Star birth

[George]

When a star finally contracts to the fusion point, what happens? For instance, will a short term chain reaction occur generating a shock wave? Will photon activity greatly increase? Will the random walk become a temporary run? Will the contraction momentum be a big factor during this birth period?

[edit: I said collapse when I meant contract]

[Ken G]

These are pretty detailed questions, I think you'd have to peruse supernova models or direct the question to those who do them. Keep in mind also, no one has every completely successfully modeled a supernova yet-- it is still quite hard to get the star to actually blow apart! Still, some of the basic ideas are likely to hold-- a collapsing core, the "bounce" that drives material away from the collapse (and yes, I would expect that to be a shock wave), the role of neutrinos in transporting the energy from the fusing collapsing core to the expanding envelope. I'm afraid I can't say more, I don't really know anything more about supernovae than you can read in a standard astronomy text or online. The random walk of photons won't be a run, that's why you need neutrinos, which do run quite well. And yes, after the supernova blasts through the surrounding medium, there will be contraction and star birth. I'm not sure "momentum" is the key issue, but the increased density certainly is. But a lot depends on where you are talking about, and when in the supernova.

[George]

These are pretty detailed questions, I think you'd have to peruse supernova models or direct the question to those who do them.

Why the heck did a say "collapse"? I meant a contracting star. Sorry. [I'd better edit it]

I am wondering what most pre-"main sequence" stars do when they start fusion.

The random walk of photons won't be a run, that's why you need neutrinos, which do run quite well.

I was wondering if the low density portion of a possible shock wave would allow photons to keep up with it due to the increase in the mean free path? Obviously, this would depend on the intensity of the shock wave (if one even exists).

[Kaptain K]

When a star finally contracts to the fusion point, what happens?

I think that it has been pretty wel established that thermonuclear fusion ignition is a gradual event (at least on human timescales). It is also a fairly "bumpy" process. T Tauri stars (very young stars, in the process of igniting) are extremely and erratically variable.

[George]

I think that it has been pretty wel established that thermonuclear fusion ignition is a gradual event (at least on human timescales). It is also a fairly "bumpy" process. T Tauri stars (very young stars, in the process of igniting) are extremely and erratically variable.

[We are watching our 11 month old niece on weekends. She is truly beautiful but is now throwing temper tantrums that are classical T Taurible. ]

Would their fusion event be eventful? [Hard to say to no when asked this way, huh? ]

[Ken G]

I think that it has been pretty wel established that thermonuclear fusion ignition is a gradual event (at least on human timescales). It is also a fairly "bumpy" process. T Tauri stars (very young stars, in the process of igniting) are extremely and erratically variable.

But gradual implies smooth, while bumpy implies the opposite. It sounds like there could be a lot of violent dynamics going on, despite the inherent stability of fusion in a stellar core supported by gas pressure, if the bumpiness is due to fusion irregularities and not envelope instabilities that have more to do with photon escape.

[George]

Instability seems to come from both internal disruptions and accretion disk interactions. T Tauris seem to be quite intersting and, like most babies, fussy.

From the link...

Brightness changes detected in these stars are not due to evolutionary effects, per se, but may be due to such processes as instabilities in the disk, violent activity in the atmosphere of the star, and may also be due in part to moving clouds of dust and gas from which they were conceived.

Apparently, 5 optical magnitudes have been observed in at least one T Tauri; which makes them hard to classify.

Unlike many other types of variable stars, classification of T Tauri variables cannot be based upon their lightcurves. The behavior of these young stars is just too erratic as found by Alfred Joy in 1945: "The variations in light of the T Tauri stars are so irregular and unpredictable that classification by means of their lightcurves is practically impossible....
The belief still holds true today since the lightcurves of T Tauri stars continue to show variability over a wide range of amplitudes and durations....

Who really knows what these young stars will be up to next? [per Herbig]

Also, there are the FU Orionis objects...

