## sun size

Got a beginners' question? No matter how elementary, our friendly forum community and magazine writers will answer it.

### sun size

Hi all,
newbie here so forgive me if this is truly a silly question.
Reading the article about the impossible star I find it hard to come to terms with how a star can become so large,
I thought that when the gas and dust start collapsing under gravity to the point when the nuclear furnace switches on would be
similar throughout the universe ( ie a critical point ) therefore stars would all be similar,which clearly is not the case?.
I would be extremely gratefull if anyone could give me what is probably a very simple answer.

Jim
m00g7h00

Posts: 2
Joined: Mon Feb 13, 2012 7:07 pm

### RE: sun size

Welcome to the forum!

[quote]I thought that when the gas and dust start collapsing under gravity to the point when the nuclear furnace switches on would be
similar throughout the universe ( ie a critical point ) therefore stars would all be similar,which clearly is not the case?. [/quote]
Hmmm, there's at least a graduate level course in astrophysics there.

The point at which nuclear reactions will start depends on the temperature and the density. Lower temperature and higher density will work (up to a point). When a lot of material is infalling inder gravity, its temperature will rise faster ... and nuclear reactions can start at lower densities. The amount of dust (metal in stellar physics terms) affects the rate at which heat can escape, more dust traps radiation & helps keep the pressure low. The angular momentum also has a major influence on the balance between gravitational collapse & radiation pressure. Sorry if this is all a bit "hand waving" but to really understand it you need a system of partial differential equations a couple of pages long.

There is an excellent book "An Introduction to the Theory of Stellar Structure and Evolution" by Dina Pralik (2nd edition, Cambridge University Press, ISBN 978-0-521-86604-0) which covers the ground in fair detail, but even if you have advanced mathematics it still isn't exactly light reading.

brianb

Posts: 5513
Joined: Mon Feb 13, 2012 6:49 pm

### RE: sun size

Wow that was quick and thanks for the explanation, for such a short reply you really said quite a lot i'm impressed.
I can get my head around what you said about heat and pressure enough to finally grasp why there is such a difference
in size without getting blown away with the physics of it all.
Once again thanks
forever gratefull

Jim
m00g7h00

Posts: 2
Joined: Mon Feb 13, 2012 7:07 pm