The interstellar medium is the matter that exists between stars in galaxies, consisting of gas and microscopic dust particles. About 1% of a galaxy's mass is made up of interstellar medium. In the Milky Way, the density of the interstellar medium is about 1 atom per cubic centimeter, much less dense than Earth's atmosphere. Molecular clouds within the interstellar medium can be enormous, many light years across and containing thousands of times the mass of the Sun. The interstellar medium plays an important role in the formation of stars, planets and other astronomical objects.

1. GALACTIC DUST
ANDCLOUDS
BRYAN NOZALEDA

2. What is the
interstellar
medium?
is the matter that exists within
galaxies, between and
among—but not including—
the stars. Almost all of the
interstellar medium is
comprised of gas and
microscopic dust particles.

3. How much
interstellar
medium is
there in
galaxies?
About one percent of the
luminous mass of a galaxy like the
Milky Way (that is, excluding the
non-baryonic dark matter) is
interstellar medium.

4. How denseis
the
interstellar
medium?
The interstellar medium in our region of
the Milky Way galaxy has a density of
about one atom of gas per cubic
centimeter. By contrast, Earth’s
atmosphere at sea level contains about
1019 gas molecules per cubic centimeter.
There is also about one dust particle per
10,000,000 cubic meters in the local
interstellar medium.

5. What does the
interstellar
medium
look like?
It can appear in an amazing variety
of forms and colors. Much of the
interstellar medium is invisible; in fact, it
will block the view of distant
astronomical objects.

6. What is a
molecular
cloud?
A molecular cloud is a cloud that
contains molecules—constructs
of multiple atoms.

7. Are molecules
in the
interstellar
medium only
found in
molecular
clouds?
No. They exist in the interstellar
environments surrounding stars,
too. Gas molecules in space,
however, are much more fragile than
atomic gas.

8. If the
interstellar
medium is
so thin,
how can we
see
nebulae at
all?
Even though interstellar gas clouds are
incredibly thin by terrestrial standards,
they make up in size what they lack in
density. Interstellar nebulae can be
many light-years wide, so the total
amount of gas we see from a distance
can far exceed even the thickest cloud
in Earth’s atmosphere, making them
quite visible.

9. How big are
molecular
clouds?
Molecular clouds can be enormous
compared to stars. The largest ones are
called “giant molecular clouds” and can
be many light-years millions of
times as much mass as the Sun
across. Giant molecular clouds can
contain thousands or even; they may also
contain a number of dense core regions,
each with 100 to 1,000 Suns’ worth of
gas.

10. Where can the
interstellar
medium be
found in
galaxies?
In most galaxies, the
majority of their interstellar
gas and dust collects in the
disks of the galaxies rather
than in bulges or halos.

11. How does the
interstellar
medium affect
astronomical
observations?
The interstellar medium is, of
course, itself a target for
astronomical study. However, it can
also complicate astronomical
observations substantially. This
effect of dust is called extinction,
and it both changes the observed
colors of astronomical objects and
obscures them from view.

12. Is interstellar
dust similar
to
household
dust?
No. Interstellar dust is typically much smaller,
and it is made of very different material,
compared to house dust here on Earth. While
house dust typically is made up of things like
dirt, sand, cloth fibers, crumbs, animal and
plant residue, and even microscopic living
creatures, interstellar dust is composed
primarily of carbon and silicate (silicon, oxygen,
and metallic ions) material, which is sometimes
mixed with frozen water, ammonia, and carbon
dioxide.

13. Why is the
interstellar
medium
important?
To make things like stars,
planets, plants, and people,
enough of the interstellar
medium has to come together
and interact—physically,
chemically, and even
biologically—to create them.