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Natural Science Forum / Physics / Research / June 2008Star Ignition Question
A friend of mine raised the question as to what the current theory is about how stars "ignite". That is, how hydrogen fusion inside a star gets started. If you have an initial huge ball of hydrogen, the hydrogen will become compressed through gravitational attraction. But is gravity alone sufficient to compress and heat hydrogen enough to ignite a self-sustaining nuclear fusion reaction? According to this web page http://nuclearplanet.com/stellar%20ignition%20and%20dark%20matter.htm JM Herdon made the claim in 1996 that heating due to infalling matter is not sufficient. Is Herndon's claim considered respectable? -- Daryl McCullough Ithaca, NY In article <g23qhe0kl0@drn.newsguy.com>, stevendaryl3016@yahoo.com (Daryl McCullough) writes: > A friend of mine raised the question as to what the > current theory is about how stars "ignite". [quoted text clipped - 3 lines] > JM Herdon made the claim in 1996 that heating due to infalling matter is not > sufficient. Is Herndon's claim considered respectable? No. Note that the page is also maintained by the same JM Herndon. This is something which, as far as I know, is simply not a problem in conventional astrophysics. Sounds similar to creationists claiming "evolution proved to be a hoax" and so on in a manner which, to a casual reader, gives the impression that an outsider (creationist) has solved a major problem in the field (which, however, is unknown to those working in the field) with a radical hypothesis. A quote from: http://nuclearplanet.com/index.html "Earth, says geophysicist J. Marvin Herndon, is a gigantic natural nuclear power plant." Look at http://nuclearplanet.com/Important%20Discoveries.htm and draw your own conclusions. Thus spake Daryl McCullough <stevendaryl3016@yahoo.com> >A friend of mine raised the question as to what the >current theory is about how stars "ignite". That is, [quoted text clipped - 3 lines] >But is gravity alone sufficient to compress and heat hydrogen >enough to ignite a self-sustaining nuclear fusion reaction? yes >According to this web page >http://nuclearplanet.com/stellar%20ignition%20and%20dark%20matter.htm >JM Herdon made the claim in 1996 that heating due to infalling matter is not >sufficient. Is Herndon's claim considered respectable? I have not heard of this theory, but the basic theory of stellar evolution works extremely well. One can calculate the mass required to generate sufficient heat to ignite fusion, and one finds good agreement with observed sizes of stars. Regards  SignatureCharles Francis moderator sci.physics.foundations. charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces and braces) http://www.teleconnection.info/rqg/MainIndex > A friend of mine raised the question as to what the > current theory is about how stars "ignite". That is, [quoted text clipped - 8 lines] > JM Herdon made the claim in 1996 that heating due to infalling matter is not > sufficient. Is Herndon's claim considered respectable? > A friend of mine raised the question as to what the > current theory is about how stars "ignite". That is, [quoted text clipped - 8 lines] > JM Herdon made the claim in 1996 that heating due to infalling matter is not > sufficient. Is Herndon's claim considered respectable? "Heating by the in-fall of dust and gas is takes place at the surface of the forming star." THERE'S your problem. The star will contract and pseudoadiabatically heat from compression. Gravitation is relentless. Core conditions will eventually reach fusion ignition conditions because temperature is the only thing that inflates a star against collapse. When you crush Fermi exclusion you cannot help but generate heat well beyond chemistry. Look up core conditions in the sun. It's heat output/volume is less than mammalian metabolism. There is a lot of volume and a lot of time. SignatureUncle Al http://www.mazepath.com/uncleal/ (Toxic URL! Unsafe for children and most mammals) http://www.mazepath.com/uncleal/lajos.htm#a2 > A friend of mine raised the question as to what the > current theory is about how stars "ignite". That is, [quoted text clipped - 7 lines] > JM Herdon made the claim in 1996 that heating due to infalling matter is not > sufficient. Is Herndon's claim considered respectable? Let us put a few numbers in. Let us assume that the nebula is at about 4K. How much does it have to be compressed to get to 100 million K Now TV(gamma-1) is a constant http://en.wikipedia.org/wiki/Adiabatic_process Temperature goes up by a factor of 25 million. For a monatomic gas gamma = 5/3. Probable average for interstellar gas thoughout its range is 1.3. V^-0.3 = 25 million Hence V contracts by about 4*10^24. If we take the cube root of this we have a contraction of 1.5*10^8 in radius. As stars start of from a nebula some light year across this seems an eminently reasonable answer. The volume contraction is quite a bit above what would be needed by adiabatic equations. There is a lot of complexity. The star starts spinning, planets form. There is a great deal of complexity in what actually happens but there is no a priori reason for assuming shock waves. In fact angular momentum and planet formation tends to make the compression slower than it would otherwise be. - Ian Parker Ian Parker says... >Let us put a few numbers in. Let us assume that the nebula is at about >4K. How much does it have to be compressed to get to 100 million K [quoted text clipped - 14 lines] >As stars start of from a nebula some light year across this seems an >eminently reasonable answer. The point made by Herndon is that stellar contraction is not adiabatic, since energy at the surface of the star radiates away into space (the energy lost per second is proportional to the fourth power of the temperature). The consensus here seems to be that Herndon is a crackpot, so I assume that this effect has been taken into account in models of stellar ignition. -- Daryl McCullough Ithaca, NY > Ian Parker says... > [quoted text clipped - 23 lines] > seems to be that Herndon is a crackpot, so I assume that this > effect has been taken into account in models of stellar ignition. Physics Today 61(6) 70 (2008) "For objects with mass less than about 0.072[solar mass], degeneracy pressure halts contraction before the critical H fusion temperature is reached. Hydrostatic equilibrium but not thermal equilibrium is achieved." Surface radiation is a minor factor. It requires centuries for a core photon to diffuse to the surface. Stellar core compression is pseudoadiabatic as