Thursday, August 27, 2009
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. It was independently noted as a "nebula" by John Flamsteed about 1680, who cataloged it as his No. 2446. 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 on 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. 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.
One of the remarkable features of the Lagoon Nebula is the presence of dark nebulae known as 'globules' which are collapsing protostellar clouds. 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).
Within the brightest part of the Lagoon Nebula, a remarkable feature can be seen, which according to its shape is called the "Hourglass Nebula". 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.
The young open cluster NGC 6530 associated with the Lagoon Nebula M8 was classified as of Trumpler type "II 2 m n", 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.
These are three Nebula found in Sagittarius. M8 (the Lagoon Nebula) and M20 (the Trifid). The Nebula in the upper right hand corner is NGC 6559 (which did not make Messier's list). This is a false color photo, with a color scheme apparently made common by the HUbble Telescope. The color scheme works for land based telescopes because it allows the astronomer to ignore urban light pollution. M8 is 30 light years LONG and around 5000 light years away.
The centre of the Lagoon Nebula is a whirlwind of spectacular star formation. Visible on the upper left, at least two long funnel-shaped clouds, each roughly half a light-year long, have been formed by extreme stellar winds and intense energetic starlight. The tremendously bright nearby star, Hershel 36, lights the area. Vast walls of dust hide and redden other hot young stars. As energy from these stars pours into the cool dust and gas, large temperature differences in adjoining regions can be created generating shearing winds which may cause the funnels. This picture, spanning about 5 light years, was taken in 1995 by the orbiting Hubble Space Telescope.