Star Formation

[Rodstar]

Just wondering if anyone can give me a simple answer to the following question:

I have been reading, as novice, about how stars are believed to form when:
1. Gravity causes hydrogen gas to clump together, then
2. Over time the process "snowballs", until
3. Critical mass is reached, where the building heat and pressure within the growing ball of gas spontaneously initiates nuclear fushion, and
4. A star is born.
I also understand that the temperature of a star relates to its size when this first moment of spontaneous combustion occurs.

Does anyone know why all stars aren't exactly the same if this theory is correct, ie, wouldn't the critical moment of combustion occur as the same mass is reached for all stars if the constituent is always hydrogen? It is not as if there would be a significant influence from the surrounding material / conditions as we are talking about the expanses of the universe here!!

Looking forward to your thoughts....

Rodstar

[gaa_ian]

With my limited understanding ...you are correct on the theroy of formation.
The size of the star will depend on the amount of hydrogen (& other elements) present in the molecular cloud the star forms from.
Often stars are formed when the shockwave of a super nova compresses the gas clouds.
There are many ways a star can form and die , I think we will all spend a lifetime learning about this :)

[silvinator]

Hi Rodstar, welcome to the forum. I'll try my hand at answering your question. Here goes:

1. Gravity does cause things to clump together but it may not necesarily be hydrogen gas. Since the universe has evolved, exploding stars in the form of supernovas have enriched the universe with higher elements. What you have to note here is that all these elements make up what is known as the interstellar medium - the gas and dust between stars, but yes, most of it is hydrogen. Still this is one small reason why stars do not follow the same evolutionary track as not all of them form out of the same starting material.
2. I'm not sure what you mean by the term "snowballs" so if you could elaborate on that, it would help :) The next step here in star formation is a triggering of the collapse of this interstellar medium. Astronomers believe several processes may trigger the contraction of a cloud of gas. These include, but are not limited to, shockwaves from supernovaes or colliding gas clouds, or the spiral arms of a galaxy itself.
3. Here you mention a "critical mass" that a star must reach. In fact, the starting material can have any mass. Once this cloud of gas and dust is triggered into a gravitational collapse, it will keep on collapsing until the internal pressure and temperature are increased and reach a critical point where the collapsing star can no longer keep itself stable. This is known as the protostar stage. The more massive this protostar is, the faster it contracts. Once the protostar reaches 10,000,000K, that's when the core of the protostar is hot enough to start hydrogen fusion and that is how it stabalizes its contraction.
4. A star is born :)
Now the main reason why all the stars aren't the same has to do with the mass of the starting material. The mass of the protostar has a lot to do with how long it will continue to burn hydrogen as fuel. The more massive the star is, the faster it will consume its hydrogen and start burning other materials. So it is incorrect to say that all stars reach the same critical mass. It is the temperature of the star that is the most important trigger of star formation.
I hope this has helped :)

[gaa_ian]

Silvinator ... very true & correct
I think you have just nominated yourself as our science writer
:atom: :D

[rumples riot]

Rather well put Silvie, and I concur that temp of the core is the critical factor. That way large amounts of mass can be built up very quickly prior to the fusion point. Once fusion commences all remaining gas is expelled and this can if the conditions are right cause another contraction of stellar clouds to form another star. Which is yet another way of creating a star. The only thing that I would question is the critical temp which I think is in fact 1 million degrees K and not 10 million degrees K. I think that figure is too high. Still I am sure that this has answered Rodstar"s question.

[Rodstar]

To the Silvinator and other contributors, my thanks.....there is so much to learn, and the more I learn, the more aware I am of how little I actually know!

It sounds like you don't want to be around a protostar when it hits 1millionK/ 10millionK!

Signing off,

Rodstar

[silvinator]

Sorry rumples but I have to put my foot down and say that I am certain the temperature for hydrogen fusion is actually 10 million kelvin :) This figure has been hammered into my brain since I persuaded my yr12 class to study astronomy as an elective (hehe, they all thanked me for it later as they actually enjoyed it and found it easier to learn than the other electives).
I'm just glad to help out rodstar. One other thing to mention is that the protostar stage is shrouded with a lot of gas and dust so astronomers don't have actual visual proof of the processes going on within the contracting cloud. But you're right, no matter what is going on in there, you'd best stay away! :)

[[1ponders]]

Onya silvie, spot on

Silvinators explaination also helps to explain why stars in one particular cluster tend to be of very similar type (OB) yet stars that form in a nearby cluster form as another sort (AF). The difference in distribution of interstellar "impurities", density of the surrounding medium, "initial" gravitational collapse initiation type and competition for gas and dust from surrounding developing stars all play an important part in the process of determining what sort of star will eventually form

Its a fun and interesting topic thats for sure.