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NGC 104
47 Tucanae, GCL-1, ESO050-SC009
RA 00:24:05.2
Dec -72°
04' 51''
Globular cluster |
This spectacular globular cluster was
discovered by Lacaille and included in his 1755 catalogue as Class I No. 1. In
his half-an-inch 8x telescope he saw it as being "like the nucleus of a
fairly bright comet."
James Dunlop observed it from Paramatta, New
South Wales, and included it as No. 18 in his catalogue of 1827. Using a 9-inch
f/12 telescope, he described it as "a beautiful large round nebula, about
8' diameter, very gradually condensed to the centre. This beautiful globe of
light is easily resolvable into stars of a dusky colour. The compression to the
centre is very great, and the stars are considerably scattered S.p. and N.f."
Sir John Herschel observed it frequently
during his stay at the Cape of Good Hope. He used an 18-inch f/13 speculum
telescope, and observed it for the first time on 11 April 1834. On that occasion
he wrote of it as "the great cluster preceding the Nubecula Minor.
Estimated diameter 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 observations of
12 August 1834 read: "A most glorious cluster. The stars are equal, 14th
mag., immensely numerous and compressed. Its last outliers extend to a distance
of 2 minutes, 16 seconds 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 (Pos. 226.5 - 6.5 arcsec in RA from centre
of neb.)" On 21 September 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 seconds 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 made on 5 November 1836 when he called it "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 arcsec diameter 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." He sketched the cluster, showing its appearance "as seen
on a great many other occasions." He noted that "the contrast between
the rose-coloured light of the interior and the white of the exterior portions
cannot, of course, be represented in an engraving, but of the phenomenon itself,
I have no doubt. The double star on the S.p. edge of the more condensed portion
has probably no connexion with the cluster." Referring to its overall
shape, he notes "its figure is round, and not elliptic - (at least not so
elliptic) as described and figured by Mr. Dunlop."
Burnham writes: "In larger instruments
the cluster becomes a shimmering globe of thousands of star points, crowding
toward a rich central blaze. The diameter is about 25' visually, but on the best
photographic plates the full size is close to 45'."
Agnes Clerke wrote that "the sheeny
radiance of this exquisite object appeared of uniform quality from centre to
circumference . . . Perhaps no other cluster exhibits an equal degree of
compression . . . The blankness of the surrounding sky renders 47 all the more
obvious to unaided sight; it was, indeed, for several nights . . . mistaken by
Humboldt for a comet."
ASV Journal, Vol 24, No 3, June 1971: "hazy
to naked eye, good in binoculars, superb in telescope."
The mean blue magnitude of the 25 brightest
stars, excluding the 5 brightest, is 13.54. From Harris: Integrated V magnitude
3.95 Central surface brightness, V magnitudes per square arcsecond 14.43
Integrated spectral type G4 Central concentration, c = log(r_total/r_core); a
'c' denotes a core-collapsed cluster 2.04 Core radius in arcmin .37. [“Catalog
Of Parameters For Milky Way Globular Clusters”, 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)
]
11x80: At 4th magnitude,
this cluster is visible to the naked eye as a blurred star about 2.5 degrees
west of the Small Magellanic Cloud. In binoculars, under these dark skies, it is
breath-taking! The nucleus 1.5 arcmin wide, is a perfectly round bright disc,
surrounded by a round soft halo at least 12 arcmin wide. (pristine skies, SAAO
plateau, Karoo) [AS]
10-inch f/5 Newtonian: At
30x, the 10-inch shows this incredible object as an innumerable host of bright
points gathered up into a round blaze of unresolved stars in the centre. The
central blaze looks like a bright button. In the wide-field eyepiece, many
unevenly distributed field stars are visible, and looking towards the cluster,
the star density gradually increases, and then all of a sudden the stars gather
into a nucleus. This nucleus is a very prominent feature and it draws the eyes
attention right away; with the result that you see the outer region as a widely
scattered starfield with a bright disk in the centre which glows warmly with a
slight tinge of yellow; if you avert your vision onto one of the bright field
stars, the outer regions fill up with stars, becoming less resolvable, the whole
looking like Omega Centauri. (suburban skies) [AS]
12-inch Meade Newtonian:
What an excellent, beautiful globular cluster! Very large, compressed and bright
towards the middle. It resembles a soft light embedded in a haze of crowded
faint and brighter stars, going out to the far edges of the field. Two bright
stars along the sides round it off beautifully. (40mm eyepiece) (suburban skies)
[MS]
15.5-inch f/9 Newtonian: The
first impression of this cluster is simply the overwhelming multitude of stars,
and then the very tight, knotted nucleus. Studying the object, it seems to be
divided into three layers: (1) there is the central compact core with some stars
resolved, then (2) a surrounding halo, twice as wide as the core, and then (3) a
completely dissociated matte sprinkling of stars. There are also a number of
foreground stars which seem to lie over the globular, the most prominent one
lies southeast of the cluster, just outside the fringe, and appears red.
Observing at 200x, the cluster exhibits two interesting features. The most
prominent is a lesion on the eastern side of the nucleus. This is a dark gap,
about a half to a third of the width of the core, extending west-east across the
core, causing it to look like a Pac-Man. Also present are conspicuous streamers
of stars leading from the nucleus into the 2nd zone. There are at least four of
these streamers, two starting from the lesion, and two originating directly
opposite. At 260x, the dark gap appears less conspicuous, although the gap
itself seems to have a chain of stars lying on its southern side. Upon close
examination, however, the dark gap seems to extend right across the core,
dividing it into two uneven halves, the smaller half being to the south and
about a third of the total area in size. (suburban skies) [AS] |
