|How can a star be formed? From the big bang the
collapse of gas clouds under gravity is supposed
to generate the heat required for nuclear fusion.
However, a gas cloud would be too hot and the
outward pressure would prevent the collapse. I
understand that molecules in the cloud collide
and radiate enough heat away, but according to
the theory, the bang is made mainly of hydrogen,
with a little helium with other elements formed
inside stars. Helium can't form molecules at
all, so the only molecules formed would be
molecular hydrogen. Even this is easily
destroyed by ultraviolet light and needs dust
grains to form.
You and I know dust grains require heavier
elements. The only coolant left is atomic
hydrogen, and this would leave gas clouds over a
hundred times too hot to collapse.
|Question Date: 2003-05-23|
You are right to be a little worried about
whether a cloud of gas gets too hot to form a
star. However, astronomers have already worried
about it and have determined that there is no
problem. Let me explain.
To start off, the "big bang" theory is
the theory of the moments after the beginning of
the universe. It contains in it a theory of
how matter was first created and you are correct
that our current understanding is that most of the
matter created after the big bang was in the form
of hydrogen. Of course, you don't need to know
this to understand how a star forms.
form all the time, and have been observed in
various stages of their formation by telescopes so
astronomers are able to check their ideas. If they
couldn't, it wouldn't be science!
begins with a large cloud of molecular
hydrogen - hydrogen is always molecular if the
cloud is cool enough. The cloud begins to collapse
because the density of the cloud is not exactly
the same everywhere - some places have higher
density and higher gravity. Once a cloud begins to
collapse, it has more mass concentrated in a
smaller area which increases the gravitational
force that it feels and causes it to collapse
you are also correct that as the gas collapses, it
gets hotter and because of this, there will be an
outward pressure that pushes against the
gravitational collapse. The hotter the gas, the
stronger the pressure. This is good - without it
all the hydrogen would never stop collapsing and
never form stars. The denser the cloud is when it
starts, the stronger the gravity because there is
more mass, and the hotter the cloud is, the
stronger the pressure. A star is formed if a
collapsing cloud gets so hot at its center that it
starts nuclear fusion. So in fact, you *want* the
cloud to get hot to form a star. The catch is, you
want the gravity to be stronger than the outward
pressure long enough for fusion to start. There is
a condition called the "Jeans condition" that
astronomers can use to determine whether a cloud
of gas is dense enough and cool enough before it
begins collapsing, to form a star. Certainly not
all clouds of hydrogen can form stars. But
calculations show that some can. And, telescopes
show us that some do. Of course, there is still a
lot to understand about star formation, especially
right after the big bang.
The last thing I want
to say is that things cool off by radiation of
light, not just through matter, so you don't need
other matter around. By light, I don't mean
just the light that we see but also infrared
light, ultraviolet light, x-rays, gamma rays,
radio waves, and so on. Also, as the universe
expands the temperature of the hydrogen gas will
decrease though I'm not sure how to explain this
in a way that is easy to understand (or
short). I hope that answers your question.
Please feel free to let me know if I just confused
you or if you have other questions about the big
bang or star formation.
The distribution of energy and matter in the early
universe was not uniform. Initial irregularities
in the early universe were intensified by their
gravity, spinning matter into an immense web or
network, drawn into what are now clusters and
sheets of galaxies that encircle gigantic
voids. With enough mass to cause gravitational
collapse of the primordial elements, the clouds of
the primordial plasma aggregated together into
globules in which the pressure at the centers of
which were conducive to nuclear fusion: the first
The first stars were massive, still running
from the abundance of matter in the early
universe, massive enough that after a short life
span of but a few million years, the stars went
supernova, spreading the heavier elements
the cosmos. These heavier elements such as carbon,
oxygen, silicon, and iron, make up the bulk of the
In short, if there is
enough mass or mass-density, gravity will overcome
all of the other forces combined. In the most
extreme examples this happens, creating a body
from which nothing, not even light, can escape.
Needless to say, this does not describe any star
that we can see, but you get the point.
Most of interstellar space is quite empty with
only a FEW atoms of Hydrogen or Helium per cubic cm.
(cm3). But there are regions called
GIANT MOLECULAR CLOUDS or GMC , where the
of particles is a little bit higher...on the order
of a few 100 or 1000 atoms per cm3.
Now, what determines if these so called
"dense" MC's will collapse? Well, there is a
theorem called the VIRIAL theorem that
conditions whereby a large tenuous MC will
DISPERSE or CONTRACT. If it disperses, then the
game is over and that's that. If the cloud
contracts, it will shrink in volume, but increase
in density and undergo FRAGMENTATION, so that
eventually after some tens of Millions of years
(roughly) a bunch of separate STARS will form.
The basic balance is between the
temperature of the cloud (high T(temperature)
makes the cloud
DISPERSE) and the amount of MASS contained in the
There is an equation which lets you
calculate the Mass needed for collapse if you know
the SIZE and the Temperature and a few other
things that we can determine from telescopic
observation on Earth.
There is a region in
ORION, a constellation in the night sky, where new
stars are being born right now as we speak.
Wow, this is a hard question to answer, hard
enough that I think no one can answer it
completely yet. By that I mean that I don't think
the way stars form is totally understood. Also,
if you start at the Big Bang, everything is spread
out very evenly, so you have to have some way to
form clumps of material, which can form stars and
galaxies (collections of stars and other matter).
It's only recently (the last few years) that
scientists have been starting to figure out how
that can happen. But I think I can give you
part of an answer.
No matter the
temperature, large amounts of matter always like to
collapse due to gravity. This is known as a "Jeans
instability" after the scientist who discovered
Another part of the answer is that clouds
of matter, even hydrogen atoms, can cool down some.
For example, some atoms will eventually collide
with each other, and eventually one will go into an
excited state. The excited atom can then radiate
a photon away, which releases some energy and
cools the whole system down. Unfortunately, I
don't know a whole lot about the process of real
life gravitational collapse in astrophysics,
especially when the universe was young (I do know
that there can be very complicated situations),
but I hope that I can encourage you to keep
studying, so you can learn more about the
formation of stars -- and the answers to any other
questions you have.
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