Molecular Clouds
The galaxy is full of great clouds of hydrogen. The adjacent image shows a map of neutral
atomic hydrogen (HI) plotted in
galactic coordinates, as inferred from the 21 cm radio frequency line (see Chapter 23).
Molecular Hydrogen
If these clouds become very cold, the hydrogen forms molecular hydrogen (two atoms of hydrogen joined
together in a molecule).
Cold molecular hydrogen is very difficult to detect because it does not emit or absorb characteristic
wavelengths that can be used to easily identify it. But such cold clouds of
molecular hydrogen are the birthplace of stars.
These clouds often contain dust, which makes them dark (see the right panel).
Dusty Nebulae
The adjacent image shows one of the most famous images in astronomy, the dark nebula called the
Horsehead Nebula. It is about 1500 light years away in the constellation Orion,
and is part of a larger molecular cloud called Barnard 33.
The darkness of the nebula is primarily
because
of dust in it that obscures the light from behind it. The red glow is caused by hydrogen in the region
behind the Horsehead that has been ionized
by the nearby hot star sigma Orionis.
This
animation illustrates the effect of dust on light passing through it.
