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View Full Version : Object of the Week, September 27, 2015 -- The Magnificient 47 Tucanae (NGC 104)



Steve Gottlieb
September 27th, 2015, 11:32 PM
Object of the Week: September 27, 2015 – The Magnificent 47 Tucanae (NGC 104)


NGC 104 = 47 Tucanae
RA: 00h 24m 05.2s
Dec: -72° 04' 50"
Type: Globular Cluster
Diameter: 30.9'
Concentration Class: III
Mag of brightest stars: 11.7
Mag of horizontal branch: 14.1
Distance: ~15,000 light years
Age: ~12 billion years

Since I'm heading to Australia in less than 2 weeks for a week of observing, I thought I would pay homage to 47 Tucanae, one of the grandest spectacles in the sky -- though only well seen from the southern hemisphere.

Nicolas-Louis de Lacaille discovered the nebulous appearance in 1751-1752 with a 1/2-inch telescope at 8x during his expedition to the Cape of Good Hope. It appeared "like the nucleus of a fairly bright comet." Lacaille placed it in his group I, consisting of nebulae without stars. The cluster was noted, though, as a "star" in Bayer's Uranometria, which was first published in 1603. The designation "47 Tuc" comes from Bode's extension of Flamsteed numbers to the southern constellations (these are not used today except for 47 Tuc and 30 Dor).

With his 9-inch speculum reflector from Paramatta (now a suburb of Sydney), James Dunlop logged "this is a beautiful large round nebula, about 8' diameter, very gradually condensed to the centre. This beautiful globe of light is easily resolved into stars of a dusky colour. *The compression to the centre is very great, and the stars are considerably scattered south preceding and north following." *His sketch in figure 1 of his catalog shows a beautifully resolved, elongated cluster.

John Herschel first observed the magnificient globular from the Cape of Good Hope on 11 Apr 1834 with his 18-inch speculum reflector. He logged "The great cluster preceding the Nubecula Minor. Estimated dia of the denser portion 5'; of the whole (not, however, including loose stragglers) 8'. Stars 14..16 mag. and one of 12th mag N.p. the centre. Excessively compressed. (N.B. In a sweep below the pole, when of course owing to the low altitude much of the light was lost.)" *His observation from 12 Aug 1834 reads: "A most glorious cluster. The stars are equal, 14th mag., immensely numerous and compressed. Its last outliers extend to a distance of 2 min, 16 sec in RA from the centre. It is compressed to a blaze of light at the centre, the diameter of the more compressed part being 30 arcsec in RA. It is at first very gradual, then pretty suddenly very much brighter in the middle. It is completely insulated. After it has passed, the ground of the sky is perfectly black throughout the whole breadth of the sweep. There is a double star 11th mag. preceding the centre." On 21 Sep 1835 he wrote: "Fills the field with its stragglers, condensation in three distinct stages, first very gradually, next pretty suddenly, and finally very suddenly very much brighter in the middle up to a central blaze whose diameter in RA is 13.5 sec and whose colour is ruddy or orange-yellow, which contrasts evidently with the white light of the rest. The stars are all nearly equal (12..14 mag). A stupendous object." *His final record of the object was on 5 Nov 1836: "A most magnificent globular cluster. It fills the field with its outskirts, but within its more compressed part, I can insulate a tolerably defined circular space of 90" dia wherein the compression is much more decided and the stars seem to run together; and this part I think has a pale pinkish or rose-colour."

I find it interesting that Herschel described the core of the cluster as "ruddy or orange-yellow" and later as "pinkish or rose-colour"! To me the core generally appears pale yellow (no problem seeing color).

47 Tucanae is an easy 4th magnitude naked-eye blur just west of the Small Magellanic Cloud. *It's visible from a dark sky while very low in the sky as well as suburban locations when higher in the sky.
1810

I've made numerous observations of 47 Tucanae from Australia -- several times I've just soaked up the view as words can't really capture the experience, but here's a sample from July 2002 in an 18-inch f/4.5 from the Southern Highlands.
"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 relatively small 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. *A 9 mm Nagler (229x) did a better job of busting apart the stars in the core, although the cluster overfilled the field at this power. Although the total visual magnitude is just slightly fainter than Omega Centauri and the size slightly smaller, 47 Tucanae is certainly equal if not surpassing Omega Centauri in visual impact due to its dazzling central blaze."

Another memorable view was 5 years back in November 2010 using a 30-inch f/4.5 from Coonabarabran.
"Absolutely stunning view in the 30" at 163x and 264x. *Even in the 37' field of the 21mm Ethos, the stars appeared to fill the entire field, only thinning out near the edge. *The pinpoint stars were amazingly packed, but increased in intensity to a relatively small, blazing core, which was plastered with resolved stars. *The very center of the nucleus contained a small, intense knot overlaid with packed stars giving a strong impression of layers. *I immediately noticed the core had a pale yellowish tint."

Now I'm psyched up for another view soon again!

