NGC 5139, omega Centauri

Primary identifier: NGC 5139
Other identifiers: Omega Centauri, HD 116790, ASSA 056, Ben 61
Type: Globular cluster
Magnitude: 3.7 V
Diameter: 36'

RA: 13h 26m 46s
Dec: –47° 28' 37''
Constellation: Cen
Charts: U2000 403, MSA 2:953, SA 21, Discover 02
Visibility: Feb-Sep

Historical notes: General comments

It was plotted in the Almagest of Ptolemy over 1800 years ago, and in the early 17th century it was catalogued by Bayer as a star. The first telescopic observer to see it as a cluster was Halley, who observed it from St Helena in 1677.

Historical observations: Lacaille

It was observed by Lacaille and included in his 1755 catalogue as Class I No. 5, classifying it as a nebula. He wrote: "Naked eye, a 3rd mag star [10 Cen] in a fog. Telescope, [Half-an-inch aperture, 8x magnification] like a big diffuse comet."

Historical observations: James Dunlop

James Dunlop observed it from Paramatta, New South Wales, and included it as No. 440 in his catalogue of 1827. Using a 9-inch f/12 telescope, he described it as "a beautiful large bright round nebula, about 10' or 12' diameter, easily resolvable to the very centre; it is a beautiful globe of stars very gradually and moderately compressed to the centre; the stars are rather scattered preceding and following, and the greatest condensation is rather north of the centre: the stars are of slightly mixed mags, of a white colour. This is the largest bright nebula in the southern hemisphere." Dunlop observed it 8 times.

Historical observations: John Herschel

Sir John Herschel observed it at the Cape of Good Hope with an 18-inch f/13 speculum telescope. He wrote of it as "the noble globular cluster w Centauri, beyond all comparison the richest and largest object of its kind in the heavens. The stars are literally innumerable, and as their total light when received by the naked eye affects it hardly more than a star of the 5th or 5..4th magnitude, the minuteness of each may be imagined: it must however be recollected that as the total area over which the stars are diffused is very considerably (not less than a quarter of a square degree), the resultant impression on the sensorium is doubtless thereby much enfeebled, and that the same quantity of light concentrated on a single point of the retina would very probably exceed in effect a star of the 3rd magnitude. On a consideration of all the sweeping descriptions, as well as from a great many occasional inspections of this superb object, I incline to attribute the appearance of two sizes of stars of which mention is made to little groups and knots of stars of the smaller size liying so nearly in the same visual line as to run together by the aberrations of the eye and telescope; and not to real inequality. This explanation of an appearance often noticed in the descriptions of such clusters, is corroborated in this instance by the distribution of these appearently larger stars in rings or mesh-like patterns, chiefly about the centre where the stars are most crowded. An attempt has been made to imitate this appearance in the drawing, but partly from the difficulty of its execution, partly from defect of engraving, the plate fails to convey a just idea of it. Two such rings on an oval crossed by a kind of bridge is especially conspicuous in the central part." In the records of his telescopic sweeps he recorded it as "Diameter full 20'. It much more than fills the field. When the centre is on the edge of the field, the outer stars extend fully half a radius beyond the middle of it. The stars are singularly equal, and distributed with the most exact equality, the condensation being that of a sphere equally filled. - Looking attentively, I retract what is said about the equal scattering and equal sizes of the stars. There are two sizes 12th mag and 13th mag, without greater or less, and the larger stars form rings like lace-work on it. One of these rings, 1.5' in diameter, is so marked as to give the appearance of a comparative darkness like a hole in the centre. There must be thousands of stars. To the naked eye it appears as a star of 5th mag or 5..4th mag, rather hazy. There is a 9th mag star on the S.p. border of it, about 4' or 5' south of centre, and several 8th mag are scattered far away. My attendant (J.S.) called up, who saw the hole and darkness, and described it as I have done above. On further attention the hole is double, or an oval space crossed by a bridge of stars. Position of axis = 150 ." His second observation recorded it as "very very bright, very very large, very very gradually much brighter in the middle; all clearly resolved into stars of two mags, viz. 13 and 15; the larger lying in lines and ridges over the smaller. Near the centre are two distinct darkish spaces formed by a deficiency of the larger stars 13 mag within, and an excess without. This most glorious object fills the whole field with its most condensed part, and its stragglers extend three-quarters of a field beyond it either way. It is very conspicuous to the naked eye as a dim cometic looking star, 4th mag or 5th mag."

