Omega Centauri - HST
This is the first part in a series based on my trip to Australia last summer, covering observations of a few southern
showpiece objects. The other parts in the series are:
Southern Planetaries
Southern Galaxies
Two Southern Galaxy Groups
These observing notes were made in early July while my family was staying at the Magellan Observatory (astronomical farmstead) for eight nights. The observatory is in the southern tablelands of New South Wales between Goulburn and Canberra (roughly 3.5 hrs from Sydney) and is hosted by Zane Hammond and his wife Fiona. Viewing the showpiece southern globulars was high on my observing priorities for Australia. Because the center of the Milky Way wheels overhead from -35° latitude, the globular system is much better placed and several of the best globulars in the sky which are completely inaccessible from the north are well placed.
In the August issue of S&T, Les Dalrymple (who I observed with one evening), ranked M13 no better than 8th among the best globulars visible from Australia. I'd still jack up its ranking a couple of notches, but it's just one of the weak runnerups to 47 Tucana and Omega Centauri viewed over 75° up in the sky and behind NGC 6752, 6397 and M22. Several of the less publicized clusters were quite a surprise -- NGC 362, 2808, 3201, 4372, 4833, 6541 were all spectacular sights in the 18". I didn't have an opportunity to chase after the challenging Terzan and other obscured globulars best viewed from the south, but still upped my total to 123 out of the 150 currently confirmed milky way globulars (ignoring local group globulars in Fornax, LMC and SMC as well as M31). These are observations of some of my favorites ...
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NGC 104 = 47 Tucana = E050-SC009
00 24.1 -72 05
V = 3.8; Size 30.9
20" (7/8/02): this was the best view I had during the week of 47 Tucana at 212x. The entire 23' field was packed edge-to-edge with pinpoint stars and the blazing core was resolved into a mesmerizing dense mat of stars. 18" (7/6/02): at 171x, this breathtaking globular was viewed at over 50° elevation and was stunningly resolved into several thousand stars out to a diameter of over 25'. The star density steadily increases towards the center. The roughly 4' core was blazing and highly resolved right to the edge of a very small compressed nucleus. A 3-dimensional affect was very strong with layers of stars forming a dense mat over the core. Many of the stars in the halo are connected in chains and lanes. The 9 Nagler does a better job of busting apart the stars in the core, although the cluster overfills the field at this power. The total visual magnitude is just slightly fainter than Omega Centauri and the size slightly smaller, but certainly equal in visual impact. 12" (6/29/02): While at Bargo, I observed 47 Tucana still very low in the sky but an obvious 4th magnitude naked-eye blur just west of the SMC. At 186x, it filled the 26' field with an uncountable number of stars. Strongly concentrated to an intense, blazing core which was only partially resolved at a low elevation. The highly resolved outer halo extends ~25' in an irregular outline while the central halo was very symmetric. |
NGC 362 = E051-SC013 01 03.2 -70 51 V = 6.6; Size 12.9 18" (7/6/02): this bright globular is situated just north of the SMC (it's much closer at ~20,000 ly) but unfortunately is overshadowed by 47 Tucana on the west side of the SMC. At 228x, it appeared very bright and well-resolved into a couple of hundred stars! The rich halo is plastered with stars and extends to nearly 8'. The 2' compressed core is well-concentrated to a blazing center. Stars appear to stream out of the core in spiral curving lanes. This globular has a classic symmetric appearance with prominent, round core and halo.
NGC 1261 = E155-SC011 03 12.3 -55 13 V = 8.3; Size 6.9
18" (7/8/02): bright, symmetric globular ~5' diameter, with a large very bright condensed core (concentration class II). A bright star is ~5' away. At 171x, the halo just resolves into a large number of very faint stars.
20" (7/8/02): at 212x, the resolution was a bit better in the halo than with the 18" NGT, but the blazing core was still unresolved.
NGC 2808 = E091-SC001 09 12.0 -64 52 V = 6.1; Size 13.8
18" (7/8/02): at 171x, this bright globular (ranked 10th brightest at V = 6.1) is fairly large, ~10' diameter and very compressed with a blazing 2' core. The halo is noticeably elongated, nearly 3:2. A dense swarm of mag 14 and fainter stars are resolved in the halo and around the edges of the core but the inner central core is unresolved. At 228x, there appear to be some very faint stars lanes streaming into the halo which are just unresolved but look like small tentacles. A mag 10/10.6 double at 16" is outside the cluster ~10' ESE. This is by far the brightest concentration class 1 GC and the only one easily resolved.
