OPEN STAR CLUSTERS : 2 of 10

The Open Star Clusters

Galactic or Open Star Clusters are exquisitely beautiful in either small or medium sized telescopes, and as their name suggests, they are relatively easy to resolve in a telescope into individual stars. Each are gravitationally associated collections of stars, numbering between twenty and one thousand components.

Some open star clusters display very young stars giving the cluster its distinct blue colour. Most commonly known examples include The Pleiades (M45) and The Hyades both in the zodiac constellation of Taurus, or the southern examples known as The Southern Pleiades (IC 2602 surrounding θ Car) in Carina, and the Velorum Cluster (IC 2391 and around ο Vel). Some other clusters visually have some impressive colour variations that may contain gems of several blue, yellow and red stars. This includes the famous Jewel Box (NGC 4755), NGC 3293 in Carina and NGC 3532 also popularly known as the Football Cluster.

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↑ Image of NGC 3293 :
20cm. 127× 26mm Plössel (32′)

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Such observed colour differences transpire because when the star cluster is formed, all the stars start on their evolutionary paths simultaneously. As the cluster ages, the larger stars are first start run out of fuel, and begin their individual evolution upon such energy crises and soon the star rapidly swells to becoming a red giant. The other cluster stars will then follow, each evolving in turn by order of decreasing mass. If any star cluster contains one or more red stars among their number, then they will be likely be the heaviest, and sometimes the oldest, of all the cluster stars. Most of these coloured clusters do appear rather attractive in telescopes. As mentioned, one of the best example for southern observers is the so-called Jewel Box (NGC 4755) quite near the bright star Mimosa (β Crucis) in the Southern Cross. This A-shaped asterism contains the full compliment of star colours and is truly magnificent. (See Detailed Southern Astronomical Delights Article on this site “The Jewel Box — NGC 4755”)

The Loose Sub-Clusters
Associations and Moving Clusters

The common used terminology of the sub-classes of star clusters are;

As the name associations infers, they are groups of stars that are loosely associated together. It was V.A. Ambarcumjan who discovered this class of object in 1952, and who was to eventually subdivided them into their modern designations. (1) It was only later that the astronomers W.W. Morgan and K.H. Schmidt called them stellar aggregates. (2) Unlike the open and globular clusters, the gravitational attraction between these stars is much weaker. Most associations are considered as gravitationally quite unstable objects that can only exist for periods up to ten million years or so before literally falling apart into individual star that just seemingly moving in the same direction through anagalactic space of our Milky Way. Most subtend anywhere between thirty and two hundred parsecs (30 to 200 pc.) across. Sometimes they may contain multiple stars within their own environs and are often associated with long chains of stars of similar spectral types. Associations divide in sub categories; OB Associations, T Associations, C Associations and Moving Clusters.

OB Associations

Any OB Association normally has anywhere between ten and several thousand stars. The majority of stars are comprised of hot O-type or B-type spectral classes. OB Associations types are far more numerous and diverse. Some may have bright open star clusters as their nuclei, though this is not always the case. Rarely, some are associated with bright nebulae. Most OB Associiations appear as seemingly an amorphous collection of stars, and thus are impossible to identify except by using of finder charts. Most of the stars attached to any group were found by spectroscopy. Then by determining those with similar radial velocities, we can sorting out those which appear to exceeded some particular range of criteria. For the nearest associations, some have been determined by measuring all the similar stellar parallax shifts. Of these loose stellar groups we know more than one hundred and forty (140), with most typically lying along or near the galactic plane. (See Fig. 2-1.)

Figure 2-1 : DISTRIBUTION of the ASSOCIATIONS. This shows the places as Associations from the galactic plane, which deviates only ± 20°. The x-axis is the longitude (L of the galactic plane, where 0° is the galactic centre in Sagittarius. The y-axis is galactic latitude b, giving the distance above or below the galactic plane.

