Cocoons for Young Stars
The initial collapse phase of star formation
before fusion reactions are initiated is termed the
protostar phase.
When collapsing protostars
form inside nebulae, they are surrounded
initially by the gas and dust cloud from which they
condensed. These "cocoons" of gas and dust
are opaque to visible light, but we can see evidence for the
protostars hiding inside
in IR light.
The Eagle Nebula
The visible-light
image adjacent right shows starbirth regions in the Eagle Nebula (M16),
which is about 7000 light years away in the
constellation
Serpens.
The yellow rectangle in the inset indicates the region of the nebula blown up in the main part of
the image.
The clouds are
columns of cool molecular hydrogen and dust that that serve as
incubators for new stars. The tallest is almost a light year in length!
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Erosion in Astronomy and Geology
The process producing EGGs by "erosion" with UV photons
is similar to the water and wind erosion that produces
many striking landscapes in the Western United States. For example,
a feature like Ship Rock in New Mexico represents the throat of an old
volcano that contained much harder rock than the surrounding region. Thus,
when the surrounding softer rock has been eroded away, Ship Rock remains.
EGGs are similar; they remain after the surrounding "soft" gas has been
eroded away.
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The Eagle's EGGs
Nearby hot stars
(approximately 100 young stars can be counted in
this part of the nebula) are emitting a flood of UV photons.
The strong flux of
photons is heating the gas of the nebula and causing it to be driven off in a process
called photoevaporation.
This photoevaporation
uncovers small globules of dense gas buried within the cloud that are less susceptible to evaporation.
These are termed EGGs ("Evaporating Gaseous Globules"). They are evaporating too, but much more
slowly than the surrounding lower-density gas (see the adjacent box).
The finger-like structures at the tops of the clouds are larger than the Solar System
and hide embryonic stars that are forming within them. They are produced
by the shadows of the EGGs, which protect the gas behind them from the intense UV flux.
Starbirth in the Trifid
The image adjacent left shows a region of star birth in the Trifid Nebula, which is approximately
9000 light years away in the constellation Sagittarius.
There is a hot, young star above and to the right
of this image. When it formed, its radiation and stellar wind swept away much of the gas in this region
and only low-mass stars are presently forming here.
The pillars visible in the image were more dense than their surroundings and thus have survived, though
they too are being eroded by the radiation and wind from hot stars. The unusual jet of material that can
be seen in the upper left is not well understood, though its source presumably lies hidden in the large
left pillar. The red dots are newly formed low-mass stars.