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NGC 3201 = E263-SC026
10 17.6 -46 25
V = 6.7; Size 18.2
18" (7/8/02): beautifully resolved globular at 171x and 228x. The cluster is fairly large, ~8'-10' diameter with more careful viewing. Roughly 150 stars are resolved, though the number grows with magnification and averted vision. A fairly dense layer of brighter mag 11.5-12 stars are resolved right over the bright core. The halo, which has a large number of mag 13 stars, has a scraggly, irregular edge and seems elongated - possibly partially obscured by dust. This concentration class X cluster is 7th in ranking of brightest stars and 10th in horizontal-branch mag (high resolution). |
NGC 4372 = E064-SC006 12 25.8 -72 40 V = 7.5; Size 18.6
18" (7/7/02): at 228x, this large, loose globular is well-resolved into ~100 stars mag 12-15 with a 12' diameter. The roundish core is ~4' in size with many faint stars sprinkled across the core. The halo is elongated and irregular in shape. A mag 6.8 star lies 5' NW of the core and interferes with viewing. A starless dark lane appears to wind into the cluster towards the core from the NW side angling roughly NW-SE (verified on DSS). Located 44' SW of mag 3.8 Gamma Muscae.
NGC 4833 = E065-SC004 12 59.6 -70 53 V = 7.0; Size 13.5
18" (7/10/02): This was a surprising find for a little-publicized globular! At 171x, it appeared as a bright, fairly large globular of low concentration class. The loose halo was highly resolved and numerous stars were splashed across the brighter core. Many of the stars formed large loops and chains. A single bright star (mag 8.7 SAO 256996) is superimposed on the northern side of the halo. The core spans roughly 3', while outliers increase the overall halo to at least 10' [John Herschel gave similar dimensions]. Located 42' NNW of mag 3.6 Delta Muscae.
NGC 5139 = E270-SC011 = Omega Centauri Cluster 13 26.8 -47 29 V = 3.7; Size 36.3
12" (6/29/02): this was the first object I viewed at Bargo with Les Dalrymple's 12" and was not disappointed. At 140x, it filled 2/3 of the field (over 25') and resolved into a few thousand stars down to the center. The cluster seemed almost 3-dimensional with a lattice of brighter mag 12 stars superimposed on a dense background layer of mag 13-14 stars. The density was generally uniformly high in the halo except near the edge although the core shows more non-uniformity.
20" (6/29/02): at 208x with a 24' field, the cluster overfilled the field with edge-to-edge stars mag 11.5 and fainter and was a breathtaking sight. Broadly concentrated with large, brighter core of at least 10' in diameter which is extremely densely packed with layers of stars. The elongated halo gradually thins but with no definite border up to 30'. This is the largest and brightest globular in the sky (V = 3.7) and an obvious naked-eye blur, but lacks the strong central condensation of NGC 104 = 47 Tucana.
IC 4499 = E022-SC005 15 00.3 -82 13 V = 10.0; Size 7.6
18" (7/7/02): at 171x this globular has an unusual appearance as a round glow of nearly even surface brightness, ~3.5' diameter with a single brighter mag 11.5 star superimposed at the very center. A brighter mag 9.5 star is just off the south edge, 2' from the center. Appears similar to a large, fairly low surface brightness planetary or reflection nebula. This observation was possible hindered by some clouds as the skies completely clouded up soon afterwards, but there was no evident resolution at this power.
NGC 6101 = E069-SC004 16 25.8 -72 12 V = 9.2; Size 10.7
12" (6/29/02): at 140x this moderately faint globular is ~4' diameter with just a weak concentration. A scattering of mag 14 stars pepper the face of the cluster and at the edges of the irregular halo.
NGC 6266 = M62 = E453-SC14 17 01.2 -30 07 V = 6.6; Size 14.1
18" (7/9/02): at 171x this is a very striking globular set in a fine star field. The halo is very irregular and elongated due to a flattening along the SE side. The halo is easily resolved into several dozen faint stars, many in strings and chains. A long string of stars extends from a mag 10.5 star off the SE side along the east edge of the halo. The center is very compressed with a large, blazing core.
NGC 6352 = E228-SC003 17 25.5 -48 25 V = 8.2; Size 7.1
18" (7/10/02): fairly bright, fairly large. Even at 128x, fairly well-resolved into ~75 stars. Rather loose concentration class with a broad concentration to a moderately bright 3' core. The irregular halo extends to nearly 8' diameter. Set in a rich milky way field.
