The Eagle Nebula, Messier object 16 (M16), NGC 6611, as seen by the Kitt Peak 4-meter Mayall telescope in 1973. This picture shows an interesting emission nebula and its associated galactic star cluster in the constellation of Serpens (the Serpent). The star cluster was discovered in 1746 by P.L. de Cheseaux, but he missed the surrounding nebulosity, leaving it to be noticed by Charles Messier some twenty years later, while he was compiling the catalog that brought him permanent fame in astronomy and provided the `M' numbers we use to designate so many of the bright, fuzzy patches visible at night. M16 is one of the more unusual objects in the sky, and although it is unfortunately not a naked-eye object, it is a fine sight through a telescope of low to moderate power. M16 is an example of a galactic HII region, which is to say that most of the emission comes from the red light of ionized (electrically charged) hydrogen gas. The nebula shines because of the energy provided by the cluster of hot blue and white stars. These stars are about two million years old, which is quite young for a star (our own middle-aged Sun clocks in at over four billion years). However, these O and B stars are considerably heavier than our Sun, since they contain some thirty times as much matter, and this extra weight shortens their lifetime to just a few million years in total. The brightest stars, such as the conspicuous double of O stars, are concentrated toward the north-west (the top right of this picture). The cluster also contains a large number of faint red stars, which are probably reddened by absorption in the dust surrounding them. M16 is in a late stage of its evolution, where the hot stars have blown away the closer material and now shine in a cavity surrounded by an expanding complex of neutral lumps, which show bright edges due to shock waves created as the outwardly flowing material hits other gas. On the southern side there are several bright-edged dark lanes or striations crossing the nebula: these are often called `elephant trunk' structures. M16 also contains many small black globules, which are believed to be compact dust clouds on their way to collapsing into proto-stellar objects, and then later into new stars. Current estimates suggest that several new stars are formed somewhere in our Galaxy every year. In fact, the current glowing red appearance and the dark features are, respectively, the gas and dust out of which the existing stars originally condensed. There is also evidence for rapid motions in the nebula, with suggestions of turbulence around the dark lanes to the north-east (the top left). The nebula also contains some neutral gas clouds, which have no electrical charge and are only detected by radio astronomers. M16 is located, along with its neighbor, the Omega Nebula, M17, in the Sagittarius-Carina spiral arm of our Milky Way galaxy. We also have a spectacular 0.9-meter emission-line image. Location: 18h18.8m -13deg47min (2000). Distance: about 2000 parsecs (6500 light-years). Size: 8 arc minutes across. Magnitude: 6.4. Power source: O and B stars.
