What Occurs First: Nuclear Fusion or Fission in a Star?

[Rader]

What occurs first, nuclear fusion or fission inside of the core of a star, or both?

Stars are powered by nuclear fusion deep in their cores.
If fusion was first, the star would begin to radiate light immediately.

Nuclear fission is where the centers of atoms (nuclei) are split and broken apart. When they split, nuclear energy is released.
If fission was first, the star would begin to radiate over a period of time.

Would the solar mass then determine how a star begins to give light?

 

[Labguy]

What occurs first, nuclear fusion or fission inside of the core of a star, or both?

Stars are powered by nuclear fusion deep in their cores.
If fusion was first, the star would begin to radiate light immediately.

Nuclear fission is where the centers of atoms (nuclei) are split and broken apart. When they split, nuclear energy is released.
If fission was first, the star would begin to radiate over a period of time.

Would the solar mass then determine how a star begins to give light?

Fission can only occur when heavy elements "break down" into lighter elements with a release of energy. Since most stars form with only H and He, there are no heavy elements for fission, so fusion is the only alternative, which is triggered by temperatures of ~11.5 million to 12 million degrees K. Even late generation stars with "heavy" elements, such as our Sun, have so few metals (anything heavier than He) that the proportion is too small to have any significant effect on a star's formation.

IOW, it is fusion only that is significant in any star formation.
As Oppenheimer said:
"Nuke 'em if they can't take a joke." ...

 

[franznietzsche]

A star is born when fusion is initiated in the core. Fission is not a normal process for stars, although it could happen, but it would only be by random chance, the normal driving process for all stars is hydrogen fusion, followed by helium fusion, and so on, until iron is produced,and then the star starts to break down because iron fusion takes up energy rather than producing it.

 

[Rader]

Fission can only occur when heavy elements "break down" into lighter elements with a release of energy. Since most stars form with only H and He, there are no heavy elements for fission, so fusion is the only alternative, which is triggered by temperatures of ~11.5 million to 12 million degrees K. Even late generation stars with "heavy" elements, such as our Sun, have so few metals (anything heavier than He) that the proportion is too small to have any significant effect on a star's formation.

IOW, it is fusion only that is significant in any star formation.
As Oppenheimer said:
"Nuke 'em if they can't take a joke." ...

So can ~11.5 million to 12 million degrees K be equated to specific size of solar mass?

 

[Labguy]

So can ~11.5 million to 12 million degrees K be equated to specific size of solar mass?

Yes, that starts a star of minimum size to fuse H to He. The Sun has a core temp of ~16 million K. A star with about 1.2 solar masses has a core about 19 million K. Etc., etc. At about 19 million K, the burning process changes from the Proton-Proton Chain, predominant in the sun, to the CNO Cycle which doninates fusion in "high" temperature cores. Helium won't fuse to carbon, etc. until about 100 million K.

Core temperature is very dependent on mass with high mass causing high pressure (gravitational) causing high temperatures.

 

[Rader]

Yes, that starts a star of minimum size to fuse H to He. The Sun has a core temp of ~16 million K. A star with about 1.2 solar masses has a core about 19 million K. Etc., etc. At about 19 million K, the burning process changes from the Proton-Proton Chain, predominant in the sun, to the CNO Cycle which doninates fusion in "high" temperature cores. Helium won't fuse to carbon, etc. until about 100 million K.

Core temperature is very dependent on mass with high mass causing high pressure (gravitational) causing high temperatures.

Can the math pinpoint a specific mass with a gravitational pressure in which fussion starts, or is this a quantum process that could start ~11.5+ million K.

What happens in the core below -11.5 million K, before fussion starts?

 

[chroot]

There isn't a specific temperature where fusion starts in all its glory -- fusion occurs very slowly at low temperatures, and becomes more and more fervent as the temperature goes up.

When the core is too cool to support fusion, nothing happens. It's just a ball of (ionized) gas.

- Warren

 

[Nereid]

You may have heard the term 'failed star' Radar, and also 'brown dwarf'. The stellar models used by astrophysicists today are pretty good, also pretty complex. Basically they seek to model all the known physical processes that could take place in a ball of gas of a given composition, mass, and starting radial profile; they are written to handle 'phase' changes (e.g. transition from molecules to atoms, from atoms to ions, etc; from radiative to convective transport of heat; ...) automatically (where the model suggests a change takes place - either in time or space - the regime is automatically adjusted; this includes such tricky situations as when changing from radiative to convective transport causes the plasma/gas to change - e.g. another ionisation state - causing the dominant transport mechanism to change ... you can think of it as a kind of instability).

The border between stars and brown dwarves is approx 0.08 sol (in mass); apart from some deuterium burning, self-gravitating objects below this can't start their nuclear fires; above this they can.

A very interesting situation apparently occurred in the early universe - there were no 'metals' (astronomers, in their desire to assist the widespread understanding of their craft delightfully call all elements heavier than He 'metals'!) - so the first self-gravitating objects were huge, and many of the fusion reactions (plural) that are found in today's stars simply didn't happen (e.g. you can't have a CNO cycle if you have no C, N, or O!)

 

[Rader]

Well listening to the Richard Feynman lecture, he brought up a comment about some observations and photographic plates in the 40s, that showed the apparent position of a new star in the same place where there was none a few months earlier. He seemed to have his doubts about if it was true. Technology has changed in 45 years. Has there even been a sighting of a birth of star in a rather short period of time examining photographic plates?

It seems like from what you all have commented that the birth of a star is a process that occurs within a set of parameters which involve a lot of complex variables that would affect the outcome.

 

[Nereid]

Well listening to the Richard Feynman lecture, he brought up a comment about some observations and photographic plates in the 40s, that showed the apparent position of a new star in the same place where there was none a few months earlier. He seemed to have his doubts about if it was true. Technology has changed in 45 years. Has there even been a sighting of a birth of star in a rather short period of time examining photographic plates?

It seems like from what you all have commented that the birth of a star is a process that occurs within a set of parameters which involve a lot of complex variables that would affect the outcome.

I was talking more about stellar models in general than the formation of stars ... the details of how stars form is an active area of astrophysics ... lots of questions ... observations are beginning to help, with data from new telescopes such as Spitzer.

 

[Nereid]

As for 'new' stars ... highly unlikely that one would form in just 45 years. However, there have since (Feynman's time) been many interesting discoveries that may account for cases like this (e.g. BL Lac objects, novae, supernovae, GRBs, ...)