Medium—What is it?
Most of us, at one
time or another, have learned that space is a perfectly empty vacuum. But
the vast space between the stars is really not empty, it is sparsely sprinkled with
and dust. This ‘stuff’ in the
vast spaces between the stars is known as the Interstellar
Medium (or ISM).
99% of the ISM is gas (hydrogen and helium), the remaining one percent
consist of heavier elements and dust. The gas is extremely dilute, with an average density of about 1 atom per cubic
centimeter. The air we breathe is approximately 30 quintillion (30,000,000,000,000,000,000,) times more dense than the ISM.
Picture this: an "empty" coffee mug in the ISM would contain
about 500 hydrogen molecules. The same "empty" coffee mug sitting on
your desk contains about 1500 quintillion gas molecules - mostly nitrogen,
hydrogen and oxygen.
The dust in the ISM is made of
tiny, irregularly shaped particles of silicates, carbon, ice and
iron. In areas where the dust is thick, the light from nearby
stars can be completely blocked - similar to the way dark clouds block light
from the sun.
The picture at left shows the Horsehead Nebula - the dark area
in the center of the photograph (shaped like a horse's head) is a thick cloud of interstellar dust blocking
the light. Thinner clouds of interstellar dust may dim the light passing
through, without completely blocking it. This is know as extinction.
The interstellar dust scatters blue light more effectively than red light -
which means that most of the light that reaches us through the interstellar dust
is reddish. This is known as interstellar reddening. A similar
process happens here on earth at sunset - which is why sunsets often appear red.
Light from nearby stars
can also be reflected from the interstellar dust, similar to the
way light from a car's headlights can reflect off fog. The
picture (right) shows a reflection
nebula - a cloud of interstellar gas and dust illuminated by and reflected light
from the star it surrounds. The picture was taken by
the Hubble Space Telescope, and is of NGC 1999, a nebula in the
the dust in the interstellar medium, which can only reflect or block light, the
gas in the interstellar medium glows in visible and many other wavelengths. In the region of
hot, newly-formed stars, clouds of hydrogen gas are ionized
by the ultraviolet radiation emitted from
the stars. When free electrons recombine with the ionized hydrogen,
visible red light is emitted from the hydrogen gas. This accounts for the red colors in
photographs of emission
nebulae, such as the Trifid
and Orion Nebulae.
The hydrogen gas in
the ISM can also form relatively cool clouds that emit light in the radio band of the electromagnetic
spectrum. These cool clouds of hydrogen can collapse - it is here,
in these dense, collapsed clouds known as stellar nurseries, that new stars are born.
This image (left) shows a stellar nursery in the spiral galaxy M33,
about 2.7 million light years away in the constellation Triangulum.
(If you are
interested in learning more about the ISM - check out this collection
of links to various sites with more information.)