20th century visual observations

Doig, P. (1925) Journal BAA, 36(3), December, p91

Magnificent globular cluster, 31' diam. over denser parts; ... a considerable number of giant stars is visible in this cluster, however, with a moderate aperture.

E. J. Hartung

Hartung writes: "... the main region is about 20' across and its myriad stars are broadly compressed towards the centre. It is powdered with faint stars with a three-inch, and with a 4-inch looks like delicate tangled threads of beaded gossamer. Larger apertures show a pronounced lace-like pattern which seems to be made of small crossing curved lines of stars. Dark lanes and streaks are evident with moderate magnification and the star distribution is far from uniform. On a clear dark night it is a most impressive and beautiful sight."

Harrington

Harrington writes that this "magnificent 3.7 mag globular is visible to the naked eye. And what a marvelous sight it is. A pair of 7-power binoculars begins to reveal the unparalelleled splendor of Omega Centauri. Observers with exceptional eyesight may just perceive some of the globulars estimated one million stars, but 11x80 glasses will reveal scores ... an 8-inch instrument can resolve stars across nearly all of the clusters half degree disk. Through a 25-inch reflector, I once cracked the core of Omega, seeing countless suns pouring across the field!"

Harrington (1986)

Harrington, P. (1986) More globulars for observers. Sky & Telescope, Sep, 310.

".. the view was stunning. Using a 4.25-inch f/4 rich field telescope, I could resovle countless stars across the cluster, which appears as large as the Full Moon. Some of its stars could even be seen in 11x80 binoculars and several were suspected in 7x50s. The cluster is distinctly oval."

Simon Tsang

Simon Tsang notes that it appears oval with binoculars; a 13-inch reflector at 120x showed "no obvious dark lanes, and the core was partially resolved. In appearance it was comparable to my view of M13 at 350x in a 16-inch telescope in Ottawa but with three to five times the density of stars ... a breathtaking sight."

Tony Cecce (astro-l listserv, 1995)

I finally did it last week, I got my first view of Omega Centauri. I was on vacation in Louisiana then on to Cocoa Beach and Orlando. Knowing that this globular would be visible in early evening this time of year I brought along my 10x50 binoculars. I went out to find this object while north of New Orleans. It was difficult to find skies down to the horizon, many trees with the only open areas being parking lots (and lots of lights). I finally ended up on an interstate overpass. I wasn't too hopeful since there was plenty of atmosphere (moisture laden), it was less than 15 degrees above the horizon, and the light dome from New Orleans 30 miles south extended well into the region of centaurus. I began scanning with my binoculars when all of a sudden there it was. Wow! Words escape me in being able to describe what this looked like (but you know I'll try). This was a glow almost the size of the full moon. Faint at the edges, but getting very bright at the center. I had the impression it was more extended than I could see in those skies with my binoculars. I also seemed to be on the edge of being able to resolve it. I viewed this every chance I got after that first night. I also swung over to M3 and M13 for a comparison. Those fabulous Messier globulars don't hold a candle to Omega Centauri. At home again last night I pulled out an 8" f4.5 dob with a 32mm eyepiece and M13 looked the same through that as Omega Centauri did through my binoculars. I can't wait for my next trip south to see this again, and I will set aside time to find a local astronomer who is willing to let me view through their scope. Clear Skies, Tony

Todd Gross, IAAC (1998)

