Open Clusters NGC 6755 and NGC 6756
History
The open cluster NGC 6755 was discovered by William Herschel on 30 July 1785. He listed it as VII 19 and recorded: «A pretty compressed cluster of pretty scattered stars of variable sizes, magnitudes, and colours. Irregular figure and unequally compressed 12 or 15' diameter.» [464] John Herschel first observed the cluster on 7 July 1827 (sweep 77) and last on 13 August 1830 (sweep 278) where he noted: «A very large, very rich cluster, composed of 2 or 3 clustering groups running together; place that of the most condensed part.» [466]
The open cluster NGC 6756 was discovered by William Herschel on 21 August 1791. He listed it as VII 62 and wrote: «A small pretty compressed cluster of stars, not very rich.» [465] John Herschel observed the cluster four times, first also on 7 July 1827 and last on 13 August 1830 where he wrote: «Pretty rich; small; much compressed; oval or rather fan-shaped. The stars 11..12 magnitudes, 4' in extent; the north following side most compressed.» [466]
Physical Properties
| Name | RA | Dec | Type | vMag | Dim | MD | Dreyer Description | Identification, Remarks |
|---|---|---|---|---|---|---|---|---|
| NGC 6755 | 19 07 49.0 | +04 16 00 | OCL (IV2m) | 7.5 | 15 | 1.785 | Cl, vL, vRi, pC, st 12…14 | WH VII 19; h 2030; GC 4470; OCL 96 |
| NGC 6756 | 19 08 42.5 | +04 42 21 | OCL (I2m) | 10.6 | 4 | 1.507 | Cl, S, Ri, lC, st 11…12 | WH VII 62; h 2031; GC 4471; OCL 99 |
Finder Chart
The open clusters NGC 6755 and NGC 6756 are located in constellation Aquila. On 8 July these are in opposition with the Sun and is therefore highest in the sky at local midnight.
Visual Observation
762 mm Aperture: In the open cluster NGC 6756, the stars are arranged very loosely and scattered. The brightest stars lie slightly outside the center, giving the cluster more the appearance of a triangle with a bright tip. — 30" f/3.3 SlipStream Dobsonian, Hasliberg, 20. 9. 2025, SQM-L 21.01, Eduard von Bergen