The Lagoon Nebula, Messier 8 (NGC 6523) is one of the finest star-forming regions in the sky, and is faintly visible to the naked eye. It is a giant glowing cloud of interstellar gas, divided by a dark lane of dust, containing a cluster of young stars (NGC 6530) that have formed from it.
Discovery and History
The earliest observations of this object were made by Giovanni Battista Hodierna before 1654. He classified it as "nebulosa" of intermediate brightness. It was independently noted as a "nebula" by John Flamsteed around 1680. The object was again seen by Philippe Loys de Chéseaux in 1746, who could resolve some stars and consequently classified it as a cluster. One year later, in 1747, it was observed by Guillaume Le Gentil, who found the nebula along with the cluster. Abbe Nicholas Louis de la Caille cataloged it in his 1751-52 compilation.
When Charles Messier cataloged this object in 1764, he primarily described the cluster, and mentioned the nebula separately as surrounding the star 9 Sagittarii. Although most sources identify only the nebula as "Messier 8", it is clear from Messier's description that he found both the nebula and the cluster.
William Herschel assigned separate catalog numbers to two parts of, the Lagoon Nebula: H V.9 (GC 4363, NGC 6526) and H V.13 (GC 4368, NGC 6533), which are described in the NGC as "large and faint nebulae". John Herschel eventually cataloged the open cluster NGC 6530 separately as h 3725 (GC 4366); he has M 8 as h 3723 (GC 4361, NGC 6523).
Amateur Observation
At magnitude 6, the Lagoon Nebula is faintly visible to the naked eye under good conditions. It is one of only two star-forming regions visible to the naked eye from mid-northern latitudes (the other is M 42 in Orion).
The Lagoon is a spectacular object in small telescopes, nearly as impressive as M 42. Even binoculars show an oval wisp of light split, into distinct sections by an irregular dark lane (the "Lagoon"). M 8 has apparent dimensions of 90' x 40', and covers three times the area of the full Moon. The nebula's faint extension to the East has its own IC number: IC 4678. The open cluster NGC 6530, centered in the eastern wing of the nebula, contains thirty members in a 10' area.
Like most nebulae, the Lagoon appears gray to the eye peering through binoculars or a telescope, since human vision has poor color sensitivity at low light levels. But M 8 is a magnificent object for the amateur astrophotographer, appearing pink in long-exposure color photos. M 8 is situated in a very conspicuous Milky Way field near the Trifid Nebula M 20 and the open star cluster M 21. This region, and the rich star field and faint nebulae surrounding it, is a favorite of astro-imagers.
The brightest parts of the Lagoon Nebula contain a feature known as the "Hourglass Nebula", discovered and named by John Herschel. This is in a region where vivid star formation is taking place, and its bright emission is caused by heavy excitation from very hot, young stars.
The nebula also contains a number of dark globules which represent collapsing clouds of protostellar material. The most prominent of these dark patches were catalogued by E. E. Barnard: B 88, a comet-shaped globule extended North-to-South in the nebula's eastern half; B 89, a smaller dark nebula near the cluster NGC 6530; and B 296, a long, narrow black patch at the nebula's south edge.
Properties and Cluster
The Lagoon Nebula lies in the heart of the galaxy's Sagittarius-Carina spiral arm, but its distance is a bit uncertain. Estimates range from 4,800 to 6,500 light years, with 5,200 quoted by many sources. A 2006 study found a distance of 4,100 light years, which would make its true size about 110 x 50 light years. The nebula probably has a depth comparable to its linear dimensions. The dark "Bok" globules of collapsing protostellar material have diameters of about 10,000 AU.The western half of M 8 is primarily illuminated by the magnitude 5.97 star 9 Sagittarii, an extremely hot O5 star which radiates 44 times more high-energy ultraviolet than visual light. At visual wavelengths, 9 Sagittarii is 23,000 times brighter than our Sun! The illuminator of the "Hourglass" feature is the hot star Herschel 36 (mag 9.5, spectral class O7). In 2006, four Herbig-Haro objects were detected within the Hourglass, providing the first direct evidence of active star formation by accretion within it.
The hot O-type stars of the young open cluster NGC 6530 are fluorescing the eastern part of the nebula. As their light shows little reddening by interstellar matter, this cluster is probably situated just in front of the Lagoon Nebula. Its brightest star is a hot, mag 6.9 class O5 star, with an age around 2 million years. The cluster also contains one extremely hot, peculiar star of spectral type Of, with spectral lines of ionized Helium and Nitrogen.