Messier 23

Discovered on June 20, 1764 by Charles Messier.

Messier: M23.

June 20, 1764. 23. 17h 42m 51s (265d 42' 50") -18d 45' 55"
A star cluster, between the end of the bow of Sagittarius & the right foot of Ophiuchus, very near to 65 Ophiuchi, according to Flamsteed. The stars of this cluster are very close to one another. Its position was determined from Mu Sagittarii. (diam. 15')

[Mem. Acad. for 1771, p. 444 (first Messier catalog)]
In the night of June 20 to 21, 1764, I determined the position of a cluster of small stars which is situated between the northerb extremity of the bow of Sagittarius & the right foot of Ophiuchus, very close to the star of sixth magnitude, the sixty-fifth of the latter constellation [Oph], after the catalog of Flamsteed: These stars are very close to each other; there is none which one can see easily with an ordinary [non-achromatic] refractor of 3 feet & a half, & which was taken for these small stars. The diameter of all is about 15 minutes of arc. I have determined its position by comparing the middle with the star Mu Sagittarii: I have found its right ascension of 265d 42' 50", & its declination of 18d 45' 55", south.
[p. 456] 1764.Jun.20. RA: 265.42.50, Dec: 18.45.55.A, Diam: 0.15. Cluster of stars between the extremity of the bow of Sagittarius & the right foot of Ophiuchus, very near to the star 65 Ophiuchi, according to the catalog of Flamsteed.

Bode: Bode 42

A star cluster.

Caroline Herschel

May 5, 1783. Observed M23, M4, and M9.

William Herschel

[Unpublished Observations of Messier's Nebulae and Clusters. Scientific Papers, Vol. 2, p. 652]
1784, June 18 [Sw. 230). A cluster of beautiful scattered, large stars, nearly of equal magnitudes (visible in my finder), it extends much farther than the field of the telescope will take in, and in the finder seems to be a nebula of a lenthened form extending to about half a degree.

John Herschel (1833): h 1990.

h 1990 = M23.
Sweep 276 (July 29, 1830)
RA 17h 46m 41.8s, NPD 108d 61m 45s (1830.0) [Right Ascension and North Polar Distance]
A v L, p rich, coarse scattered cluster which fills the field. Stars 11 and 12 m.
A very large, pretty rich, coarse scattered cluster which fills the field [of view]. Stars of 11th and 12th magnitude.

Sweep 269 (July 15, 1830)
RA 17h 46m 61.0s, NPD 108d 57m 48s (1830.0)
A star 10m in the centre of a beautiful discrete cluster of 60 or 70 stars 10 and 11 m and one of 9.10. They run in lines and arches. It is loose and straggling, and the sky around it has a dotted appearance.
A star 10th magnitude in the centre of a beautiful discrete cluster of 60 or 70 stars of 10th and 11th magnitude and one of 9th or 10th. They run in lines and arches. It is loose and straggling, and the sky around it has a dotted appearance.

Sweep 33 (July 24, 1826)
RA 17h 46m 61.9s, NPD 108d 58m 18s (1830.0)
A v large, coarse straggl cluster of about 100 stars 9.10...13m. It is announced several minutes before by an increased number of stars in the field.
A very large, coarse straggled cluster of about 100 stars 9.10...13m. It is announced several minutes before by an increased number of stars in the field.

Smyth: DCXXVI [626]. M23.

DCXXVI. 23 M. Ophiuchi [now Sagittarii].
AR 17h 47m 32s, Dec S 18d 58'.2
Mean Epoch of Observation: 1835.54 [Jul 1835]
A loose cluster in the space between Ophiuchus's left leg and the bow of Sagittarius. This is an elegant sprinkling of telescopic stars over the whole field, under a moderate magnifying power; the most clustering portion is oblique, in the direction sp to nf [south preceding to north following, SW to NE], with a 7th-magnitude star in the latter portion. The place registered it that of a neat pair, of the 9th and 10th magnitudes, of a lilac hue, and about 12" apart. This object was discovered by Messier 1764, and it precedes a rich out-cropping of the Milky Way. The place is gained by differentiating the cluster with Mu Sagittarii, from which it bears north-west, distant about 5 deg, the spot being directed to by a line from Sigma on the shoulder, through Mu at the tip of the bow.
After having examined this object, I lowered the telescope a couple of degrees, and gazed for the curious trifid nebula, 41 H. IV [H IV.41]; but though I could make out the delicate triple star in the centre of its opening, the nebulous matter resisted the light of my telescope, so that its presence was only indicated by a peculiar glow. Pretty closely preceding this is No. 20 M., an elegant cruciform group of stars, discovered in 1764, which he considered to be surrounded with nebulosity.

John Herschel, General Catalogue: GC 4346.

GC 4346 = h 1990 = M23.
RA 17h 48m 40.9s, NPD 108d 59' 43.7" (1860.0) [Right Ascension and North Polar Distance]
Cl; B; vL; pRi; lC; st 9.10,11...13 6 observations by W. & J. Herschel.
Cluster; bright; very large; pretty rich; little compressed; one star of 9th to 10th magnitude and stars of 11th to 13th magnitude.

Dreyer: NGC 6494.

NGC 6494 = GC 4346 = h 1990; M 23.
RA 17h 48m 41s, NPD 108d 59.7' (1860.0) [Right Ascension and North Polar Distance]
Cl, B, vL, pRi, lC, st 10...; = M23
Cluster, bright, very large, pretty rich, little compressed, stars of 10th magnitude and fainter.