While T Tauri variations may result from instabilities within and interactions with the surrouding accretion disk, FU Orionis (also known as FUors) activity results from a dramatic increase in instability and the dumping of large amounts of matter on the accompanying star (Kaler 1999). "Roughly half of the 11 commonly accepted FUors have been observed to rise 3-5 magnitudes in optical or near-IR brightness on timescales of 1-10 yr" (Kenyon et al. 2000). The remaining half show some additional features including association with optical jets and HH objects. It is speculated that perhaps all T Tauri stars probably go through FU Orionis-type behavior one or more times in their development (Herbig 1987)

And, we haven't gone nuclear yet!

Added....
I also wonder if much of the fuss is found in the star's transition to the ineveitable zones. The outer convective zone is differential in rotation, whereas the radiative zone and core is not. The boundary of the these two zones is known as the tachocline and is believed to be the region where tornadic(?) magnetic storms are generated (the trailer parks of stars ).

[Ken G]

Yes, all this points to the fact that when you see variability in stars, it usually does not relate to anything happening in the core. Ironically, one things of nuclear fusion as unstable and dramatic, but in stellar cores held up by gas pressure, nuclear fusion is the most stable aspect of the star.

[George]

Yes, all this points to the fact that when you see variability in stars, it usually does not relate to anything happening in the core. Ironically, one things of nuclear fusion as unstable and dramatic, but in stellar cores held up by gas pressure, nuclear fusion is the most stable aspect of the star.

Oh the irony. I was expecting some nuclear fussing.

However, looks like the birth pains are quite dramatic enough to be of real interest.

I wonder what happens if a planet sized object gets gulped? Considering Shoemaker-Levy's marks, this should create real havoc. It is my understanding pre-nuclear stars are not much larger than Jupiter, so the impact energy should be considerable.

Are bipolar flows a given during this T Tauri period?

[I apologize for a thread question of such broad scope.]

[trinitree88]

[QUOTE=Ken G] Keep in mind also, no one has every completely successfully modeled a supernova yet-- it is still quite hard to get the star to actually blow apart!

Snippet; Hi Ken. This unfortunately is not true. The authors Leinson and Oraevskii, Soviet Journal of Nuclear Physics ...circa 1988..found that there was a bimodal scattering in supernovae. One mode is spherically symmetric, and involves neutrino/ quark-gluon bag scattering. There is an additional mode for neutrino/magnon scattering (where magnons are spin-waves)...for an additional 30%. This gives sufficient momentum and energy to put the kinetic energy over the gravitational potential well....and the supernova is spontaneous from there. It was my suggestion that there be an approximate 40% parity asymmmetry in this secondary mode, coupled to the ambient B field, that gives the pole-sensitive ejection of the nascent pulsar and the correct energetics OOM. I referred to the model as the Leinson-Oraevskii model, with proper attribution, as they pioneered the thinking, in my talk at Harvard in 94 at the Olney Science Center...."Gamma Ray Bursts...A Halo of Neutron Stars at 400 Kiloparsecs?"
Since then, the community leans towards beamed supernovae for GRB's...but the given kinematic supernova model of the talk, with OOM parity effects, and the use of Weak Asymmetric Recoil Effects, or WARP Drive..... was a first. Pete I'd be happy to crank out the numbers again for you, or Fermilab (Dr. Laurence?)....this time without Matt Damon and Ben Affleck in the back row of the auditorium.
Pete

[Ken G]

It sounds like ideas have been presented that could make supernovae work, but the physics is not yet widely accepted. All lines of inquiry must be pursued vigorously. The OP was actually not talking about supernovae, so it was a nonsequitur for me to bring it up here.

[George]

I am surprised at the blatant difference between docile first fusion in a star going main sequence versus one with fusion toward near annihilation.

[Ken G]

If by annihilation you mean supernova, note that supernovae are not powered by fusion, they are powered by the gravity of the core collapse. It is core collapse, not fusion, that is the dramatic process. Even when fusion is unstable, like when a red giant first starts to fuse helium, the effect on the star is fairly gradual despite the incredible suddenness of the "helium flash", though it does cause the star to shrink down quite a lot after awhile (and not be a giant anymore).

[George]

I find this strange applied to both Type I and II.

I assumed the core collapse was more of a triggering mechanism causing a large shell region outside the core to rise in temperature and pressure enough to "blow".

