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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
Answer 1:

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.

>Stars 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!

Star formation 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 faster.

Now, 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.

Answer 2:

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 stars.

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 through the cosmos. These heavier elements such as carbon, oxygen, silicon, and iron, make up the bulk of the universe's dust.

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.

Answer 3:

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 density 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 gives the 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 cloud.

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.

Answer 4:

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 it.

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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