Associations are normally attached to some central open star cluster, which are usually bright, young and prominent NGC or IC objects, also typically containing many hot blue stars. Examples include;

NGC 4755 in Crux, attached to Cen OB1.
NGC 3293 (and possibly IC 2581), attached to Car OB1
NGC 3572 (Eta Carina Nebula), attached to Car OB2
Trapezium (and the Orion Nebula), attached to Ori OB1
NGC 3572 (Eta Carina Nebula), attached to Car OB2
IC 2944 (around λ Centauri), attached to Car OB2
NGC 6231 (in Scorpius), attached to Sco OB1
h & χ Per (Double Cluster in Perseus), attached to Per OB1

Not all the associations are attached to known open clusters, and a few of these are only just suspected.

By size, the smallest may cover only ½°, like Cyg OB2 (23324+4217). The largest is Lac OB1, which subtends some 15°×9° of sky. Of most interest to southerners is probably Cen OB-1, which is joined with the Jewel Box (NGC 4755), and whose Association surrounds a circular region of about 6° around this cluster. Most of the other known southern associations are typically some 5° to 10°.

General distances for most of the brighter OB Associations are mainly found between 500 and 4000 parsecs from the Sun. Southern associations are related to either the Crux-Norma arm of the galaxy, being roughly 2.0±0.5 kiloparsecs away from us; and the Carina and Sagittarius Arm, which is just fractionally closer. The northern Associations lie in the Perseus Arm, whose distances exceed >3.0 kpc. Most distant known is Cep OB1 that includes the bright star β Cephei, whose distance is about 3.5 kpc.

OB Association distances are notoriously unreliable, and are often quoted using the open cluster attached to it. No doubt more OB Associations are within the Milky Way, but we know little about them because of the difficulty in measuring the smaller stellar motions they may contain.

Associations are normally listed by combining three-letter constellation abbreviation followed by the letters ‘OB’ and have attached an additional Roman numeral — based on the order of their discovery. This particular system was first introduced by the observer Markarjan — correctly called the Markarjan designations. I.e. Cassiopeia V, etc. Morgan, and later Schmidt, who also arranged these associations by a given Roman numeral then followed by the constellation. Each group was also placed in order of galactic longitude within the constellation. Such designations caused needless problems to the IAU, and by 1962, Commission 37 “Designation of Astronomical Objects” recommended that the system needed significant improvement. (3) The final system was suggested and entrusted to J. Ruprecht in 1966 at the IAU Commission 37. (4) Based on the nomenclature report, associations are now named as; I.e. CMa OB1, Sco OB4 etc. (5)) Some are also named by the closest bright star that lies near the association. I.e. β Persei Association. (7),(8)

Some OB associations are truly huge in size. Some are designated by constellation to constellation like the so-called Sco-Cru Association that extends from Scorpius to Centaurus — including many bright blue stars throughout Centaurus, Crux and many other bright stars in Lupus, Ara and Scorpius. Another is less known southern example is the Pup-CMa Association. (9)

List of Known Associations

The following Tables list the all the known OB Associations and those south of −30° declination. Although most Associations are fairly uninteresting for the amateur observer, often their central clusters can be very attractive.
(Note: The southern ones I hope to add some more data in due course.)

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ASS  Name     RA (2000) Dec.  Gal. Co-Ord | ASS  Name     RA (2000) Dec.  Gal. Co-Ord
No.  Name     hh mm.m   o  ′    L     b   | No.  Name     hh mm.m   o  ′    L     b  
******************************************|******************************************
35   Cas II   00 21.4 +62 34  119.5 -00.1 | 65.2 Nor OB3? 16 24.0 -52 21  332.2 -02.0
36   Cas OB4  00 28.4 +62 42  120.3 -00.1 | 64.5 Cir-Nor? 16 36.1 -59 11  322.6 -02.8
37   Cas OB14 00 28.8 +63 22  120.4 +00.6 | 66   Ara OB1  16 39.5 -46 46  338.0 +00.0
38   Cas OB7  00 56.1 +63 42  123.5 +00.8 | 66.1 Ara OB2  16 44.2 -47 29  338.0 -01.0
39   Cas OB1  01 00.8 +61 30  124.1 -01.4 | 67   Sco OB1  16 53.5 -41 57  343.3 +01.2