[Here are my green thoughts, that will probably make me red with embarassment then blue with mild depression. Being light-headed should also explain my signature. Though the above is not a prerequisite. ]

Gravity energy is far weaker than the nuclear force.
If all the energy is in the gravitional collapse of only a few thousand miles, why are they billions of times brighter than young contracting stars?
If the star is a giant, then the speed of gravity limits the bang, maybe.
Since there is a rebound of falling gas off the newly formed neutron core, how could one not expect more than just a hot bounce?
If the core becomes a blackhole, then isn't a large portion of the energy lost to the abyss, and what of the rebound here?

How solid are the sn models? Can 1% of the neutrino's be absorbed in the contracting shell? Is this 1% enough to explain the billion fold gain in the visible portion?

Assuming you are right, and you always are, how does this understanding of the star contraction (limited collapse) relate to star births?

[Ken G]

I find this strange applied to both Type I and II.

I'm only talking about core collapse.

I assumed the core collapse was more of a triggering mechanism causing a large shell region outside the core to rise in temperature and pressure enough to "blow".

No, the core collapse is the energy driver. Keep in mind, it is collapsing into something that can release not too much less than c^2 energy/gram, you just can't get that kind of energy release from nuclear reactions (maybe 1%).

Gravity energy is far weaker than the nuclear force.

Yes but energy is force times distance, and the nuclear force is very short range. The gravity extends over several kilometers in a core collapse, so that's why it's effects are more important.

If all the energy is in the gravitional collapse of only a few thousand miles, why are they billions of times brighter than young contracting stars?

Even less, a few miles. But remember, gravity is an inverse-square force, so the smaller the object, the stronger the gravity. What matters is the size it ends up, not the size it starts at, and the smaller the better for energy release.

Since there is a rebound of falling gas off the newly formed neutron core, how could one not expect more than just a hot bounce?

I'm not sure I understand the question, I thought we were talking about core collapse, in which case there is more than just a hot bounce.

If the core becomes a blackhole, then isn't a large portion of the energy lost to the abyss, and what of the rebound here?

Yes, a lot is lost, but roughly half the energy is not lost to the black hole, but rather shows up in the bounce.

How solid are the sn models? Can 1% of the neutrino's be absorbed in the contracting shell? Is this 1% enough to explain the billion fold gain in the visible portion?

I don't know any details about the SN models, really. I'm not sure what fraction of the neutrinos get absorbed, but I do think they carry away a lot of the SN energy. Note that the problem in a supernova is not the energy, there is plenty of that, it's in the momentum-- getting the heavy envelope to actually blow off.

Assuming you are right, and you always are, how does this understanding of the star contraction (limited collapse) relate to star births?

Thanks, but I wish I was always right! (Then again, that would be boring.) I'm not sure there are any lessons in core collapse that relate to the gradual contraction of a forming star. The former is not in hydro equilibrium, while the latter is, so the former happens on the dynamical time while the latter happens on the much longer photon diffusion timescale. They're pretty different, so that's why it might be a non sequitur.

[George]

INo, the core collapse is the energy driver. Keep in mind, it is collapsing into something that can release not too much less than c^2 energy/gram, you just can't get that kind of energy release from nuclear reactions (maybe 1%).

How is this so? Are you saying the collapse of several kms releases this much energy? I can't see it. Or, is there a circumstance, due to extreme temp. and press., that cauases nearly all matter to energy conversion? [I did a little googling and found little in clear explanation.]

Yes but energy is force times distance, and the nuclear force is very short range. The gravity extends over several kilometers in a core collapse, so that's why it's effects are more important.

Yet gravity is not significantly higher near the core if only a few miles are contracted, I'm guessing. Maybe my problem is seeing it as a release of potential energy only in the collapse.

Even less, a few miles. But remember, gravity is an inverse-square force, so the smaller the object, the stronger the gravity. What matters is the size it ends up, not the size it starts at, and the smaller the better for energy release.

I can see this when the core contracts to neutron material, though I thought it would be more than just a few miles to get there. Is your assesment iron clad?

I'm not sure I understand the question, I thought we were talking about core collapse, in which case there is more than just a hot bounce.

