My impression of how stars form is that hydogen accumulates until a critical mass is reached and the thing sorta implodes due to the effects of gravity and energy is released.
My question is this:
The line is reached and the rection starts - grand.
Obviously the reaction must sustain itself afterwards, even if the mass drops below the amount needed to start the reaction initially.
My question is ‘How do the stars grow beyond that initial size?’
Surely the heat from the star should start burning off the hydrogen, and the solar winds push it away, so that the remaining hydrogen does not actually reach the star surface of the star?
Or does gravity overcome this?
What you start with is a cloud primarily made of hydrogen. Now this cloud is likely rotating and has a region with a higher mass of gas/dust and so a nucleus forms. This starting pulling in more and more gas. Now not much happens to the gas, though it does start to heat and convection waves form. Slowly the proto-star becomes large enough that the hydrogen at its core begins to fuse together releasing energy.
BUT the star is still massive and still pulling in hydrogen gas. The hydrogen doesn’t burn (there’s no oxygen) so it “joins” the star further increasing the star’s mass. But more mass means a larger fusion region and so the star becomes hotter and expands but continues to grow and absorb even more gas.
Eventually the star exhausts the gas it can get a hold of and settles into hydrostatic equilibrium, balanced between gravity and thermal expansion.
I think one of the problems is that people think of the star’s “ignition” as being some kind of instantaneous on/off event. It is estimated that photons in our sun take hundreds of thousands of years to reach the surface (though estimates vary widely). Thus, the process Grey describes could continue for maybe a million years before the star’s fusion energy reaches the surface in enough volume to throw off the excess gas.
Also consider the force of gravity of a star half the mass of Sol at 10 AU (roughly where Saturn is). F[sub]g[/sub]=GM[sub]star[/sub]M/r[sup]2[/sup], so a proton experiences GM[sub]star[/sub]/r[sup]2[/sup] acceleration towards the star of about 12x10[sup]-6[/sup] m/s[sup]2[/sup] which is pretty big really and likely big enough to overcome a stellar wind. Though I’m not sure completely how to check that.
Also if a star loses mass, its fusion reaction rate drops since there is less compression due to reduced mass. Actually I remember seeing proposals for extending the sun’s life span by judicious “siphoning” off of mass to give earth a few more billions of years.
We did a similar thread about a year ago. The main problem I see in your post is here:
It’s not a critical mass, it’s a critical density. As the gas in the protostellar cloud collapses under its own gravity, it gets denser and denser, and thereby gets hotter and hotter; this increase in temperature is what causes thermonuclear fusion to “turn on”. As noted above, it’s a gradual process — but more importantly, a cloud that is physically larger when the critical density is reached will result in a more massive star.