NGC 6362 = E102-SC008 17 31.9 -67 03 V = 7.5; Size 10.7
12" (6/29/02): at 186x, this fairly loose globular is moderately bright and large and broadly concentrated. It was resolved into 25-30 stars, several arranged in a line bisecting the cluster. The ragged halo is roughly 8' diameter. Located 1.2° NE of mag 4.7 Zeta Apus.
NGC 6388 = E279-SC002 17 36.3 -44 44 V = 6.8; Size 8.7
18" (7/8/02): at 171x, this bright globular appeared as moderately large, round and granular but with no definite resolution. The appearance was very symmetric and strongly concentrated with a blazing core which ranks as one of the highest surface brightness gc cores.
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NGC 6397 = E181-SC004
17 40.7 -53 40
V = 5.8; Size 25.7
18" (7/8/02): amazing view of this bright globular at nearly 70° elevation at 171x. It extended across 2/3 of the field - perhaps 20' in diameter. Very highly resolved into hundreds of stars, many mag 10-11, and overall noticeably brighter than most globulars (1st in ranking of brightest stars). The globular appears fully resolved in the halo and even the blazing core is covered wall-to-wall with stars. Many stars are in small groups, chains, curves, etc. The core is ~3.5' diameter and at 228x has a 3-dimensional effect with the mag 10.5-11.5 stars layered over a dense, uneven background mat. 20" (7/8/02): I also had a fantastic view in the 20" f/5 at 212x - the stars seemed to radiate out from the core in spiral curves and completely filled the 23' field! |
NGC 6496 = E279-SC013 17 59.0 -44 16 V = 9.0; Size 6.9
18" (7/10/02): moderately bright, low surface brightness globular of 4'-5' diameter. Loose concentration class with little central condensation. About a dozen stars are resolved over the face although some of these may be foreground stars as it is set in a very dense star field. Three of the stars are 12th magnitude with the rest mag 14-15. Situated on the Sco/CrA border.
NGC 6541 = E280-SC004 18 08.0 -43 42 V = 6.1; Size 13.1
18" (7/9/02): this beautiful globular is set in a rich star field and is strongly compressed with a blazing core. At 171x, the halo is ~8' and highly resolved into a couple of hundred stars mag 12 and fainter. A few brighter stars (field?) are mixed in including a mag 11 stars on the E and SW side of the halo. The stars are densely packed towards the core - another southern showpiece globular!
NGC 6584 = E229-SC014 18 18.6 -52 13 V = 8.6; Size 7.9
18" (7/9/02): at 171x this globular appeared fairly bright, moderately large, ~4' diameter with a broad concentration and a fairly symmetric appearance. Resolved into a couple of dozen faint stars, mostly in the halo which is a bit ragged. The central core is very mottled but unresolved. A few brighter mag 11 stars are just outside the halo, but these appear to be field stars. A mag 7.5 star is 13' NW and mag 7.0 star 15' NNE.
NGC 6723 = E396-SC010 18 59.6 -36 38 V = 7.3; Size 11.0
18" (7/10/02): very bright, beautiful symmetric globular at 128x. The halo measures 6'-7' with sharply concentrated, blazing 3' core. Roughly a 100 stars are resolved with a strong impression the core would more fully break apart using higher power. A single mag 10.5 star is at the NE edge of the halo. In a remarkable region with a complex of unusual reflection nebulae (N6726/27/29) and a huge dark cloud (Bernes 157).
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NGC 6752 = E141-SC030 19 10.9 -59 59 V = 5.5; Size 20.4 18" (7/10/02): this is one of the top globular clusters (4th brightest in integrated magnitude and 2nd in terms of brightest members) was spectacular from Magellan Observatory. At 128x, this naked-eye cluster seemed fully resolved with uncountable stars densely packed over a large region. Streamers in the halo greatly extend the outer extend to roughly 20'. The central region is well-compressed to a very bright core. There is a strong 3-D effect as the core is covered with scores of fairly bright stars seemingly superimposed over a rich mat of fainter stars and all set over a background glow. The nucleus is small and very bright. Many stars, 11th-12th magnitude, in the halo form complete loops and longs chains. Fascinating object! 12" (6/29/02): this naked-eye cluster ranks second in terms of brightest members (mag 11.5). The 4' core has several dozen mag 12-13 stars, many arranged in streaming curves and chains layered over a dense central glow. The halo is large but irregular with knots of mag 11-12 stars over a diffuse background. The brighter members, particularly in the outer halo give the visual appearance of a globular embedded in a larger, open cluster! |
