Lagoon Nebula — The Lagoon Nebula was discovered by Le Gentil in 1747.
As often for diffuse nebulae, the cluster of young stars which has formed from the nebula’s material was discovered first.
In this case the young open cluster NGC 6530 in the Eastern half of M8 was discovered by Flamsteed about 1680, and again seen by De Ch’seaux in 1746, before Le Gentil found the nebula in 1747.
Abbe Nicholas Louis de la Caille has cataloged it in his 1751-52 compilation as Lacaille III.14.
When Charles Messier cataloged this object on May 23, 1764, he also primarily described the cluster, and mentioned the nebula separately as surrounding the star 9 Sagittarii.
His original position is closer to the modern position of the cluster than to that of the nebula.
Nevertheless, it is the nebula which is now generally regarded as “Messier 8″.
According to Kenneth Glyn Jones, the Lagoon Nebula has an apparent extension of 90×40 minutes of arc, which is 3 x 1 1/3 the apparent diameter of the full moon.
This corresponds to about 140×60 light years if our distance of 5,200 light years should be correct, which is a bit uncertain; newer sources have 4850 (Glyn Jones) to 6500, but David J. Eichler gives the value of 5,200 light years (Eichler 1996).
One of the remarkable features of the Lagoon Nebula is the presence of dark nebulae known as ‘globules’ (Burnham) [see expanded image] which are collapsing protostellar clouds with diameters of about 10,000 AU (Astronomical Units). They can also be seen, along with other detail, in the DSSM image of M8.
Some of the more conspicuous globules have been cataloged in E.E. Barnard’s catalog of dark nebulae: Barnard 88 (B 88), the comet-shaped globule extended North-to-South (up-down) in the left half and near top of our image, small B 89 in the region of cluster NGC 6530, and long, narrow black B 296 at the south edge of the nebula (lower edge of the image).
According to David Eichler, the nebula has probably a depth comparable to its linear extension indicated above.
Within the brightest part of the Lagoon Nebula, a remarkable feature can be seen, which according to its shape is called the “Hourglass Nebula” (see our detailed photos).
This feature was discovered by John Herschel and occurs in a region where a vivid star formation process appears to take place currently; the bright emission is caused by heavy excitation of very hot, young stars, the illuminator of the hourglass is the hot star Herschel 36 (mag 9.5, spectral class O7).
Closely by this feature is the apparently brightest of the stars associated with the Lagoon Nebula, 9 Sagittarii (mag 5.97, spectral class O5), which surely contributes a lot of the high energy radiation which excites the nebula to shine.