M16-2


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.

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. 

Minimum credit line: Bill Schoening/AURA/NOAO/NSF (for details see
Copyright Statement)