A question - in a slightly different form - was asked on the APOD forum, but hasn't been answered yet.

During formation of a star, once nuclear fusion starts, how long before the star is blazing away? I understand it takes hundreds of thousands of years for a nebular mass to coallesce into a solar mass, but I would think that once the density and temperature reaches that required for fusion, things might progress rapidly.

As I understand it, the star will be shining brightly before the fusion starts in the core, because of the heat from the gravitationally driven compression. When the fusion does start, the star counterintuitively fades as the fusion halts the contraction and eliminates further compression heating.

Most stars are difficult to observe in this stage because they are surrounded by dust in the protostellar nebula. A few can be seen as T Tauri variables. They generally are more luminous than they will be after settling down on the main sequence.

My sources are mostly articles in Sky and Telescope over the last four decades.

That's basically right, but I don't think the star actually fades when fusion starts, but you are right that fusion doesn't brighten the star either. What fusion does, above all, is halt the contraction of the star (a process that had already made the star very bright indeed but less and less bright with time, prior to the onset of fusion), allowing the star to be in equilibrium for a very long time with relativity minor changes.

Perhaps "fade" is a poor choice of a word. The stellar evolution plots I have seen on a Hertzsprung-Russell diagram show the T Tauri stage as being somewhat more luminous than on the main sequence, but larger in diameter and cooler. As it settles down on the main sequence the temperature and surface brightness increase, and it stabilizes as the fusion comes up to full power.

Yes, the general trend prior to fusion is gradual dimming. The fusion eventually arrests that trend, but doesn't reverse it. Those are the broad terms, what happens in detail I could not say.

I'll offer just a piece of the puzzle. Exposure of the protostar to a GRB and a prompt neutrino burst. Nearing the point of ignition by pressure/temperature/density gradients...a GRB would only heat the outer layers unable to ignite the core, but a prompt neutrino burst can penetrate to the core, and interact via the three weak currents..W⁺, W^-, and W⁰...or Z⁰. The W⁺'s are key. As a neutrino strikes a proton, it converts an up quark to a down, changing it into a neutron and emitting a W⁺. The W⁺ annihilates into a positron and a neutrino. The positron annihilates a local electron in a two or three gamma ray interaction that scatters and random walks,supplying heat, and the star begins emitting neutrinos.
If only campfires could do that.... pete

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A third rate theory forbids
A second rate theory explains after the fact
A first rate theory predicts...A. Lomonosov

Are you suggesting that a star can only start fusion if ignited by radiation from a GRB and/or a burst of neutrinos? It's a wonder there are as many stars as there are!

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Microsoft is over if you want it.

The bar has been lowered for the promotion of ATM ideas; the bar for the acceptance of ATM ideas must remain high.

Celestial Mechanic. No,not quite what I'm thinking. The three constraints of time/temperature/density required for fusion remain the complete criteria, and they can reach that state by contraction. What I'm suggesting is that in a galaxy laden with stars having cores near the edge of the requirements for ignition, a supernova prompt neutrino burst might push a number of them over the edge at the same time...like an expanding event horizon, and that we would see a burst of star ignitions, like a light echo, as the burst spreads outward. Otherwise, their births would be more sporadic. pete

see:http://www.spacetelescope.org/news/html/heic0708.html

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A third rate theory forbids
A second rate theory explains after the fact
A first rate theory predicts...A. Lomonosov

Is there any peer-reviewed astrophysics and/or particle physics opinion that an influx of neutrinos will set off fusion under what otherwise would be insufficient temperature and pressure, or is it mere conjecture?

Suppose, for the sake of argument, that a brief burst of neutrinos from an outside would hasten the onset of fusion in a protostar. How do we know it would not fizzle out after the burst has passed?

Some one reported that it takes 50,000 years for the energy of fusion in the core of our sun to reach the photosphere, after which it takes 8 minutes for the photons to reach Earth. One of the reasons is the mean free path of gamma photons produced at the core is very short due to the very high density. the photons are absorbed and re-emitted very many times before they reach the surface.
If so a period of dimming seems reasonable, shortly after fusion begins. Neil

I am no expert, but I do not think the time lag is the reason for the dimming. When the protostar was contracting, the ongoing compression was generating a lot of heat, which made the star glow brightly. When the fusion came up to enough power to halt the contraction, that source of heat went away leaving the fusion as the only source.