Date and UT of observation: 1/1/98 11:00 GMT. Location & latitude: Cancun, Mexico. Site classification: Urban/Suburban. Limiting magnitude (visual): 5.1 zenith (est) 4.0 (est) in vicinity of object. Weather: Clear to pt. cloudy. Instrument: 80mm APO refractor f/6 - f/l 480mm

This HUGE globular cluster is almost comically big. It is easily swept up by just fishing around in approximately the right spot in 8x32 binoculars. Even in poor conditions, with limiting magnitude around 4, I was able to make it out naked eye with averted vision. In the 80mm scope, it is roughly the size of the full moon, and a perfectly symetrical, bright cotton ball, which is partially resolved at 16x, and esp. 32, 48, and 69x. About 4 foreground stars are particularly bright near the edges, however, the cluster itself only breaks up a little, in these conditions and with this aperture, not breaking into diamond - like points throughout like I can get M13 to do in perfect conditions near the zenith at home. Nevertheless, impressive, and best with averted vision.

Tom Lorenzin

Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "3.6M; 36' diameter! Humongous Globular Cluster! An incredible sight when resolved from a low latitude observing site; uneven texture across its broad condensation; never seen as more than a large, soft blur from latitude of 35.5' N; culminates at May's end; plan a trip to North Africa, Mexico or the Caribbean at that time, and take me with you."

AJ Crayon

AJ Crayon, using an 8" f6 Newtonain, notes: "is the most magnificent globular cluster of the heavens and much better known as Omega Centauri. It is visible in 8X50 finder as large round and bright. It is 6m 20', it is well resolved into hundreds of stars. With averted vision and good seeing conditions it is resolved into a MOST glorious object at 100x!

Steve Coe (1996)

Steve Coe, in "SACNEWS On-Line for May 1996", observing with a 17.5" f/4.5 Dobsonian, notes: NGC 5139 is Omega Centauri, I saw it as very bright, very, very large, extremely rich and very compressed at 100X. What can be said about the KING of the Globulars? This fantastic object was overwhelming from Australia when I went to visit Jim Barclay while Halley's Comet was at its best in 1986. The globular filled the field at 140X in his 12.5" f/6. There were chains of stars that meandered outward in all directions from a blazing core. A dark area was seen on the south side of the central section. It can be seen from even mediocre skies, but if you need to dial it up then go to 13 26.8 and -47 29.

Steve Coe (unpublished)

Steve Coe, using a 17.5" f/4.5, notes: "Very bright, very, very large, extremely rich, very compressed at 100X. What can be said about the KING of the Globulars? This fantistic object was overwhelming from Australia when I went to visit Jim Barclay while Halley's Comet was at its best in 1986. The globular filled the field at 140X in his 12.5" f/6. There were chains of stars that meandered outward in all directions from a blazing core."

Other notes and comments:

Shapley, H. & Paraskevopoulos, J.S. (1940)

Galactic and Extragalactic Studies, III. Photographs of thirty southern nebulae and clusters. Proc. N.A.S., 26, 31-36.

Laustsen, S., Madsen, C. & West, R.M. (1987)

Exploring the Southern Sky: A pictorial atlas from the European Southern Observatory. Springer-Verlag. Plate 174.

Melotte, P.J. (1915)

A catalogue of star clusters shown on Franklin-Adams chart plates. Mem.R.A.S., 60(5), 175-186.

RNGC

The RNGC (Sulentic and Tifft 1973) notes that this is a 4.0 mag globular cluster.

Photo Index by Jim Lucyk

Sky & Tel. 3/85 p236, Sky & Tel. 3/86 p239, Sky & Tel. 5/72 p325, Sky & Tel. 8/77 p93, Sky & Tel. 9/84 p259, Sky & Tel. 12/80 p477, Astronomy mag. 4/82 p9, Deep Sky #10 Sp85 (back cover), Burnhams V1 p34, Burnhams V1 p563, Ast.Obj.for South.Tel. (Hartung, 1984), Universe Guide to Stars & Planets (Ridpath & Tirion) p106.

