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RA: 20h 06m 4.75s
Dec: −21° 55′ 16.2″
Ch: MSA:1386, U2:343, SA:23
Ref: SIMBAD, Archinal&Hynes (2003), SEDS
Type: globular cluster
Mag: B=10.03, V=9.18
The globular cluster near the border of Sagittarius and Capricorn was probably first seen by Mechain in August 1780.
This cluster was confirmed by Messier within two months of Mechain discovering it, describing it as "nebula without stars... possibly composed of very small stars and containing nebulosity."
William Herschel, in 1784, described it as "a miniature of M3 and pale to the gaze."
In the Philosophical Transactions, 1818, William Herschel wrote: "1799, 7 feet finder. It is but just visible. 1799, 7 feet telescope. There is not the least appearance of its consisting of stars, but it resembles other clusters of this kind, when they are seen with low .. magnifying powers. 1810, 10 feet telescope. With 71 it is small and cometic. 1784, 1785, 20 feet Newtonian. Easily resolvable, some of the stars are visible. 1810, 20 feet, front view. It is a globular cluster. 1799, 1810, large 10 feet. Its diameter with 171 power is 1' 48 seconds; with 220 it is 2'."
In the NGC it is described as bright, pretty large, round, very much brighter in the middle forming a bright nucleus, partially resolved, some stars seen.
Admiral Smythe described the cluster as "a lucid white mass among some glimpse stars" and thought Messier's claim of suspected resolution was distinctly "bold".
Burnham calls it a small but very rich cluster, and notes that it is one of the more compact globulars. It is claimed by some observers to be the equal of the better known M80 in Scorpius, which was listed by Herschel as the most compressed cluster he had ever observed. It is only clearly resolved in large telescopes, the average magnitude of the 25 brightest stars being 17.1 as reported by Hogg, who also gives a diameter of 4.6' and total photographic magnitude of 9.5.
Burnham notes that there are four or five 12-14th magnitude stars in the field, surrounding the cluster in a neat semi-circle on the N, E and S; the brightest star of this arc is 1.5' out from the cluster on the SE side. Very possibly these stars were glimpsed by Messier and gave the impression of partial resolution.
Doig, P. (1925) Notes on the nebulae and clusters in Webb's 'Celestial Objects for Common Telescopes' (Sixth edition, Vol.ii). Part IV. M.N.R.A.S., 36(2), 58.
RA 20 06 04.8 (2000) Dec -21 55 17 Integrated V magnitude 8.52 Central surface brightness, V magnitudes per square arcsecond 15.55 Integrated spectral type F9- Central concentration, c = log(r_total/r_core); a 'c' denotes a core-collapsed cluster 1.88 Core radius in arcmin .10. ["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) ]
The RNGC (Sulentic and Tifft 1973) notes that this is a 9.5 mag globular cluster.
"globular cluster, extremely condensed"
Bailey, S.I. (1908) A catalogue of bright stars and nebulae. Ann.Harv.Coll.Obs., 60(8), 199.
Harrington, P. (1986) An observer's guide to globular clusters. Sky&Telescope, Aug, 198.
It can be reoslved in only the largest of amateur telescopes. Messier thought he had glimpsed several of the clusters individual stars, but this seems quite impossible. Most likely he saw the 12th mag foreground stars that are unrelated to M75 but are seen superimposed on it.
Jack Bennett, observing with a 5" Moonwatch telescope at 21x, classified this globular as C1, which means the cluster is bright and well seen, but is angularly small, almost stellar and can easily be missed when sweeping.
Walter Scott Houston calls it a "hazy spot about 2' in diameter" in his 4" Clark, and says that an excellent night is needed to detect any trace of stars. K.G. Jones, observing with an 8-inch at 120x noted only slight mottling at the edge of the cluster, whereas Houston, using a 10-inch reflector could begin to resolve only the outermost part.
William P. Clarke (San Diego, California, USA) writes in the The Webb Society Nebulae and Clusters Section Report No. 10, July 1992: "Bright and concentrated towards the centre. Only partial resolution achieved. (21-inch f/20, x140, x350)."
Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "8M; 3' diameter; pearl-like! small and beautiful with much brighter center; appears as core of a face-on spiral galaxy! not resolved."
Observer: Dave Mitsky
Your skills: Intermediate (some years)
Date/time of observation: 06/19/98 05:55 UT
Location of site: ASH Naylor Observatory, Lewisberry, PA (Lat 40.1d N, 76.9d W, Elev 570')
Site classification: Exurban
Sky darkness: <5.0 Limiting magnitude
Seeing: 7 1-10 Seeing Scale (10 best)
Moon presence: None - moon not in sky
Instrument: 17" f/15 equatorial classical Cassegrain
Magnification: 144x, 259x, 381x
Object(s): M75 (NGC 6864)
Category: Globular cluster.
Data: mag 8.6 size 6.0'
Position: RA 20:06.1 DEC -21:55
M75 is a relatively small but highly concentrated globular cluster in eastern Sagittarius. It is actually the most concentrated and distant (78,000 light years) of the Messier globulars and has a diameter of about 88 light years.
At 144x M75 was small, circular, and unresolved. When I observed M75 at 259x it was only partially resolved and a brighter central condensation was noticeable. M75 appeared almost stellar at 37x through the 5" f/5 finder scope.
Steve Coe, observing with a 13" f/5.6, notes: "(M 75) Bright, pretty large, round, much, much brighter in the middle at 165X. This globular has a blazing core and is very mottled, but I could not resolve a star in it on a 7/10 night, even with powers up to 270X."
8-inch Newtonian, 66x: 1995-06-19 "Like a point; small; from the inner point outwards the brightness falls abruptly." [Gabriel Giust, San Isidro, Argentina]
Location: Campsite (South 23 16 East 29 26).
Sky conditions: Clear, steadiness good.
Instrument: Meade 8 inch, Super wide-angle, 18mm eyepiece; 36.2' fov
DSO Report N
Bright, medium size, round compressed globular cluster with a bright core. Faint stars just want to become visible, with bright stars just outside the cluster in the field.
16-inch f/10 SCT (EP: 2-inch 40mm SW 102x 26' fov; 2-inch 32mm SW 127x 32' fov; 2-inch 14mm UW 290x 17' fov)
Very small but bright and prominent. This globular show a small though very bright compact core that comprises about one third of the whole appearance. Soft outer envelope can be seen. No stars reveal themselves but slightly mottled towards the edges. At (290x) the inner core appears somewhat compact. Many stars are visible in the south of the field of view. Discovered by Mechain in 1780.
Sky Conditions:The fainter parts of the Milky Way are barely visible.Haziness only visible on the horizon.Atmosphere stable with little interference.
M75's stars are not resolvable and that this cluster is strongly condensed and looks like a bright roundish snowball.This globular cluster is well condensed as a halo of white light and that the central core of this cluster is extremely condensed.This globular cluster measures 3'x 2.5'.Chart No:331,NSOG Vol.2.
Instrument:12-inch Dobsonian Reflector Telescope.
Sky Conditions:The fainter parts of the Milky Way are barely visible on the horizon.
Transparency of the Sky:Haziness only visible on the horizon.
Seeing:Atmosphere stable with little interference.
Object Type:Globular Cluster.
First Impression:This object looks like a globular cluster.
Chart Number:No.19(Extract taken out of "Atlas of the Night Sky").
Size:26mm Eyepiece:Field Of View:57'/15= 3.8'.
20mm Eyepiece:Field Of View:50'/13= 3.8'.
3.8'+ 3.8'= 7.6'.
Size in Arc Minutes:3.8'(Nucleus).
Globular Cluster is 3.8'* 1.2'.
Brightness Profile:From the far outskirts of this globular cluster it grows brighter in the nucleus.
Challenge Rating:Moderately Difficult.
This globular cluster is moderately faint to observe and no individual stars are seen in this cluster.The stars in this cluster look like a round neat snowball.This cluster is moderately small however the stars are centrally concentrated towards each other in a halo.
The Messier objects
The Bennett objects
The Caldwell list
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