Hornblower. Point 1. I'm searching the literature and will chase this for a bit. It seems there has been a concensus in the particle physics community that the T-Tauri mechanism for synthesis of light elements is a failure (Ryder, Schramm)and that another mechanism should be found... I'm still preliminary on this so I'll say the point is conjecture until sources are found.
Point 2. If a T-Tauri or young protostar is near to the point of compression ignition/the same random walk of photons that confines the energy for that ignition equally applies to the energy stimulated by a burst of neutrons interacting in fusions after being created in a passing prompt neutrino burst, No? pete

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A third rate theory forbids
A second rate theory explains after the fact
A first rate theory predicts...A. Lomonosov

I would certainly enjoy seeing details on the early phases leading up to hydrogen fusion, including any deuterium fusion phase.

Since fusion requires ~15 million degrees in the core, then the star would already have had to be hot enough to initiate fusion. As has been stated, this happens due to contraction. However, once fusion begins, then I have assumed it would generate more heat than the rate of heat generated from the contraction process. This should swell the star and increase the luminosity. Am I not correct?

[Added: I only stated heat for the purpose of increasing the momentum of the stars gases that would cause such a swell idea (pun intended, as usual). The photon pressure factor, however, needs to be included, so I should have stated energy in lieu of heat, I suppose.]

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

Shouldn't this be in the "Astronomy Forum"?

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

Maybe, but I wasn't sure so I figured this would be a good place.

This is a most interesting and counterintuitive topic. Once again I am no expert but I have seen various accounts of stellar evolution in Sky and Telescope over the past four decades. Searching upwards of 400 back issues would be a beastly job, and browsing with search engines is a hit-and-miss affair. I will wing it with what I can halfway remember.

First, the fusion in the core of the Sun is heating it at the rate of a small fraction of a watt per kg. It is nothing resembling what happens when we set off a hydrogen bomb. If a small laboratory object was being heated at that rate in thermal equilibrium we would not even feel it. The Sun's great size, with its vastly larger mass to surface area ratio, causes it to remain so hot at such a low rate of energy production.

Suppose a brief burst of outside neutrinos started some scattered fusion in a T Tauri star when none was occuring before because of insufficient heat and pressure. That would be over in a few seconds or minutes, and would heat the star only a tiny amount, even if the fusion briefly reached the solar rate. Unless a nuclear physics expert comes forth and says that scattered fusion events such as these, should they occur at all, would set off a chain reaction in an otherwise subcritical environment, my inclination is to disregard this hypothesis as insignificant.

I cannot do the math necessary to explain what happens in the interiors of stars at different stages in their evolution. I would have to resurrect someone such as Sir Arthur Eddington to help me. All I can do is take his word and the words of his successor astrophysicists as being reliable. When a star has stabilized on the main sequence, the fusion in its core is its sole continuing source of heat, and the calculations by the experts have shown that its total power is less than the maximum heating power of the gravity-driven compression that was occuring before the onset of fusion. I have seen it illustrated on H-R diagrams more than once in the aforementioned Sky and Telescope articles.

Hornblower. I'm also not an expert here, but am acting on intuiton till corrected. The rationale goes (temporarily)..the energy of supernova neutrinos in the prompt burst runs to several Mev each. (Sn1987a) Scattering of neutrinos if they are massless, can only be forward scattering, and they typically lose ~ 10% of their energy per scatter (James Losecco, Notre Dame). They can also interact via charged weak currents....changing protons to neutrons and vice versa...(up quarks to down and vice versa...as the exchange is a few Mev). Free neutrons will decay in ~ 1000 seconds if not captured...but protons are ideal for "cooling" neutrons with high velocities, as they have a very similar mass..(which is why wax and water were early moderators for chain reactions). So I'm thinking a flurry neutrons generated, and producing a denser concentration of deuterium in the core as they are swept up by available protons.
The other player here is in-situ muons created in the same burst. Muons can catalyze ~100 fusions before decaying. So if the first burst through doesn't enrich the Deuterium/Protium ratio sufficiently, then successive bursts should ~ 1/century in this arm of the galaxy....with one of them causing ignition.
The players with a more structured knowledge of Eddington's particulars will set me straight here. pete

see:http://prola.aps.org/abstract/PRC/v45/i2/p532_1

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A third rate theory forbids
A second rate theory explains after the fact
A first rate theory predicts...A. Lomonosov

Here we have some tidbits from particle physics experts which suggest the possibility that a burst of neutrinos from a nearby supernova might result in some deuterium enrichment in the core of a protostar. Since deuterium starts fusing into helium at a lower temperature than does ordinary hydrogen, perhaps the star will warm up somewhat more quickly than it would have otherwise, get the main charge fusing a bit earlier, and settle down on the main sequence a bit sooner. If this does happen, we still do not know whether the effect is substantial, modest, slight, or vanishingly slight. We do know that neutrino interactions are few and far between in proportion to the total number of neutrinos passing through the star. My educated guess is that it will be negligible unless a supernova is really close.

I can't do the math, either. I just kno