M 16 - Associated with the Eagle Nebula
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M 16 - Associated with the Eagle Nebula

February 18, 2005
Open Cluster M16 (NGC 6611), type 'e', in Serpens

Right Ascension: 18 : 18.8 (h:m)
Declination: -13 : 47 (deg:m)
Distance: 7.0 (kly)
Visual Brightness: 6.4 (mag)
Apparent Dimension: 7.0 (arc min)

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 our photograph, as well as in AAT pictures and other images of M16. A deeper insight in the star formation process could be obtained from the HST images of M16, published in November 1995; moreover, they were used for an animation simulating the approach to this star forming region, and we provide some screen sized images (suitable as backgrounds for your computer screen).

This stellar swarm is only about 5.5 million years old (according to the Sky Catalog 2000 and Götz) with star formation still active in the Eagle Nebula; this results in the presence of very hot young stars of spectral type O6. The cluster was classified as of Trumpler type II,3,m,n (Götz). The brightest star of M16 is of visual magnitude 8.24. 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.

Some sources have smaller distances for M16: Kenneth Glyn Jones gives 5,870. Götz 5,540 light years. Götz states that this is one of the intrinsically most luminous open clusters, at an absolute magnitude of -8.21.

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.

M16 is found rather easily, either by locating the star Gamma Scuti, a white giant star of magnitude 4.70 and spectral type A2 III, e.g. from Altair (Alpha Aquilae) via Delta and Lambda Aql; M16 is about 2 1/2 deg (19 min in RA) west of this star. Or, in particular with a pair of binoculars, locate star cloud M24, and move northward via a pair of stars of 6th and 7th mag, followed by small open cluster M18 1deg North of M24, the magnificient Omega Nebula M17 another 1deg N, and finally another 2deg N, M16.

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).

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