My focus is on the bounce itself. Wouldn't fusion due to bounce intensity augment, at least, the explosion?

Yes, a lot is lost, but roughly half the energy is not lost to the black hole, but rather shows up in the bounce.

Thanks. 1/2 would be a lot as the star would necessarily be extra massive and a bigger boom than some.

[quoe]I don't know any details about the SN models, really. I'm not sure what fraction of the neutrinos get absorbed, but I do think they carry away a lot of the SN energy. [/quote]
From the little I've read, 99% of the energy flys off with neutrinos. It is believed 1% does intereact with the massive shell but there does not seem to be a clear understanding just how this transpires.

Note that the problem in a supernova is not the energy, there is plenty of that, it's in the momentum-- getting the heavy envelope to actually blow off.

Is this the outward momentum, reversed from the inward fall by the neutrino heating and from the bounce's shock wave?

Thanks, but I wish I was always right! (Then again, that would be boring.)

Not with me, it can be entertaining, and challenging, trying to learn me good.

I'm not sure there are any lessons in core collapse that relate to the gradual contraction of a forming star. The former is not in hydro equilibrium, while the latter is, so the former happens on the dynamical time while the latter happens on the much longer photon diffusion timescale. They're pretty different, so that's why it might be a non sequitur.

Again, I seem to be thinking potential energy release, so the difference is in time unless there are greater dynamics involved in newborns.

[added: TalkOrigins seems to do a nice job of supernova processes. The type II does seem to collapse from an Earth sized core to about 50km at low relativistic speeds (is this an oxymoron?) . Outer layer fusion does take place but I get no sense as to how much this augments the explosion. Apparently, the neutrino production comes from a energy producing process > mere potential (gravitational) energy change. ]

[bigbluestar]

George if you are wondering how a supernova's energy level or the energy required to give it the luminosity that it has, in most cases that of millions of stars our size. The energy really all comes from the core callapse and not from fusion.
Fusion has stopped in the star. One annology that I have heard and will steal appropriatly now is take a basketball and a tennisball. place the tennis ball ontop of the basket ball and drop them. What this experiment will show is that the tennis ball will be rocketed away when the both hit the floor. Now the basketball is the core, the tennisball the envelope of the star. The bounce from them hitting the floor is the rebound the energy you are looking for.

The violence from the rebound does two important things that dramatically increases the luminosity of the ill fated star. The increase of activity in the envelope increases the tempreture, and second expands the envelope of the star. luminosity or energy is determined by L=4śr^2s(Teff)^4

L=luminosity
r=radius
s=Stefan Boltzman constant (5.67e-8)
Teff=tempreture

With that equation its actually easy to see were that increase in luminosity comes from. The radius is squared, increases greatly with envilope expansion. And the tempreture is to the fourth power, well we can deffinatly say that there will be a great tempreture increase in the event.
So the rebound is the cattalyst, everything else is a reaction to the core callapse but the energy is accounted for.

[George]

The energy really all comes from the core callapse and not from fusion.

This seems to be the consensus, though I am non-sequituring. [My way of implementing your word, Ken. [/cornjunktive] ]

Fusion has stopped in the star.

This is where I am surprised. It is obvious the core would quit once the iron ash accumulates and the endothermic core collapses at an astounding rate, [just how much core volume is reduced seems to be uncertain], and infalling shells would have lost pressure enough to continue fusion, I would assume. However, this should be only temporary during the fall.

One annology that I have heard and will steal appropriatly now is take a basketball and a tennisball. place the tennis ball ontop of the basket ball and drop them. What this experiment will show is that the tennis ball will be rocketed away when the both hit the floor. Now the basketball is the core, the tennisball the envelope of the star. The bounce from them hitting the floor is the rebound the energy you are looking for.

Thanks. I like the analogy. This makes sense. Allow me to add my spin...if the tennis ball also converts 1% of its mass to energy, how much higher would sections of it fly? Also, there are many more other tennis balls falling down on top of the rebounded one. Wouldn't the rebound energy create a shockwave great enough to trigger the pressure and density necessary for fusion, in particular - hydrogen fusion? Wouldn't this have to be the case to forge elements > iron?

