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

by alancj
Tags: explode, holds, stars
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SpaceTiger
#19
Nov11-05, 10:39 PM
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Quote Quote by scott1
When the photons form at the Sun's core it take it about 1,000,000 years to get to the surface(so the sun's core is 1millon light-years away).
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.


I think whatever takes light to get 1 millon years to get to the surface is probally whould the thing that's holding it togther.
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
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Nov11-05, 11:32 PM
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Quote Quote by 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.
That was just a joke lol
SpaceTiger
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Nov11-05, 11:50 PM
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Quote Quote by Danger
True at our sun's level of activity. Farther down the line, the helium-3 will fuse into helium-4, then lithium, etc..
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
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Nov12-05, 12:53 PM
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Thanks, Tiger. I knew that there was something wrong with my post. Those sequential thingies always mess me up.
Labguy
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Nov12-05, 06:42 PM
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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:
However, the lack of stable nuclei with atomic weights of 5 or 8 limited the Big Bang to producing hydrogen and helium. Most lithium and beryllium is produced by cosmic ray collisions breaking up some of the carbon produced in stars.
And from Berkeley we find that formation of Lithium first requires tritium and deuterium:
In a short time interval, protons and neutrons collided to produce deuterium (one proton bound to one neutron). Most of the deuterium then collided with other protons and neutrons to produce helium and a small amount of tritium (one proton and two neutrons). Lithium 7 could also arise form the coalescence of one tritium and two deuterium nuclei.
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
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Nov12-05, 07:38 PM
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Quote Quote by 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.
How are you distinguishing this from the cosmic ray spallation I referred to? Or are you just expanding on what I said?


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.
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
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Nov12-05, 07:48 PM
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Quote Quote by alancj
I do know that stars explode on occasion... but I don't understand how gravity could possibly have enough force (attraction or whatever you want to call it) to hold trillions and trillions and trillions of tons of hydrogen undergoing nuclear fusion. It seems to me that there would be vastly more pressure to expand outward than to hold it together.
Can anybody point me towards some math that would show that a Sun's estimated mass would have enough gravity to keep things together?
Well those trillions and trillions and trillions of tons (~1.98911030 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
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Nov12-05, 08:59 PM
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Quote Quote by SpaceTiger
How are you distinguishing this from the cosmic ray spallation I referred to? Or are you just expanding on what I said?
Just expanding when looking for probable sources for Li found. No distinction from your post.
Quote Quote by SpaceTiger
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".
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
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Nov12-05, 09:04 PM
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Quote Quote by Astronuc
Re: Li production (is it necessary that most(?) Li was formed around time of BB as opposed to stars later on or novae?)
See posts # 21, 23 and 24.
SpaceTiger
#28
Nov12-05, 09:17 PM
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Quote Quote by Labguy
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!)..
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
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Nov12-05, 09:36 PM
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Quote Quote by Space Tiger
but what does we even mean?
I doesn't know what we does mean....

(slow night)
rocketman7
#30
Dec27-05, 05:30 PM
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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
#31
Dec27-05, 07:25 PM
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Quote Quote by 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.
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
#32
Dec27-05, 10:13 PM
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Quote Quote by 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.
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
#33
Jan12-06, 02:50 PM
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Quote Quote by 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.

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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pressuregraph.jpg  


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