GIVE IT A GO AND LET US KNOW!
1809

Ivan Maly
September 28th, 2015, 02:06 AM
I don't have much in my notes as far as observational description of this cluster, but here is the entire paragraph from a report that at least contains the obligatory "stunning" and "warm tinge" - and allusions to the Australian exotics:

At the Ayers Rock resort, a cluster of modern buildings in the middle of the desert, after dinner I went into the ”bush” “out back” of the hotel. Just steps away from our own balcony, the ground rose above most of the lights, and the dark sky was terrific. From there I revisited some highlights in the SW with my 18x50 IS Canons, which I also used during the first trip to Oz. NGC 346 and 371 stood out in the SMC, as did the nearby Galactic globular NGC 362 (Caldwell 104). 47 Tucanae (NGC 104, Caldwell 106) was stunning and displayed a warm tinge. The Tarantula Nebula (NGC 2070, Caldwell 103) dominated the “hook/arm” of the LMC. Clouds covered the SW sky before I could find the first new object, Caldwell 87 (NGC 1261), the GC in Horologium. The S Milky Way was in the clear, but I literally got ants in the pants and could not stand their biting any more. We found the Red Centre generally rather green after the unusual rains and teeming with life.

Have a great trip, Steve!

wvreeven
September 28th, 2015, 09:56 AM
I have the luck to have visited Madagascar a few years ago. Actually I flew there the day after the big Christmas tsunami that wrecked havoc all over the Indian Ocean. It was at the end of the rain season in Madagascar and I had to wait for over a week before I got a clear night. But what a night it was! I only had binoculars and I explored the southern sky, as well as part of the north where comet Macholtz (I think it was that one) was close to the Pleiades and showed off a naked eye tail of over 45º long! The location I was was at -24º, just south of the tropic of Capricorn, in a resort without artificial light. Shortly after sunset both the Milky Way and the zodiac light were as clear as I ever saw them, both before and after. The Rosetta Nebula was easily visible naked eye and very high in the sky.

I only clearly remember observing the Large Magellanic Cloud with my binos. The LMC was larger than th 5º field of view of the binos. I do recall viewing the Small Magellanic Cloud and 47 Tucanae both naked eye and with the binos, however I don't recall any specific details.


Clear skies, Wouter

Clear Skies
September 29th, 2015, 09:00 AM
Observations of 47 Tuc appear to go hand in hand with Mother Nature's dangers, judging by two of the three posts above..! Let me add a third:

1 January 2008, Welgevonden Game Reserve, South Africa

Simple observations using cheap, tripod mounted 10x50 binos, on the deck of Ibubhesi lodge (http://www.welgevonden.org/lodges/ibhubesi-private-game-lodge.html). Without doubt the darkest skies I have ever seen. Everyone's already asleep, it's just me and the stars. Well, actually it's me, the stars and the wildlife. There's really nothing between myself and everything roaming the plains.

Naked Eye: A somewhat fuzzy star NW of the Small Magellanic Cloud.

10x50 binos: Clearly recognizable as a globular. Brighter in the center and very faintly resolvable.

I observe until one in the morning. Just before wrapping it up, I hear a sound. A muffled "whoohf, whoohf". Sound travels far at night, but this sounds very close. From the lodge, I make the short walk to the small chalet where I sleep. The whoohf is definitely in the vicinity.
Before departing on that morning's gamedrive, I describe the sound to our guide and park ranger, Simon. He replies:

"Oh, thank you very much, that is the male leopard!"

Me: "Right..."


7 January 2008, Sutherland, South Africa

First visit to this magnificent site in the Karoo desert. My travelscope, a 5" Mak is tagging along.

106x / 36': An incredibly beautiful globular cluster, very bright, approximately 1/4 FoV in size. A great many stars are resolvable, truly resembles a hand of salt sprinkeled on a table. Has a clear, bright core that itself is resolvable. Directly SW of the core is a mag. 12 star (= double star hj3363), farther to the NNE and slightly farther to the SSW is a white mag. 9 star. Directly surrounding the cluster are many mag. 10 stars. The cluster appears to be slightly flattened on the SSE and west side.

48x / 58': Using this magnification the cluster fills approximately 1/6 of the FoV and already is well resolved.

284x / 18': Using this magnification the cluster fills approximately 1/2 FoV. Despite this high magnification the cluster remains well resolved.

Naked Eye: A fuzzy star that is slightly larger when using AV.


16 November 2009, Sutherland, South Africa

Third visit, I have not been back since.....
Using Jurg Wagener's (http://www.sutherlandinfo.co.za/cmsCategorys.asp?Category=STARGAZING&CHS=4&ID=13&CLID=4&LID=5&T4=4#STARGAZING) 11" SCT.

175x / 28': Very beautiful, very large with an obvious, round core surrounded by many mag. 11 and fainter stars, very bright. Using AV the central part appears somewhat three-dimensional. Resolvable into the core, not completely but a great many stars can be discerned. SSE of the central part of the cluster is an orang-red stars, resembling a carbon star. 1/4 FoV south is a yellow-orange mag. 9 star (double star Van den Bos 1908, sep. 0.4").
Rated it 10 out of 10.

Uwe Glahn
September 29th, 2015, 08:46 PM
Best glob in the entire sky and worth an OOTW Steve, good idea.

Although I have for sure visited the glob a dozen times during my three Namibia trips I found only three written comments.

naked eye
bright, maybe a little bit fuzzy star

8x30 Bino
bright an huge glow, easily concentrated to its middle

14,5" Newton
fantastic glob, unusual distribution of stars - in the middle small area with very closely packed stars, abrupt change in brightness next to the compact center with normal concentration loss to its outer regions

24" Newton
reddish to orange color tone of several stars in the center