Catalog Of Parameters For Milky Way Globular Clusters (1997)

Compiled by William E. Harris, McMaster University. (Revised: May 15, 1997; from http://www.physics.mcmaster.ca/Globular.html; Harris, W.E. 1996, AJ, 112, 1487) RA 13 26 45.9 (2000) Dec -47 28 37 Integrated V magnitude 3.68 Central surface brightness, V magnitudes per square arcsecond 16.77 Integrated spectral type F5 Central concentration, c = log(r_total/r_core); a 'c' denotes a core-collapsed cluster 1.24 Core radius in arcmin 2.58.

FitzGerald, A.P. (1954?)

"Note on the globular cluster Omega Centauri", IAJ, Vol 3, 204.

In an earlier article (IAJ, 3, 114, 1954) reference was made to a photograph of this cluster, taken by the writer during a visit to the Boyden Station .... Lindsay has pointed out that the presence of a slightl absorbing cloud on one side of the cluster can be detected by the deficiency of stars (IAJ, 2, 144, 146, 1953). So far as the writer is aware, the presence of dark absorbing matter in the region of a globular cluster has not been detected on photographs. Of special inetrest, therefore, are the thin dark 'lanes' which are discernible on the eastern side of the cluster in Plate XVII and the long serpentine 'lane' on the western side running from north-west to south-east. Whatever may be the true nature of these 'lanes' it seems unlikely thta they are an efect of the random distribution of stars. It may be that they are filaments of nebulosity lying between the observer and the cluster."

Bailey, S.I. (1908)

"A catalogue of bright stars and nebulae", Ann.Harv.Coll.Obs., 60(8), 199.

Remarks, p.217: "A magnificent globular cluster, not extremely condensed. More than 6000 stars have been counted on a photograph of this cluster, having a moderate exposure. the whole number of stars can be little less than 10,000. Of these, 128 are known to be variable."

Contemporary / ASSA observations

Observer: Richard Ford

Date: 2007 April 14/15 (Sat/Sun)
Location: Cape Town (Fairview Dairy Farm, Conterman's Kloof)
Instrument: 8-inch f/7 (Dobsonian)

Sky conditions: The sky overhead is still dark but with a brightened horizon. Atmosphere is stable and temporary turbulence might be observed. Naked-eye limiting magnitude = 4.5

Description: Globular cluster, seen as a fuzzy out of focus star by naked eye. Approximately the same size as the Full Moon, but slightly elliptical in shape. High surface brightness. It is an easy object to enjoy in every scope (including dark sky outings). The stars in omega centauri are individually resolved in an 8 inch Newtonian into cottony globes, sprinkling an out layer of individual stars using low magnification (56x). By using a medium magnification (140x), individual out layers of blue-white stars radiate from the centre. The velocity dispersion is very strongly centrally condensed in the centre.

Other notes by Richard Ford: Omega Centauri constitutes over 1 million individual stars. The angular separation of the nucleus is arranged at 20'.

Observer: Carol Botha

Date: 2007 Feb 17, 23:55
Location: Betty's Bay
Instrument: 8-inch Dobsonian, 17mm eyepiece

Sky conditions: Misty patches, good skies in open patches

Description: Large bright globular cluster. Fuzzy ball - denser in the middle. Fuzzy filaments radiating from the nucleus - almost looks like candyfloss. Four bright stars form diamond shape around the cluster. 17mm eyepiece does resolve a few stars.

Observer: Ed Finlay

Date: 1992 May 22
Location: Johannesburg
Instrument: Meade 4-inch ED APO

Sky conditions:

Description: Easy to find; quite visible; like a small round fuzzy cloud. At 184x very dim but quite spectacular with individual stars resolved at edges.

Observer: Magda Streicher

Date: 1997 April 5
Location: Campsite (23°16 South 29°26 East)
Instrument: Meade 8-inch SCT (Super wide angle 18mm eyepiece)

Sky conditions: 7 magnitude clear.

