mel 25 / Hyades 04 26.9 +16 00 0.5 46 100 α Tau
NGC 2168/ M35 06 09.0 +24 20 5.1 870 200 1 Gem
NGC 2287/ M41 06 46.0 −20 44 4.5 700 80 π CMa
NGC 2362 07 18.7 −24 57 4.1 1550 60 τ CMa
NGC 2422 07 36.6 −14 30 4.4 480 30 4 Pup
NGC 2437/ M46 07 41.8 −14 49 6.1 1660 100 4 Pup
NGC 2516 08 57.0 −60 45 3.8 400 80 ε Car
NGC 2632/ M44 08 40.4 +19 40 3.1 160 50 δ Cnc
IC 2391/ Vel 08 40.3 −52 55 2.5 180 30 ο Vel (24)
NGC 3114 10 02.7 −60 06 4.2 900 90 q Car
NGC 3293 10 35.8 −58 14 4.7 2500 93 r Car
IC 2602 10 43.0 −64 24 1.9 150 60 θ Car
NGC 3532 11 05.5 −58 44 3.0 410 150 u Cen
NGC 3572 11 10.3 −60 15 6.6 2300 35 y Car
NGC 3766 11 36.3 -61 37 5.3 1700 100 ζ Cen
NGC 4755 12 53.6 −60 21 4.2 2340 140 κ Cru
NGC 6067 16 13.2 −54 11 5.6 2100 100 κ Nor
NGC 6231 16 54.2 −41 50 2.6 1800 70 ζ Sco
NGC 6405/ M6 17 40.7 −33 16 4.2 600 80 λ Sco
NGC 6475/ M7 17 53.9 −34 47 3.3 240 80 G Sco
NGC 6613/ M18 18 20.0 −17 06 6.0 1500 40 μ Sco
NGC 6705/ M11 18 51.1 −06 16 5.8 1720 200 η Sct

Open Cluster Tables

There are another three (3) separate Tables with more clusters for the experienced observers;

A) Some Bright Open Clusters for Small Telescopes (OSC: Part 5)
1) Fifty Best Southern Open Star Clusters (OSC: Part 8)
2) Fifty Best Northern Open Star Clusters
3) Best One Hundred Open Star Clusters

References and Endnotes

23. Clusters northern edge of the Milky Way and in the western side of the constellation of Centaurus are good example of these types of clusters.
24. This is also known as Velorum or the Velorum Cluster.


Last Update : 27th November 2012

Southern Astronomical Delights © (2012)

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