with the Lagoon Nebula

Degree of difficulty 1 (of 5)

Minimum aperture Naked eye

Designation NGC 6530, NGC 6523

Type Open cluster, Galactic nebula

Class II2mn

Distance 4310 ly (K2005) 4890 ly (proper motion, 1997)

Size 9 ly; 115 ly × 50 ly

Constellation Sagittarius

R.A. 18h 3.8min

Decl. –24° 23′

Magnitude 5.8; 4.6

Surface brightness 22mag/arcsec2

Apparent diameter 7′; 90′×40′

Discoverer Flamsteed, 1680; le Gentil 1749

History In 1680, John Flamsteed noted a “nebula, preceding the bow of Sagittarius.” His small telescope simply could not resolve the star cluster (NGC 6530). De Chéseaux, in 1746, also observed the cluster.

The nebula itself (NGC 6523) was first noticed by Guillaume le Gentil in 1749. He wrote: “west of a star cluster; this nebula has the exact shape of a slightly elongated, equilateral triangle, tip pointing south-west. I have observed it with a refractor of 18 to 20 foot, and it always appeared to me nebulous and transparent; its base touches a rather beautiful star, seen in the refractor, which is the brightest of all those which form the star cluster I have mentioned.” Three years later, and with a much smaller telescope, Lacaille described M 8 as “three stars, engulfed in a band of nebulosity parallel to the equator.”

On the 23rd of May 1764, Charles Messier pointed his telescope to M 8 and noted: “Cluster of stars which appears in the form of a nebula when it is viewed with a simple refractor of three feet; but with an excellent instrument one does not notice but a great number of small stars; near this cluster is a quite brilliant star, surrounded by a very faint light; this cluster appears in an elongated shape, which extends from North-east to South-west, 30' diameter.” Hence, for Messier, the star cluster mainly makes up his object No. 8; the nebula is mentioned only in passing.

William Herschel registered the nebula as an object of its own, but skipped the cluster in consideration of the priority of Messier's published catalog.