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She burns like the sun...

6th January 2011 (15:27)

... I can't look away

[BPSBD] There's some idiot who started a thread (edit: like this) at badscience.net called pathological science in astronomy - Stellar Fusion I in which he was wrong about just about everything. This post won't make much sense unless you've read all that, but posting a compilation of responses here will hopefully make it easier to find as that thread descends into Bickering.

The gist of the original poster's argument seems to be that since the ideal gas law PV = nRT says that volume goes down as temperature goes down, that balls of gas cannot possibly contract while heating up as they would have to in order to ignite fusion and become a star. But Boyle's law has been extended to include gravity and it demonstrates that a ball of gas can collapse.1 Whether the ideal gas law applies to the sun is another matter.2

His misunderstand of the behaviour of gases comes from typical school textbook sentences such as “gases expand to uniformly fill the container they are in” but this isn't even true in something like a gas centrifuge, which is able to set up a pressure gradient in a gas and indeed cause two gases two separate by density. This isn't something which normally happens in the Earth's gravitational field on a laboratory scale, unless you have a very heavy gas such as (I'm guessing) sulphur hexafluoride, but of course the pressure of the atmosphere drops with altitude.

It gets worse, though, when we get to the subject of plasmas. Maybe it's not a coincidence, but it was Irving Langmuir who coined the term “pathological science” as well as the term “plasma” for an ionized gas. The equations which describe plasmas are of course dominated by the electromagnetic forces between the ions and the free electrons, which means again that gravity usually does not need to be taken into account in laboratory scale plasmas, but he seems to think (it gets hard to tell) that since gravity is negligible in laboratory plasmas that gravity no longer applies anywhere that there is a plasma. Plasmas, which are partially ionized gases, have just about the same mass as the same amount of neutral gas, and of course they don't actually screen gravity or anything like that. In fact, the Debye length in space is of the order of 10 metres (and much shorter in the sun). This gives you an idea of the maximum length scale over which electromagnetic forces can act before they get screened by mobile charge. Gravity is always attractive though, which is why it wins out over astronomical scales.

Anyway, solid objects (such as the sample) very obviously obey gravity's rules despite the plasma in my growth system, if they aren't mounted properly or if they break and fall off the heater.

Gravity is a very good explanation as regards the solar system anyway: the orbits of all the planets can be described, to an extremely good degree of accuracy, by

where μ is 1.32712440018 × 1020 m3 s-2 and a is the length of each orbit's semi-major axis.

Maybe the worst part, though, is the claim that the sun has a solid rock surface (although he also says that all stars are plasma) based on images such as this which show the emission from iron vapour in the corona (from TRACE.

The sun is too hot to have any solids in it, for a start. The solar spectrum corresponds to black body radiation at about 5500-6000 K, the temperature of the photosphere. The light from the hotter solar core is not seen since it does not easily pass through such a dense plasma; heat from the core actually propagates up to the photosphere by convection.

The corona is at a much higher temperature (and lower density) such that the emission from highly ionized iron can be used to visualize it. The main purpose of the TRACE mission is to explore the relation between diffusion of the surface magnetic fields and the changes in heating and structure throughout the transition region and corona. The iron is only a small fraction of the sun's composition, and its high degree of ionization means it must be at around 106-107 K in the corona.

As for nuclear fusion itself, which he doesn't seem to believe in, both high temperatures and confinement are needed to induce it, but it has been demonstrated on Earth with electric currents or radiation and adiabatic compression. So it all makes sense to the rest of us. More sense than the sun being a solid rock which keeps the Earth in orbit by electromagnetic forces transmitted through plasma anyway.

Thanks to the other members at badscience.net for support, by the way. Especially as expressed in this thread or this post...

  1. W. B. Bonnor. Boyle's Law and Gravitational Instability. Mon. Not. R. Astron. Soc. 116 351-359 (1956) [ http ]
  2. David B. Clark. The ideal gas law at the center of the sun. J. Chem. Educ. 66 (10) 826 (1989) [ http ]

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