![]() |
||||||||||||||
Australia: The Land Where Time Began |
||||||||||||||
Dark Nebulae Dark Nebulae, aka dense clouds, differ in nature
from all others in one major respect, they don’t shine. This type of
cloud is observed by their ability to block light that is emitted behind
them when viewed from Earth. They are clouds of vast dimensions that are
composed of molecules of gas such as H2, HCN, OH, CO and CS,
as well as dust grains. These grains don’t resemble dust on Earth, being
of microscopic size, in the region of 20-30 nm. Ice, however, either
water ice of ammonia ice, may condense on them to form a “mantle,” which
then increases their size to 300 nm. The dust grains are in the shape of
long spindles, some of which rotate. There is much debate concerning the
actual composition of these dust grains, though it is believed they are
composed of various unknown amounts of carbon in the form of graphite,
as well as silicon carbides and magnesium silicates and aluminium.
It is believed the dust grains form in the outer
regions of stars – in particular the cool supergiants and the R Corona
Borealis-type stars. Therefore it is believed the dense molecular clouds
are another possible site of star formation. The grains are believed to
have a temperature of 10-100 K, which is cool enough to allow molecules
to form. There may be anywhere from 104 to 109
particles consisting of atoms, various molecules and dust grains in a
typical dark nebula. The nebulae appear dark due to their vast size and
so are very effective at scattering all of the light, the result of
which is that hardly any reaches the naked eye. The light scattering
process is so effective, for instance, that the visible light emitted
from the centre of the Milky Way galaxy, the home galaxy of Earth, is
almost 100 % extinguished by the dust clouds between the centre of the
galaxy and the Earth. This is the reason the central region of our
galaxy in visible light is still unknown.
Extinction is the term used
for the scattering and absorption of light. The dark nebulae are not dense dust clouds in which
the grains are very close together. Most of the cloud material is
molecular hydrogen, as well as carbon monoxide, which is responsible for
their radio emission, and the density is therefore very low. According
to Inglis there is also some evidence that suggests the properties of
the dust in the clouds are different from those of the dust in the
interstellar medium. Many dark nebulae have been shown to be interacting
with their environs, as can be seen in the spectacular images of M16 in
Serpens taken with the Hubble Space Telescope. Dust clouds are shown by
the images to contain dense regions, or globules, resisting the
radiation pressure from close, young stars, with the result that many of
the globules are trailing long trails of material. The area near the
Horsehead Nebula in Orion is famous for its image that shows the
radiation from the supergiant stars of Orion’s Belt impacting on the
dark clouds on either side of the Horsehead, resulting in the ionisation
of that material that is seen streaming from the surface of the cloud. Most of the dark clouds listed in this book have
shapes that are vastly different, which is due to several reasons:
·
The original shape of the cloud may have
been spherical, therefore presenting a circular shape in an image,
though this will have been disrupted by radiation pressure and stellar
winds.
·
Shock fronts from nearby supernovae can
also affect them.
·
These effects can be caused by
gravitation from other clouds, stars or the Milky Way.
·
Some effects are thought to result from
magnetic fields.
·
Many dark clouds are part of a much
larger region of star formation; therefore the new stars will also
influence and alter the shape of the cloud. The “opacity” of a dark nebula is a measure of how
opaque the cloud is to light and therefore of how dark it will appear.
The rough classification system of dark nebulae; 1 indicates the cloud
attenuates light from the background Milky Way very slightly; a value of
6 indicates the cloud is nearly black and it is allocated a special
symbol.
|
|
|||||||||||||
|
||||||||||||||
Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |