OPEN STAR CLUSTERS : 5 of 10
Identification of Star Clusters
Our task involved identifying faint open clusters is sometimes not as easy as might first be assumed. Star numbers and the angular size placed in a catalogue can give only rough assessments its appearance in the eyepiece. This, however, assumes that you know the field of view, magnification and the telescopic limiting magnitude. Yet geometric parameters of clusters, magnitude range, or quantity of observable stars, become the major unpredictable variant. Only very few star clusters are large and resolvable to the naked-eye. Among the most famous include: The Pleiades, The Hyades, or the best southern clusters, NGC 2516 or the The Southern Pleiades (IC 2602). Visible as naked-eye objects, these clusters are readily seen on any clear nighttime sky. All too, are partially resolvable to the eye, assuming reasonably good eyesight!
Stellar numbers do increase substantially with optical aid, including binoculars. Yet, the angular resolution will never improve unless medium-sized telescopes are employed. To do justice to any tight open cluster, or even some globular star clusters, either 20cm. or 25cm. telescope will improve 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 descriptions will meet expected standards of advanced amateurs.
Amateurs should also be aware that other star cluster catalogues can also 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. Fainter stars that lie within the cluster are normally difficult to identify because they sometimes blend in with the foreground stars. Most outlying stars will typically be misidentified, as they often just merge with these same stars. If some cluster happens to lie beyond the band of the Milky Way, then identification and distinguishing outlying stars become far more easier.
Some clusters can also be seen associated with bright nebulae. In crisp skies, the reflection nebulae can also be sometimes 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 additional 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” — in Sagittarius can be even be telescopically seen with both stars and nebulosity. Other clusters like NGC 2174-2175 in Orion, NGC 2467 in Puppis and NGC 6960 / “Veil Nebula” in the northern constellation of Cygnus. All these nebulae that can be easily photographed, and are common targets for amateurs.
A couple of clusters show distinct visual colours. Most of the newer clusters display distinctly 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, making them quite attractive. A few show ranges of coloured, like that of the Jewel Box / NGC 4755 is probably the most stunning of this type.
Older star clusters are normally not seemingly favoured much 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 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 reddened because of the reddening of the starlight by significant interstellar absorption. Examples of these includes; NGC 5617, 5236 & 5281, all in Centaurus, being each suitably placed near the southern pointers of α and β Centauri.
Among the very best 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
24. This is also known as ‘Velorum’ or the ‘Velorum Cluster’.
Last Update : 19th April 2017
Southern Astronomical Delights © (2017)