40   Cas OB8  01 46.2 +61 19  129.5 -00.9 | 69   Sco OB4  17 14.7 -33 10  352.8 +03.2
41   Cas OB10 01 47.3 +55 40  130.8 -06.4 | 69.1 Sco OB6  17 30.9 -32 10  355.6 +00.9
42   Per OB1  02 14.5 +57 19  134.0 -03.8 | 69.2 Sco OB7? 17 36.3 -32 38  355.8 -00.2
43   Cas OB6  02 43.2 +61 23  135.9 +01.4 | 01   Sgr OB5  17 50.9 -29 27  000.2 -01.3
46   Per OB3  03 27.8 +49 54  147.0 -05.5 | 68.1 Sco OB5? 17 52.6 -38 38  352.4 -06.2

44   Cam OB1  03 31.6 +58 38  142.5 +02.0 | 02   Sgr OB1  18 07.9 -21 28  009.0 -00.6
47   Per OB2  03 42.2 +33 26  159.3 -17.2 | 05   Sgr OB6  18 11.0 -16 49  013.4 +01.0
45   Cam OB3  04 02.4 +56 37  147.0 +03.0 | 03   Sgr OB7? 18 14.7 -20 27  010.6 -01.5
49   Aur OB1  05 21.7 +33 52  173.1 -01.6 | 04   Sgr OB4  18 15.3 -19 03  012.0 -01.0
48   Aur OB2  05 28.3 +34 54  173.0 +00.1 | 09   Ser OB2  18 18.6 -11 58  018.6 +01.7

52   Ori OB1  05 31.4 -02 41  202.3 -17.1 | 06   Ser OB1  18 20.8 -14 35  016.5 +00.0
50   Ori OB2  05 58.4 +21 13  188.1 -01.5 | 08   Sct OB3  18 25.6 -14 14  017.3 -00.9
51   Gem OB1  06 09.8 +21 35  189.1 +01.1 | 12   Sct OB2  18 35.5 -09 08  039.0 +07.6
53   Mon OB1  06 33.1 +08 50  203.0 +00.0 | 07   Ser-Sct  18 38.1 -16 23  016.8 -04.5
54   Mon OB2  06 37.2 +04 50  207.0 -01.0 | 10   Sct I    18 50.4 -05 53  027.6 -02.5

54.1 Mon OB3  06 58.5 -04 44  218.0 -00.5 | 11   Aql OB1  19 01.5 +03 36  037.3 -00.6
55   CMa OB1  07 07.0 -10 28  224.0 -01.4 | 14   Vul OB4  19 43.1 +24 34  060.5 +00.5
55.1 CMa OB2  07 21.7 -20 58  235.0 -03.0 | 13   Vul OB1  19 44.0 +24 13  060.3 +00.2
56   Pup OB2  07 45.2 -27 55  243.7 -01.8 | 15   Vul OB2  19 47.6 +28 49  064.7 +01.8
57   Pup OB1  07 54.8 -27 05  244.0 +00.5 | 16   Cyg OB5  19 51.9 +21 05  067.1 +02.1

58.1 Vel OB2  08 09.5 -47 20  262.8 -07.7 | 17   Cyg OB3  20 04.7 +35 50  072.6 +02.3
58   Pup OB3  08 18.1 -35 46  254.0 +00.0 | 19   Cyg OB8  20 12.9 +40 58  077.8 +03.8
59   Vel OB1  08 49.9 -45 00  265.0 -00.7 | 18   Cyg OB1  20 17.8 +37 38  075.5 +01.1
59.1 Vel OB3  09 27.1 -53 26  275.3 -01.9 | 20   Cyg OB9  20 23.3 +39 56  078.0 +01.5
60   Car OB1  10 46.7 -59 05  287.5 +00.0 | 21   Cyg OB2  20 32.4 +41 17  080.1 +00.9

