Messier 8

Starforming Nebula M8 (NGC 6523), an emission nebula, with open star cluster, type "e", in Sagittarius

Lagoon Nebula

Discovered by Hodierna about 1654.

The Lagoon Nebula Messier 8 (M8, NGC 6523) is one of the finest and brightest star-forming regions in the sky. It is a giant cloud of interstellar matter which is currently undergoing vivid star formation, and has already formed a considerable cluster of young stars.

This object has been discovered by Giovanni Battista Hodierna before 1654, and classified it as "nebulosa," i.e. of intermediate brightness; it is his No. II.6. It was independently noted as a "nebula" by John Flamsteed about 1680, who cataloged it as his No. 2446. Due to reasons which are not completely clear, at least to the present author [hf], Kenneth Glyn Jones has supposed that Flamsteed may only have seen the cluster within this nebula, a view which we had formerly adopted here. However, Flamsteed's position is close to that later determined by Messier and near the center of the nebula, while the young open cluster, which was later cataloged as NGC 6530, is situated (or at least centered) in the Eastern half of M8.

This 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 together with the cluster. 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 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, until recently, most sources identified only the nebula with "Messier 8," a view we reject here: It is clear from Messier's description that he had found both the nebula and the cluster.

William Herschel assigned separate catalog numbers to two objects within, or parts of, the Lagoon Nebula: H V.9 (GC 4363, NGC 6526) and H V.13 (GC 4368, NGC 6533) which are described as large and faint nebulae in the NGC. John Herschel eventually cataloged the open cluster NGC 6530 separately as h 3725 (GC 4366); he has M8 as h 3723 (GC 4361, NGC 6523).

According to Kenneth Glyn Jones, the Lagoon Nebula has an apparent extension of 90x40 minutes of arc, which is 3 x 1 1/3 the apparent diameter of the full moon, and corresponds to about 140x60 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.

As published in January 1997, the Hubble Space Telescope has been used to study the Hourglass Nebula region in the Lagoon Nebula M8.

The Lagoon Nebula is a magnificient object for the amateur astrophotographer, as Brad Wallis and Robert Provin have demonstrated with their outstanding images, and Dr. Andjelko Glivar with his photos taken through a Celestron 8.

The young open cluster NGC 6530 associated with the Lagoon Nebula M8 was classified as of Trumpler type "II 2 m n" (see e.g. the Sky Catalog 2000), meaning that it is detached but only weakly concentrated toward its center, its stars scatter in a moderate range of brightness, it is moderately rich (50--100 stars), and associated with nebulosity (certainly, with the Lagoon nebula). As the light of its member stars show little reddening by interstellar matter, this cluster is probably situated just in front of the Lagoon Nebula. Its brightest star is a 6.9 mag hot O5 star, and Eichler gives its age as 2 million years. Woldemar Götz mentions this cluster as containing one peculiar Of star, an extremely hot bright star of spectral type O with peculiar spectral lines of ionized Helium and Nitrogen.

The nebula's faint extension to the East (top in our image, but beyond) has an own IC number: IC 4678.

M8 is situated in a very conspicuous field of the Sagittarius Milky Way. Another capture from the DSSM shows the Lagoon Nebula M8 and Trifid Nebula M20, plus the rich star field and faint nebulae surrounding them. We have also more images of the region of M8 and M20, which sometimes also include the nearby open star cluster M21.

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