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Back to Frequently Asked Astronomy and Physics Questions
What does nuclear fission have to do with the birth of stars?
Fission has nothing to do with it. Fusion does, however. Fission is the
process by which large nuclei (like Uranium) split apart releasing energy.
Fusion is the joining of nuclei to make larger nuclei, with the release of
energy. A ball of contracting interstellar gas becomes a star like the
Sun when fusion reactions start in its center. Then, we say "a star is
born."
What gases are needed to produce the new star? Why do you need these
gases? What are their functions?
No specific gases are actually needed. It's more like this: the
interstellar gas is predominately hydrogen gas. It is hydrogen that is
fused at the center of the gas sphere when the gas sphere becomes a star.
However, if the cloud were made of carbon gas, for instance, the sphere
could eventually fuse carbon nuclei together (to make larger nuclei), as
it first "turned on."
Just about any gas will do. The different sorts of gas spheres
would produced different sorts of stars (brighter or dimmer, larger or
smaller,...).
Where in the galaxy are the most stars born?
Stars appear to be born together in large "clusters" (groups) in clouds of
large enough density (so gravity can cause the stars to "condense" in
the gas). The clouds in which the conditions are right
are clouds made mostly of
molecular hydrogen, the so-called "molecular clouds."
The clusters of young stars are found in or near these molecular
clouds.
How hot must a star get before it becomes a star? Why does it need to
get so hot?
If you mean, "at what temperature does fusion start at the center?", then
the answer is about 10 million degrees Kelvin (10 million degrees above
absolute zero). In terms of Fahrenheit temperature that's about 20 million
degrees Fahrenheit, in round numbers.
In answer to why it must get so hot, consider the following.
In order for hydrogen nuclei (one hydrogen nucleus is a proton) to fuse,
they must come into contact with each other. However, a proton is a
positively electrically charged particle. Like electrically charged
particles repel each other with a strong electric force. Therefore, two
separate protons will repel each other strongly. The only way they can
come into contact is if they run at each other at very high speed (in order
to overcome the repulsive force). It's as if you were wearing a large
balloon around wrapped all around your waist, and you wished to touch a
wall, but could not reach it since the balloon was preventing you from
getting close enough to the wall. One way you could "overpower" the
balloon would be to step back, and run at the wall --- the balloon would
slow you down quite a bit, but you would get much closer to the wall, even
perhaps close enough to touch it. So, in the star, the protons need to run
around at high speeds to be able to touch during collisions. When the
particles in a gas have high speeds, we say the "temperature of the gas is
high." Actually, temperature is just a measure of the speeds of motion of
the particles in the gas.
How long does it take to produce a star?
Why does it take that long?
To make a star like the Sun, the process takes some tens of millions of
years. As to why,
that's actually a very complex question. When the gas sphere that will
become a Sun first condenses out of the interstellar gas cloud, the sphere
is quite large in radius (compared to the Sun). It must shrink in size,
squashing itself under its own gravity until, because of the pressure of
the squashing, the central temperature reaches 10 million degrees Kelvin
and fusion starts (it becomes a Sun). The shrinking is only possible since
the sphere is losing energy from its surface (light is escaping), and so the
material is settling down under gravity. Once the central temperature
rises to 10 million Kelvin, fusion starts, energy is released by the
fusion, replacing
the lost energy from the surface, and the sphere stops contracting --- it
is now a star like the Sun. The process of losing energy, and shrinking,
while the central temperature rises as the pressure builds, all takes
10 million years or more. Why so long? Well, it' a combination of two
things. First, such a large amount of gas (as is contained in the star)
must lose a large amount of energy (by light lost from the surface) before it
can settle down to the radius of the Sun. Second, it is losing that energy
at a specific rate dictated by the way energy is transported up the surface
from the interior. Both of these facts determine how long the entire
shrinking process will take place. As I said, it is complex, but one can
perhaps think of it as taking a long time because there is so much work
(energy loss) that a gas sphere must do to shrink to the size of the Sun.