61   Car OB2? 11 06.0 -59 51  290.1 +00.4 | 23   Cyg OB6  20 52.0 +45 39  086.0 +01.0
61.2 Car OB4  11 08.3 -60 31  290.6 -00.2 | 24   Cyg OB7  21 02.7 +49 43  090.0 +02.1
61.1 Car OB3  11 13.5 -58 52  290.6 +01.6 | 22   Cyg OB4  21 13.1 +37 52  082.5 -07.3
62   Cen OB2  11 35.3 -62 36  294.3 -01.0 | 27   Cep OB2  21 47.9 +61 04  102.5 +05.7
63   Cru OB1  11 58.3 -63 16  297.0 -01.0 | 26   Cep-Lac  22 13.7 +54 01  101.0 -02.0

64   Cen OB1  13 04.8 -62 04  304.5 +00.8 | 28   Cep OB1  22 24.6 +55 14  103.0 -01.9
64.1 Cen OB5? 13 27.6 -62 18  307.1 +00.3 | 25   Lac OB1  22 41.2 +39 05  097.0 -17.2
64.2 Cen OB3? 13 34.1 -61 27  308.0 +01.0 | 30   Cep OB3  23 00.4 +64 03  111.2 +03.8
64.3 Cen OB4  14 37.4 -60 58  315.4 -00.7 | 29   Cep OB5  23 02.1 +57 01  108.5 -02.8
64.4 Cir OB1  14 43.0 -62 28  315.4 -02.3 | 31   Cas OB2  23 19.3 +61 09  112.1 +00.3

65   Nor OB1  15 58.7 -54 30  328.0 -01.0 | 32   Cas OB9  23 37.3 +59 01  113.5 -02.5
65.4 Nor OB5  16 09.7 -48 56  333.0 +02.0 | 33   Cas OB5  23 58.7 +60 22  116.5 -01.9
68   Sco OB2  16 14.9 -25 55  350.1 +17.8 | 34   Cep OB4  23 59.5 +67 35  118.0 +05.2
65.1 Nor OB2? 16 16.3 -52 05  331.6 -01.0 | 70   Cas-Tau                             
65.3 Nor OB4  16 17.4 -51 03  332.4 -00.4 |                                          
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ASS  Name     RA (2000) Dec.  Gal. Co-Ord | ASS  Name     RA (2000) Dec.  Gal. Co-Ord
No.  Name     hh mm.m   o  ′    L     b   | No.  Name     hh mm.m   o  ′    L     b  
******************************************|******************************************
58.1 Vel OB2  08 09.5 -47 20  262.8 -07.7 | 65   Nor OB1  15 58.7 -54 30  328.0 -01.0
58   Pup OB3  08 18.1 -35 46  254.0 +00.0 | 65.4 Nor OB5  16 09.7 -48 56  333.0 +02.0
59   Vel OB1  08 49.9 -45 00  265.0 -00.7 | 65.1 Nor OB2? 16 16.3 -52 05  331.6 -01.0
59.1 Vel OB3  09 27.1 -53 26  275.3 -01.9 | 65.3 Nor OB4  16 17.4 -51 03  332.4 -00.4
60   Car OB1  10 46.7 -59 05  287.5 +00.0 | 65.2 Nor OB3? 16 24.0 -52 21  332.2 -02.0

61   Car OB2? 11 06.0 -59 51  290.1 +00.4 | 64.5 Cir-Nor? 16 36.1 -59 11  322.6 -02.8
61.2 Car OB4  11 08.3 -60 31  290.6 -00.2 | 66   Ara OB1  16 39.5 -46 46  338.0 +00.0
61.1 Car OB3  11 13.5 -58 52  290.6 +01.6 | 66.1 Ara OB2  16 44.2 -47 29  338.0 -01.0
62   Cen OB2  11 35.3 -62 36  294.3 -01.0 | 67   Sco OB1  16 53.5 -41 57  343.3 +01.2
63   Cru OB1  11 58.3 -63 16  297.0 -01.0 | 69   Sco OB4  17 14.7 -33 10  352.8 +03.2

