Quoted from SEDS:
Cluster M16 (NGC 6611) discovered by Philippe Loys de Chéseaux
in 1745-6.
Nebula M16 (IC 4703) discovered by Charles Messier in 1764.
The Eagle Nebula Messier 16 (M16) is a conspicuous region of active
star formation, situated in Serpens Cauda. The star forming nebula, a
giant cloud of interstellar gas and dust, has already created a
considerable cluster of young stars. The cluster is also referred to as
NGC 6611, the nebula as IC 4703.
The discoverer,
Philippe
Loys de Chéseaux, describes only the cluster when recording his
1745-1746 discovery.
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; Isaac Robert's finding brought this object into the
IC catalog.
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.
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 magnificent 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). |
