Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Lewis Thomson on 12/01/2022 10:37:29
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Donald has presented us with this fascinating question.
"The Earth's moon has a very rarified atmosphere about 4-6 molecules per cubic meter. This is similar to the density of cosmic gas clouds that can form new stars. But, those gas molecules on the moon are calculated to almost never collide with each other. Undeniably, increasing the temperature of a gas cloud by applying external energy, would increase the volume of an unconstrained ideal gas such as a cosmic gas cloud. So, I am not convinced of the current theory of star formation from some shockwave causing cosmic gas cloud collapse. Certainly there must be more than the available gravity of those individual molecules of gas to collapse into a new star, because those clouds still exist and all have not collapsed from their inherent gravity. That is, the inherent gravity in that gas cloud is NOT sufficient to form a new star. Obviously, something else must be operating, without increasing the temperature of the cosmic gas cloud. Are there competing hypothesis to stellar formation?"
Leave your answers in the comments below...
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increasing the temperature of a gas cloud
Temperature is one critical parameter. Star-forming dust clouds need a temperature of around 10 degrees above absolute zero (bearing in mind that the temperature of black space is currently around 2.7 degrees above absolute zero). If the gas is too hot, it will resist gravitational collapse.
Density is another critical parameter. If the density is too low, it won't have enough gravitational attraction to overcome thermal motion.
Diversity of density is also important: If there are more some more concentrated swirls and eddies, these will have more gravitational attraction than nearby areas, and will pull more matter to themselves, increasing their density even more - a positive feedback that eventually results in stars. The heat of the earliest stars heats and disperses the remaining gas, freezing star formation with a few very massive stars, and many low-mass stars (eg red dwarf or smaller).
https://en.wikipedia.org/wiki/Star_formation#Interstellar_clouds
There is an open question about how the first stars, galaxies and black holes formed. The temperature at that time was much higher, but the density was also much higher. Hopefully, the James Webb Space Telescope will shine some (infra-red) light on this subject...
James Webb should also allow us to peer more deeply into star-forming dust clouds today - and into the dusty region around our galaxy's central black hole.
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Hi everyone.
Let's just make it clear that I am not an expert in stellar formation.
The Earth's moon has a very rarified atmosphere about 4-6 molecules per cubic meter.
I'm not certain about that.
One of the lowest estimates is as follows:
...In the moon's atmosphere, there are only 100 molecules per cubic centimeter... - from Space.com
Wikipedia puts the estimate upto 1 000 000 (or 106) molecules per cubic centimetre.
This is similar to the density of cosmic gas clouds that can form new stars.
Typical densities of gas clouds that can form stars are thought to be 105 particles per cubic centimetre.
Undeniably, increasing the temperature of a gas cloud by applying external energy, would increase the volume of an unconstrained ideal gas such as a cosmic gas cloud.
Agreed. In fact you already have a good answer here for something you ask later. If you add energy to a gas cloud, then you can oppose a collapse. In terms of an ideal gas we have PV = nRT, Pressure and Volume of the gas are related to the numer density of particles and the average kinetic energy of those particles. If you feed energy into a gas cloud then the pressure and/or volume increases, or to say this another way, the cloud seeks to acquire a larger volume and not collapse to a smaller volume..
Let's skip to the section where you raise this issue:
Certainly there must be more than the available gravity of those individual molecules of gas to collapse into a new star, because those clouds still exist and all have not collapsed from their inherent gravity.
Some clouds of gas still exist in galaxies and haven't collapsed because they are being fed with energy. The collapse of gas clouds and the formation of stars can be inhibited by environmental factors. One that I've heard a little bit about is the presence of super massive black holes at the centre of galaxies. These can emit powerful jets of energetic particles which agitate the gas clouds, in effect energy is constantly being fed into the gas cloud.
I believe there are other environmental factors which inhibit the formation of new stars - I don't know much about them.
As far as I understand it, under a simple model of star formation, a gas cloud in open space well away from a galaxy should just collapse under its own gravity when the mass becomes too large. The typical mass for such a collapse is usually called the Jeans Mass.
So, I am not convinced of the current theory of star formation from some shockwave causing cosmic gas cloud collapse.
The simplest models of star formation don't require any shockwave. The collapse of a gas cloud under its own gravity follows from Newtoninan mechanics. I seem to recall reading something that used the Virial theorem as one of the fastest ways to establish this result. However, a lecture from SETI I've just seen seems to have covered the same results more intuitively and it took only the first 10 minutes (which is a bit embarrassing because I'm certain it took me a lot more than 10 minutes when I first studied star formation with the Virial theorem).
Anyway, this concerns a simple model of star formation and the Jeans mass does appear naturally as a result. There are other causes of star formation and yes, sending shockwaves through a gas cloud can cause a core to form and potentially catalyse star formation.
Are there competing hypothesis to stellar formation?
It would be amazing if there wasn't. I'm certain there are numerous simulations, models, theories and ideas. Stellar evolution is still an active area of research.
Best Wishes.