OPEN STAR CLUSTERS : 5 of 10
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 CCD’s 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
|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|
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
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’.
Southern Astronomical Delights © (2012)