I recently read the rebound will stall but neutrino interaction may revive it.

The violence from the rebound does two important things that dramatically increases the luminosity of the ill fated star. The increase of activity in the envelope increases the tempreture, and second expands the envelope of the star. luminosity or energy is determined by L=4śr^2s(Teff)^4.

Yes. But does the math match the observation? The temp. would drop fast, I think, reducing the initial value of luminosity from the equation, but the optical depth increases and light's random walk should become a run, too. [Of course, the temp. is still enormous even if reduced by the expansion.] Still, we want L^9. If the temp. of exposed gas is 100x greater than the original surface temp., then we are at L^8 already. So the greater area would do the rest and more. This would make sense, especially in the Type II.

Another argument for minimal, if any, hydrogen fusion during the explosion would be the lack of it in the Type 1a sn; since it is even brighter ,

So the rebound is the cattalyst, everything else is a reaction to the core callapse but the energy is accounted for.

So, if I understand, simply expanding the star, via shockwave, at a great rate is all that is necessary to get the observed results; fusion is not needed to augment the blast.

Can lesser shockwaves be generated in T Tauri's? [getting back to birth from death ]

[bigbluestar]

Can lesser shockwaves be generated in T Tauri's? [getting back to birth from death ]

Im sure a T tauri star goes through many shockwaves and siezmic activity. Is it strong enough to destroy the young star it, does not appear so as many stars survive that stage.

Lets start with the birth of the star one of your earlier post was about if its walk or a run for the photons. Remeber the core of the star is litteraly alway hot plasma and has been illuminating from the really early begginings of the star accreting matter. When fussion starts labratory says like in the H bomb the process is ratther quike. The problem is the young star is doing what alot of main stage stars do.... wich is a proccess called convection. Throuhg that is will take some time for the photons to reach the surface of the star. Neutrinoes interact less and will get to the surface much quicker.

Im pretty sure I have failed you in your question but I had a tough time following your young star questions as the post was quickly directed to stellar death. Lemmie know more if I can help any further

[George]

Im sure a T tauri star goes through many shockwaves and siezmic activity. Is it strong enough to destroy the young star it, does not appear so as many stars survive that stage.

Do some not survive?

Remeber the core of the star is litteraly alway hot plasma and has been illuminating from the really early begginings of the star accreting matter.

I learned that gas bodies will not get much larger than Jupiter due to the atmospheric compression from the additional mass. [Interestingly, the same is true of elements; adding more protons to a nucleous only pulls the additional electrons tighter so they all stay about the same radius regardless of atomic number.] However, as the protostellar core heats, this size limitation makes little sense. How big are the T Tauri's? I would assume they are a little smaller than the sun (assuming a one solar mass T Tauri).

When fussion starts labratory says like in the H bomb the process is ratther quike.

This is where confusion reigns in my mind. I would have guessed a chain reaction would occur in the core due to the elevation in temperature from its initial fusion, and a shock wave would be generated. However, it seems this is not the mainstream viewpoint.

The problem is the young star is doing what alot of main stage stars do.... wich is a proccess called convection. Throuhg that is will take some time for the photons to reach the surface of the star. Neutrinoes interact less and will get to the surface much quicker.

Is there a neutrino mini-flash announcing a stars birth into the main sequence?

Between the Sun's convective zone and raidiative zone is the tachocline. As I understand, the tachocline is very stressful as it adjoins a fixed rotational mass below and the differential mass (convective zone) above. The magnetic storms, creating sunspots and other events, are believed to be created here and what we see on the surface is the tail of these critters. I would assume this region would be even more unstable in a T Tauri's early years - The Terrible Twos To T Tauris (TTTTTT).

My stretched hope is to get a flash event announcing a new star's birth into its neighborhod.

Just to add more of my mental mess to all this...If hundreds of stars in a nebula nursery form around the same time frame and in close proximity, could their accretion disks collide on the adjoining side diminishing the angular momentum and increasing the accretion rate, as well as, create havoc for the two stars, maybe. This would not be a routine circumstance, of course.