64   Cen OB1  13 04.8 -62 04  304.5 +00.8 | 69.1 Sco OB6  17 30.9 -32 10  355.6 +00.9
64.1 Cen OB5? 13 27.6 -62 18  307.1 +00.3 | 69.2 Sco OB7? 17 36.3 -32 38  355.8 -00.2
64.2 Cen OB3? 13 34.1 -61 27  308.0 +01.0 | 01   Sgr OB5  17 50.9 -29 27  000.2 -01.3
64.3 Cen OB4  14 37.4 -60 58  315.4 -00.7 | 68.1 Sco OB5? 17 52.6 -38 38  352.4 -06.2
64.4 Cir OB1  14 43.0 -62 28  315.4 -02.3 |                                          
******************************************|******************************************

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Abb Constellation  No.| Abb Constellation No.
**********************|**********************
Ara Ara            02 | Lac Lacerta        01 
Aur Auriga         02 | Mon Monceros       03 
Aql Aquila         01 | Ori Orion          02 
Cam Camelopardalis 03 | Pup Puppis         03 
CMa Canis Minor    02 | Nor Norma          05 
Car Carina         04 | Per Perseus        03 
Cas Cassiopeia     10 | Sgr Sagittarius    07 
Cen Centaurus      05 | Sco Scorpius       07 
Cep Cepheus        05 | Ser Serpens        02 
Cir Circinus       01 | Scu Scutum         03 
Cru Crux           01 | Vel Vela           03 
Cyg Cygnus         09 | Vul Vulpecula      03 
Gem Gemini         01 | Multiple / Other   03 
*********************|***********************

NOTE: There are actually 101 OB Associations in total. Some 27 have been proven to be non-existent, at least whose stars have been proven to be statistically unlikely. I.e. Vul OB3 is a non-existent Association.

OB ASSOCIATION REFERENCES

1. Alter, G., et al., “Catalogue of Stellar Clusters and Associations”, 2nd Edition, Akademiai Kiada, Budapest, Hungary (1970)

2. Archinal, B.A., Hynes, S.J., “Star Clusters”, Pub. Wilmann Bell (2001)

3. Hirshfeld, A., Sinott, R.W., “Sky Catalogue 2000.0 : Vol.2. : Double Star, Variable Stars and Nonstellar Objects” pg. xxi; Cambridge University Press (1982)

4. Ruprecht, J., et al., “Catalog of Star Clusters and Associations, Supplement 1, Vols. I-III”, Bull.Inform. CDS, 22 (1982)

T Associations

T Associations are similar to the OB types, except they consist of newly formed stars with their embryonic nebulosity. The stars are comprise of the cooler spectral types, equal or less than that of the Sun. Most of these stars are variables of either the T Tauri or RW Aurigae type. Fifty-eight of these are known, each containing between thirty (30) to more than four hundred (+400) stars. Distances are typically between 100 and 800 parsecs.

R Associations

These are associations which are connected with the reflection nebula, and can be considered as specialist types of T Associations They were first classified by Sidney van den Bergh in 1948.

C Associations

These are associations that contain several Cepheid variables and are thought to be due to heavy mass star production during the star formation process. First listings of these association types were made by Efremov (1978) (9), contain perhaps thirty-five (35) in total. Explanation of these C-Associations and how they relate to stellar evolution is at present little understood.

Moving Clusters

The weakest type of the gravitational attached associations are the moving clusters. These are apparently unrelated stars that do have similar motions and velocities through space. Most familiar is the Ursa Major Group (Cr 285), which contains most of the bright stars in constellations of Ursa Major, Leo, CMa, Eri, Aur and CrB. This group was later suggested to have the Sun as one of its members, but in modern times this adopted theory has been generally dismissed as unlikely. Many astronomers do think the existence of moving clusters infers that the component stars may have been nurtured from similar associations, clusters and nebulae. Perhaps for the Sun this occurred about five billion years ago, but the evidence for this has long faded into history and may never be verifiable.

Last Update : 27th November 2012

Southern Astronomical Delights © (2012)

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