Description: Remarkable rich open cluster. Very dense to the middle with rich faint and bright stars almost look three-dimensional. Stars running out in chains and lanes to the outskirts of the field. Very impressive, resembles a pot full of sugar to me.

Observer: Dave Gordon

Date: 1999 Feb 27, 22:30 UT
Location: Marloth Park, South Africa
Instrument: 10-inch f/10 SCT, 15mm Plossel

Sky conditions: Lev II, good transparency

Description: Omega is a sheer powerhouse away from city lights. At 167 times magnification, individual members could be identified deep into the core of the cluster. The cluster displays a distinct oval shape, unlike most clusters which are round in shape. Colour was light dusky, fading to the edges.

Observer: Rui Henriques

Date: 1997 May 02
Location: Stellenbosch
Instrument: 10x50 binoculars, tripod-mounted

Sky conditions: clear skies, no light pollution on horizon, dew on binocs

Description: Snow white glow, 30' diameter, 10' brighter nucleus. No stars resolved in cluster. Stars in field form a cone pointing to north east - overall appearance of an ice-cream on a cone.

Observer: Auke Slotegraaf

Date: 1982 April
Location: Stellenbosch (PRG Observatory)
Instrument: 15.5-inch reflector, f/9, 220x

Sky conditions: moderate light pollution

Description: A glorious object which fills the field of view with stars. The central disk appears more or less even, whilst with averted vision it is clearly mottled. Look for the brighter knots of stars which surround this central core.

Date: 1993 April 30
Location: Stellenbosch (Die Boord)
Instrument: 6-inch f/8.6 Newtonian at 52x

Sky conditions: moderate light pollution

Description: Using drift-time method to determine size: average time for drift of brightest part = 52.4 seconds = 8.9'.

Date: 1990 March
Location: Stellenbosch (Die Boord)
Instrument: 10-inch f/5 Dobsonian

Sky conditions: moderate light pollution

Description: This spectacular cluster is wonderful in a 10-inch f/5. It is quite amazing that such an awesome object can actually exist. At 30x the cluster is grainy, and at 120x the central haze is clearly shown as stars. The nucleus appears to show some detail; the random scattering of the stars appear to form almost a small ring of stars with a dark central spot, but this is merely a trick the eye and mind play with this wonderful image.

Date: 1998 Feb 23/24
Location: Stellenbosch (Rifle Range)
Instrument: 6-inch f/8.6 Newtonian

Sky conditions: moderate light pollution; lim mag 6.0 (naked eye), seeing very good.

Description: Took this one in at the end of the session, with Omega high up. Wow! An indescribable sight, but here goes. Fills the K12.5mm (23 arcmin) field. Stars are seen all over, resolved from the fringe right into the core. Behind this vast collection of very small stars lies the bright nebulous glow of the other thousands of unresolved stars. The fringes of the cluster is an area rich in chains and clumps of stars - the detail and structure is awesome. Peering into the eyepiece creates an almost 3D effect, as if floating over this sphere of stars.

Date: 1998 April 23/24
Location: Stellenbosch (Die Boord)
Instrument: 5-inch Newtonian (Rui's telescope)

Sky conditions: moderate light pollution

Description: Observing with Luis Henriques, who described Omega Cen as having two sections; an oval outer ring, and a round inner portion. In the low-power binocular eyepiece, just to the N-NE outside the dense nucleus proper, in the fringe of halo stars, as a very small knot of stars, brighter than the other fringe stars; this concentrated region sort of gives the cluster an extended appearance.

Luis reports seeing three large extended streamers of stars, (by sketch, 1/3 of nucleus diameter) -- one a broader one out to the south, and two bowed ones (like legs) to the north and north-east. I did not notice these; examined at the same time with the 4-inch Unitron, and confirmed the N-NE knot, but not Luis' streamers.