|Designation|| Messier 8|
|Right ascension||18h 03m 37s|
|Declination||-24° 23′ 12″|
|Type of object||Emission nebula|
|Dimensions||90' x 40'|
|Magnitude|| Apparent Mag: +6.0|
Absolute Mag: -4.5
|Distance from Earth||4,100 ly|
|Radial velocity||3.8 km/s|
The Lagoon nebula (M8, NGC 6523) is an emission nebula in the constellation of Sagittarius. It contains an open cluster of stars that has a magnitude of 4.6. It is only just visible to the naked eye under good observation conditions and one of the larger nebulae in Sagittarius.
The Lagoon nebula was discovered by Giovanni Battista Hodierna at some point before 1654 and independently by John Flamsteed in or around 1680. Stars were first resolved by Philippe Loys de Chéseaux in 1746 and so Chéseaux classified it as a cluster. Then in 1747, Guillaume Le Gentil observed the nebula and the cluster simultaneously and catalogued the two together. Charles Messier added both the nebula and the cluster to his catalogue, but mainly described the cluster. William Herschel catalogued the nebula as two components, and his son John Herschel then added the star cluster and a separate entry in Herschel's catalogue.
Properties and Structure
With apparent dimensions of 90'x40', three times that of a full moon, the Lagoon nebula is a particularly large nebula. These dimensions correspond to it being 110x50 light years. Its name comes from the dark gas cloud in the centre, similar to a sandbar separating two lagoons. M8 is a H II region, meaning it is mostly composed of ionized hydrogen.
The open cluster housed inside M8 has been designated NGC 6530 and classified as Trumpler type "II m n". The 50-100 stars in the cluster are loosely concentrated in the centre, and have some range in their magnitude. It is thought the cluster is located in front of the nebula as there is little in the way of reddening that would be expected if it were behind. The hot stars cause the nebula to glow.
The Hourglass nebula is found inside the brightest region of M8. First observed by John Herschel, its name is derived from its shape. It is however a separate object from the Engraved hourglass nebula in Musca. The extremely large, hot stars illuminate this region, specifically Herschel 36. In 2006, scientists discovered four Herbig-Haro objects. These are occur where jets of gas collide with other gas and dust at high speeds and are often found near young stars.