M8 (NGC6523)

Index

1. The History
2. To find M8
3. To observe M8
4. The stars of M8, and more

The History

M8 was discovered by Le Gentil (1747); the cluster of young stars originated from the nebula, situated in the eastern region of M8, was discovered long before; until 1984, Flamsteed (1680) was credited with the discovery of this galactic cluster, later named NGC 6530. But in 1985, G. Foderą Serio, P. Nastasi e L. Indorato claimed Hodierna's original discovery of the cluster: it is, in fact, described and drawn in the "De Admirandis Coeli Characteribus", printed in 1654 in Palermo. It is also shown in some manuscript maps.

Lacaille listed it (as Lac III.13) in his "Catalogue of Nebulae of the Southern Sky". Messier described it as "Star cluster which appears as a nebula when seen in an ordinary telescope of three feet..."

William Herschel, with his great reflector, described "...an extensive milky Nebulosity divided into two parts; the north being the strongest. Its extent exceeds 15'; the southern part is followed by a parcel of stars which I suppose to be the 8th of the Connaissance des Temps ..."

To find M8

M8 lies in a very rich field of the Milky Way, in Sagittarius. You can find it some 4.7° W and slightly N from l Sagittarii; it's easily visible to the naked eye under a clear and dark sky, as a glowing patch, on the edge of the main stream of the Sagittarius Milky Way.

To observe M8

The small telescopes, and the binoculars, show a fine irregular nebulosity enveloping the cluster NGC6530. The O.R.S.A.'s 412 mm. Newtonian reflector shows the full extent of the "Lagoon Nebula", with its dark channel that cuts through the middle of the whole cloud. Its diameter is over 1/2° (according Kenneth Glyn Jones, 90'x40'), so, to observe it in its full extent, a wide-field eyepiece is needed.
In long exposure photographs M8 is a magnificent sight: a wealth of intricate detail becomes visible, a mix of bright and dark nebulosity, even several bright filaments interlacing in the dark lane that gives the name "Lagoon Nebula" to this superb object.
Another remarkable feature of M8 are the small, dark globules of obscuring matter detected by Bart J. Bok in 1946. These are thought to be sufficiently dense to be gravitationally stable against the disruptive forces of the galaxy and nearby stars, and may continue to contract and eventually become visible as protostars, shining with their own radiation: the first phases of the life of a star.

The stars of M8, and more

The west half of M8 is dominated by two bright stars, just 3' apart: the southern one being 9 Sagittarii; this star appear to be the principal illuminating star of the nebula. Baade, nevertheless, suggest that several other extremely hot stars probably exist in the cloud, obscured by masses of dark nebulosity.
3' WSW from 9 Sagittarii lies the brightest portion of the nebula, a "knot" about 30" in size, called "The hourglass nebula" owing to its shape
NGC6530, the galactic cluster associated with M8, lies in the eastern half of the nebula.The brightest members are subgiants B0-type.
M8 contains several irregular variable stars and also some "flare" stars.