surely as a diesel cylinder works to spec despite an active cooling system. You might want to be elsewhere during ignition. It will be a positive feedback big bump. Things then settle down as equilibrium spreads outward. SignatureUncle Al http://www.mazepath.com/uncleal/ (Toxic URL! Unsafe for children and most mammals) http://www.mazepath.com/uncleal/lajos.htm#a2 > Ian Parker says... > [quoted text clipped - 23 lines] > seems to be that Herndon is a crackpot, so I assume that this > effect has been taken into account in models of stellar ignition. Not to put too fine a point on it I think he is cracked. There is no reason to suppose that adiabatic compression cannot cause thrmonuclear ignition. Indeed nebulae are observed to have a temperature of 20,000K. Of course thermonuclear fusion starts in an opaque star and because of heat loss the outer layers are very much cooler than the core. To me the interesting question is about planet formation and not about adiabatic compression. As stated earlier a gas clould has angular momentum which stops the contraction. Only after planet formation can thermonuclear reactions start. Let us look at a star contracting WITHOUT angular momentum. Thermonuclear reactions start in the core, but the star is still contracting at 500-1000km/s (the core faster) this means that a detonation occurs in the core. Planet formation is therefore integral to stellar ignition and a star contracts slowly as planetary rings accelerate. We have a solar system where the gas giants are in the outer reaches of the solar system and rocky planets are near the Sun. Is this typical? "Hot Jupiters" - gas giants near a star have been observed. Their abundance may be due to the fact that a "typical" solar system cannot be observed with present instruments. Nothing the size of the Earth can be seen unless it is going round a pulsar. There are in facts "Earths" going round pulsars. Questions :- How do "Hot Jupiters" form? Are they typical? :- How do secondary planets form after supernovae? Now here there IS a role for shock waves. In a supernova SOME material stays in the vicinity of gthe star and gets accelerated by magnetic fields. To me these are the interesting questions. - Ian Parker > We have a solar system where the gas giants are in the outer reaches > of the solar system and rocky planets are near the Sun. Is this > typical? "Hot Jupiters" - gas giants near a star have been observed. > Their abundance may be due to the fact that a "typical" solar system > cannot be observed with present instruments. Do we know the abundance of hot Jupiters? SignatureDirk http://www.transcendence.me.uk/ - Transcendence UK Remote Viewing classes in London > > We have a solar system where the gas giants are in the outer reaches > > of the solar system and rocky planets are near the Sun. Is this [quoted text clipped - 3 lines] > > Do we know the abundance of hot Jupiters? http://exoplanets.org/planet_table.shtml This is a more up to date list with some discussion on selectivity. http://en.wikipedia.org/wiki/Extrasolar_planets Here is a comprehensive extrasolar table. Anything below 100 days is hot. The number of HJs depends on selection. If you assume that all HJs have been found (HJs are the easiest type of extrasolar planet to detect since they produce strong doppler signals) it means that they are rare. Set the list off against the number of stars. - Ian Parker > On 6 Jun, 15:34, stevendaryl3...@yahoo.com (Daryl McCullough) wrote:> A friend of mine raised the question as to what the > > current theory is about how stars "ignite". That is, [quoted text clipped - 34 lines] > > - Ian Parker The initial size of the cloud is actually irrelevant for the final temperature of the star. The latter depends only on the average potential energy of an atom in its gravitational field, which for an atom of mass m and a star with mass M and radius R is of the order of E_pot=-GmM/R . Because of the virial theorem of classical mechanics, this must be (on average) equal to -2*kinetic energy i.e. E_kin=GmM/ (2R) which for the sun is about 1 keV i.e. T= 10^7 K (E_kin=kT). The important point here is that in order for the cloud to collapse, it must actually permanently lose energy. This requires inelastic collision processes, which in turn will result in radiative emissions. So strictly speaking there is in fact no particular point where a star 'ignites'. The emission of radiation (due to electronic processes) goes hand in hand with the collapse. See my page http://www.plasmaphysics.org.uk/research/starformation.htm for more in this respect. Thomas [[...]] > The initial size of the cloud is actually irrelevant for the final > temperature of the star. The latter depends only on the average [quoted text clipped - 10 lines] > 'ignites'. The emission of radiation (due to electronic processes) > goes hand in hand with the collapse. True, however there *is* a fairly sharp transition when Deuterium ignites, and the radiated energy from D fusion exceeds that from gravitational collapse just before then. At that point the collapse stops and you have a roughly quasistationary protostar. > See my page http://www.plasmaphysics.org.uk/research/starformation.htm > for more in this respect. > > Thomas > [[...]] > [quoted text clipped - 17 lines] > gravitational collapse just before then. At that point the collapse > stops and you have a roughly quasistationary protostar. There is in fact no fusion required to stop the collapse. It simply will stop if the gas density becomes so high that no individual atoms (and thus no inelastic collisions leading to radiative energy loss) can exist anymore (which is of the order of 10^23 cm^-3). There will be a very small energy loss (and thus contraction) resulting from inelastic collisions in the (less dense) atmosphere of the thus formed star, but this will be too small to be noticeable over short periods of time. Thomas > > See my pagehttp://www.plasmaphysics.org.uk/research/starformation.htm > > for more in this respect. > > > Thomas > A friend of mine raised the question as to what the > current theory is about how stars "ignite". That is, [quoted text clipped - 8 lines] > JM Herdon made the claim in 1996 that heating due to infalling matter is not > sufficient. Is Herndon's claim considered respectable? Fusion is about temperature plus density plus how long matter is held at that temperature and density. What temperatures can pycnonuclear fusion occur at? The idea of all star cores being originally ignited by fission is ludicrous - where did the heavy nuclei originate? |
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