Similar to earthly sandstone buttes that have survived the erosion of their surrounding landscape, the celestial gas pillars of the eagle nebula have remained intact and relatively protected from the evaporating power of ultraviolet radiation produced by massive neighboring stars. The pillars show the effects of slow erosion by a process known as "photoevaporation" in which less dense gas is slowly evaporated from the surface of the pillars exposing denser material deep within the globule-like structures. The globules themselves are known as EGG's or "Evaporating Gaseous Globules". The flood of ultraviolet radiation from the young star cluster NGC 6611 illuminates and at the same time boils away the surface of the globules producing the fascinating three dimensional shapes we see at the center of M16. Protostars have been forming for thousands of years within the EGG's however their growth has become halted as the gaseous cocoons that feed their growth are lifted away. Early protostars are not often observed directly, being cloaked by opaque gas and dust until they emerge as main sequence stars. M16 presents a rare opportunity for astronomers to observe these infant stars as they emerge prematurely from the evaporating pillars of M16. In fact the infant stars emerging from the evaporating globules may be the earliest stages of "class 0" protostars ever observed.
The columns of M16 are illuminated by the stars of the young massive cluster NGC 6611 located some 6.5 light years from the pillars. The cluster which is about 6 million years old contains about a dozen hot O-type stars which generate an intense radiation field, greater than 7 times the ionizing force of Orion's trapezium cluster. The stars of NGC 6611 are part of the Ser OB1 stellar association and although the oldest stars are about 6 million years old, most of the cluster was born during a recent star burst period beginning some 2 million years ago. The cluster population includes one of the largest known populations of pre-main-sequence stars indicating that star formation continues within M16 to the present time. The visible nebula complex is but a small component of an immense molecular cloud known as W37 located within the Sagittarius arm of the Milky Way.
M16 is known as a photodissociation Region
(PDR). Within PDR's there exists a basic stratification structure
depending on the distance of the gas from the ionizing stars.
Close to the ionizing stars the hydrogen gas becomes highly ionized
and is maintained at an equilibrium temperature of about 10,000
degrees Kelvin. At the edge of this region where the ionizing
radiation field loses some of its intensity, a thin ionization
front exists which separates the ionized gas (HII) from atomic
gas (H1) just beyond it. Beyond the ionization front, the gas
in the photodissociation region is kept atomic (H1) by photons
which do not have sufficient energy to ionize the gas but are
energetic enough to dissociate molecular hydrogen (H2). Beyond
the PDR the gas remains molecular (H2).
This simple stratification is made more complex when situations like M16 exist where pillars of molecular material intrude into the ionized nebula.
The origin of the photoionized pillars or
"elephant trunks" of M16 is a subject of controversy
among astronomers. These "intrusions" of molecular gas
into the ionized nebula are indeed important sites of star formation
and have given rise to several competing theories about their
origin. One theory is that the pillars are relatively young structures
representing preexisting cores of molecular gas which have become
unstable and began to collapse less than 100,000 years ago. More
recent work has suggested that the columns may be fairly stable
structures which can exist in a steady state for time periods
greater than 500,000 years. The newer finds also suggest that
rather than being present in the original molecular cloud the
pillars may have had their origin by way of instabilities and
perturbations that developed within the HII region.