M16 The Eagle Nebula
Cluster M16 (NGC 6611) discovered by Philippe Loys de
Chéseaux in 1745-6.
Nebula IC 4703 discovered by Charles Messier in 1764.
Lying some 7,000 light years distant in the constellation Serpens, close to the borders to Scutum and Sagittarius, and in the next inner spiral arm of the Milky Way galaxy from us (the Sagittarius or Sagittarius-Carina Arm) a great cloud of interstellar gas and dust has entered a vivid process of star formation. Open star cluster M16 has formed from this great gaseous and dusty cloud, the diffuse Eagle Nebula IC 4703, which is now caused to shine by emission light, excited by the high-energy radiation of its massive hot, young stars. It is actually still in the process of forming new stars, this formation taking place near the dark "elephant trunks" which are well visible in the photograph above. A deeper insight in the star formation process could be obtained from the HST images of M16, published in November 1995.
This stellar swarm is only about 5.5 million years old with star formation still active in the Eagle Nebula; this results in the presence of very hot young stars of spectral type O6. At its distance of 7,000 light years, its angular diameter of 7 arc minutes corresponds to a linear extension of about 15 light years. The nebula extends much farther out, to a diameter of over 30', corresponding to a linear size of about 70x55 light years.
While De Chéseaux, in 1745-6, only discovered the cluster. Charles Messier, on his independent rediscovery of June 3, 1764, mentions that these stars appeared "enmeshed in a faint glow", probably suggestions of the nebula. The Herschels apparently didn't perceive the nebula, so that their catalogs and consequently, the NGC, only describe the cluster. The nebula was added in the IC II of 1908 as IC 4703, with "cluster M16 involved", but the NGC 2000.0 erroneously classifies this object as an open cluster.
The nebula was probably first photographed by E.E. Barnard in 1895, and by Isaac Roberts in 1897.
Star cluster M16 and the Eagle Nebula are best seen with low powers in telescopes. A 4-inch reveals about 20 stars in an uneven background of fainter stars and nebulosity; three nebulous concentrations can be glimpsed under good observing conditions. Under very good conditions, suggestions of dark obscuring matter can be seen to the north of the cluster. The Eagle nebula is best seen on photographs, but larger apertures and nebula filters (O-III) may help to trace some detail visually. The dark pillars can be seen in large amateur instruments (12-inch up).