Why don't stars explode? What holds them together?

SpaceTiger

Nuclear fusion doesn't itself provide a repulsive force, it provides energy to maintain the pressure balance inside the star. The "force" that holds up a star like the sun is the pressure of the gas of which the star is made (which, ultimately, is electromagnetic) and the magnitude of this pressure is dependent on temperature. A balance can be maintained as long as the gas is hot, but if the star is radiating energy away, then it needs an energy source to keep from cooling. In most cases, fusion is this energy source.

SpaceTiger

Let's not confuse diffusion time with a measure of distance. "Light year" refers to the distance that light travels in a year while freely propagating in a vacuum. The distance from the sun's core to its surface is actually around one ten millionth of a light year.


You could look at it that way. The random walk of a photon inside a star is a direct result of its interactions with the gas. The gas is also the ultimate source of the pressure for stars like the sun. However, for heavier stars, the pressure of the light itself can be important in supporting the star against gravity.

scott1

That was just a joke lol

SpaceTiger

Most of the lithium in the universe was produced by the Big Bang and cosmic ray spallation. After the hydrogen in its core is exhausted, a star will, if it's heavy enough, then begin the triple-alpha process, which converts helium into carbon.

Danger

Thanks, Tiger. I knew that there was something wrong with my post. Those sequential thingies always mess me up.

Labguy

Some Lithium could be produced within about three minutes of the big bang, but most is the cosmic ray-to carbon collisions after carbon is formed many years after the big bang from reactions in the cores of stars.
From: UCLA we get that: And from Berkeley we find that formation of Lithium first requires tritium and deuterium: and the neat chart near the top of that page shows one H isotope, two He isotopes and Li.
So, if that's the case and order of formation, when do we consider the "big bang" to be in process? One second, one minute, three minutes (as in the book name) ? This is just a general (and pickey) question that I haven't seen brought up before; how much time after zero is the BB still considered to be in process?.. Anyone answer.

SpaceTiger

How are you distinguishing this from the cosmic ray spallation I referred to? Or are you just expanding on what I said?


I think it's pretty arbitrary. In this case, I suspect that they were trying to distinguish it from stellar nucleosynthesis. Since it was one of the primary predictions of the Big Bang Theory, they just called it "Big Bang Nucleosynthesis".

Astronuc

Well those trillions and trillions and trillions of tons (~1.9891×10³⁰ kg (332 950 Earth masses)) produce a strong gravitational field. Remember, it is strong enough to keep planets in orbit, which are millions of miles away.

The pressure comes from the intense heat/temperature which comes from the fusion reactions.

Fusion reactions produce energy on the order of 1 MeV and at 11605 K/ev, that's about 11 billion K. However, only a fraction of mass is fusing at a given time, so that kinetic energy of the fusion reactants is dissipated quickly to the various atoms and electrons nearby. Temperature of the sun's core is estimated to be ~13.6 MK, and the corona temperature is about 5 MK, while the photosphere 'surface' temperature is about 5800 K.

For information on pp and CNO chain reactions see -
http://csep10.phys.utk.edu/astr162/l...gy/cno-pp.html
http://burro.astr.cwru.edu/Academics...s/ppchain.html

http://www.shef.ac.uk/physics/people...3_fusion3.html

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Re: Li production (is it necessary that most(?) Li was formed around time of BB as opposed to stars later on or novae?)

p + ⁹Be -> ⁴He + ⁶Li

d + ⁴He -> ⁶Li

t + ⁶Li -> ⁷Li + d (or p +n)

There is a paper "Influence of Gravity Waves on the Internal Rotation and Li Abundance of Solar-Type Stars" by Corinne Charbonnel and Suzanne Talon in Science mag, but I can't access it since I am not a member. Anyone read this?

Ann Merchant Boesgaard. (http://www.ifa.hawaii.edu/~boes/) is looking at Li abundance in stars.

Labguy

Just expanding when looking for probable sources for Li found. No distinction from your post.
That makes sense, the distinction allows two seperate, and logical, processes to be discussed. Any stellar nucleosynthesis wouldn't have much to do with the big bang.(Duh!)..

Labguy

See posts # 21, 23 and 24.

SpaceTiger

I think your point is a good one, though. In general, we talk about the "Big Bang" rather loosely, but what does we even mean? No current theory can confidently describe all the way back to t=0 (at which the term "Big Bang" would be completely unambiguous), so it must only be referring to some arbitrary set of times during which the universe was much smaller than its current size. In practice, I think people use it to describe any period of time during which we can't directly observe objects, which would mean anything before recombination. This may be the current meaning, but I suspect that, like "high redshift", the term has been evolving with time.

Labguy

I doesn't know what we does mean....

(slow night)

rocketman7

stars are in a state of hydrostatic equallibrium. outward radiative pressure from photons and nuetrinos emitted in he core are balanced by the inward force of gravity.

Spin_Network

So are we stating that the inward force of gravity, squeeze's photons out from the inner core, similar, like a "wet sponge" expelling water when squeezed, so to speak?

SpaceTiger

Photons are only the dominant source of pressure in stars much more massive than the sun. In most stars, it is the particles themselves that are doing the pushing, not the photons (and certainly not the neutrinos).

franznietzsche

While this is technically true, as SpaceTiger pointed out, radiation pressure is insignificant in the case of the sun.

I whipped this up in Matlab real quick to demonstrate. The graph is on a log scale, so the [tex]10^#[/tex] you see on the y axis is the same as that part in scientific notation. it had to be log scale for the gas pressure to even be noticeable on the graph. A difference of 10^4 is a factor of 10,000 difference between the two.

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