Composition: 74% H + 25% He + dust (silicates,
carbon and iron compounds).
The gas is generally extremely cold, only 10 degrees above
absolute zero (10 K), and, therefore, dark.
The atoms are usually bound together in molecules and concentrated
in giant molecular clouds, like the Orion molecular cloud complex,
of which the famous Orion
Nebula is a part. Sometimes, like the Orion Nebula and the gas
that envelopes the Pleiades,
these clouds reflect the light of newly created stars.
These clouds are extremely tenuous, having a density of about
200 hydrogen molecules per cubic centimeter. But the clouds are
immense: they have masses between about 100,000 to 2 million solar masses,
and are between 50 to 300 light-years across. The Horsehead
Nebula is an example of a region that appears to comprise mainly dust.
in the gas clouds can lead to gravitational compression
and star formation. Density fluctuations is just a fancy way of saying
that the density varies from place to place.
The basic idea of gravitational compression is this: if a
gas cloud contains density fluctuations, that is, some regions are denser
than others, the denser regions will tend to draw more gas towards them,
because their gravitational pull is stronger than that of the less dense
regions. Over hundreds of thousands to millions of years, this causes the
gas clouds to fragment into smaller and smaller regions. And the denser
regions get progressively denser because of gravitational compression.
But there is a snag: the denser regions can become denser
only if they are dense enough to overcome the opposing tendency of the
gas cloud to disperse. (Even at a temperature of 10K the molecules are
moving at 500 m/s, about 1100 mph.)
One effective way to create sufficiently large density fluctuations
is through shock waves. A shock wave
is any rapidly moving, thin shell, of dense fluid. A familiar example (at
least for those unfortunate enough to live near a military airbase boasting
high performance aircraft) is the highly compressed (cone-shaped) shell
of air created by supersonic jets. As the shell passes your ears the rapid
change in density, and therefore pressure, is sensed as a loud bang: a
In interstellar space, shock waves can be caused by the rapidly
expanding gas shells from the explosion of a nearby massive star. When
the shock wave hits the molecular cloud it compresses the cloud in the
region of contact thereby making that region denser. These denser regions
of gas in the molecular clouds are called dense
cores. It is from these dense cores that stars form. The
remnants of these dead stars are called supernova
Stars tend to form in groups because of the tendency of gravity
to fragment a larger dense region into smaller denser regions that remain
gravitational bound to each other.
The center of the dense cores get more and more dense by
drawing material into the core. This is called accretion.
The core heats up (to about 1500 K) because of the compression, caused
by gravity, and begins to shine in the infrared and at radio wavelengths.
After about 100,000 years of accretion the protostar has
as much mass as the Sun. Eventually, the "wind" of radiation and particles
flowing from the protostar prevents further material from accreting. The
protostar becomes a pre-main sequence star.
Gravitational compression continues until the core is hot
enough (7 million K) to fuse hydrogen into helium. At that temperature
the ionised atoms of hydrogen (that is, protons) are traveling so fast
that they have enough energy to just overcome the Coulomb
repulsion (that tends to keep the protons as far apart as possible)
and thus get close enough to fuse. The repulsion is due to the electric
force between charged particles. Recall that like charges repel, while
unlike charges attract.
If, however, the temperature at the core is too low then
the protons (hydrogen nuclei) will be moving too slowly and will have insufficient
energy to overcome the Coulomb repulsion. The electric force wins out.
Calculations suggest that stars that are less than 0.08
solar masses cannot trigger fusion reactions in their cores
because their cores are too cool. Such stars contract to planetlike objects
called brown dwarfs. Brown dwarfs are
basically giant Jupiters.
Some young stars have very powerful stellar winds and erratic
light outputs (T Tauri Stars). These appear to be protostars that
are clearing away their surrounding gas and dust.
Last updated October 4, 1998 Harrison B. Prosper
ã 1998 Harrison