If I remember correctly, what caught fire was Vader's mutilated leg at the stump, but how is that the case? The robes should be way more flammable than flesh, shouldn't it?
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Because midichlorians are so cool.– Nick TCommented Dec 14, 2016 at 5:33
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I always assumed they didn't catch fire because all platforms and installations have shields...– NeowCommented Dec 14, 2016 at 7:47
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3 Answers
The clothes do catch fire, the stump does not. You can see it happening:
The type of ignition that occurs is a self-ignition occurring on the material. Humans do not combust thanks to thermal inertia (the human body is 2/3 water).
Assuming the Jedi wear a basic cotton material, the self-ignition flashpoint (the material catching on fire) is 407 degrees C.
The lava flowing is viscous, flowing quite rapidly, and appears to go for kilometers, suggesting a rhyolite lava. This type of lava is unusually hot in the greater than 950 degrees C range.
Using the Sakimoto and Zuber: Flow and Convective Cooling in Lava Tubes Model and Anakin's slow descent down the loosely graveled walls we see that Anakin finally comes within range to trigger the self-ignition point of 470.
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Unfortunately, this seems to suggest exactly the opposite of what occurs in the movie. The question is why Anakin’s robe did not catch fire before his body, and this answer suggests that the flesh should indeed be harder to ignite than the robes.– AdamantCommented Dec 14, 2016 at 6:19
Unfortunately, the answer, as far as I am aware, is instead of a canon explanation, just bad movie physics.
The surface temperature of lava is anywhere from 700-1250 degrees Celsius (Francis), while the temperature at which human skin is burned away is around 72 degrees Celsius (Fire Dynamics). Actually the point at which synthetic fabrics is higher than that, but still nowhere near the 600 degrees mark at around 300 degrees Celsius (Fire Dynamics), and that's for equipment designed to resist heat. During the entire Mustafar scene, unless Mustafar's lava was for some reason much, much less hot than any known lava on Earth both Obi Wan and Anakin should have been dead long before the final scene of the fight.
Citations:
Francis, P., 1993, Volcanoes: a planetary perspective: Clarendon Press, 443 p. (accessed from http://volcano.oregonstate.edu/how-hot-lava)
Nov. 7, 2010, Fire Dynamics (accessed from https://www.nist.gov/%3Cfront%3E/fire-dynamics)
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2Which also raises the question, How was the duel even possible if they should have catched fire immediately? Every canon source takes the duel for granted but doesn't explain how they actually managed to fight so close to the lava without catching fire. Commented Dec 14, 2016 at 5:41
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1There's something called "suspension of disbelief" that is often used in movies. Another classic example is the space fighting in Star Wars, and most other sci-fi series, is completely unrealistic and based off of WWII era dogfights. Unfortunatley real space battles would probably be boring affairs waged with combatants millions of miles away from each other firing high powered cannons and missiles at each other until one side scored a lucky hit Commented Dec 14, 2016 at 5:44
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@NeoHerakles - Indeed, "the Force" is quite likely the right explanation.– AdamantCommented Dec 14, 2016 at 6:02
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Though there is also shielding: scifi.stackexchange.com/a/82765/51379– AdamantCommented Dec 14, 2016 at 6:22
Warning, TV Tropes link, abandon all hope, etc: http://tvtropes.org/pmwiki/pmwiki.php/Main/ConvectionSchmonvection
The entry for Episode III mentions Lucas placing priority on the symbolism of the scene, at the cost of realism and plausibility, though I cannot find a source for this.
As Daniel Miller notes above, it appears that the clothing does catch fire first, but only when it was most dramatic in Lucas' eyes for it to do so. Even if the flesh managed to somehow burn first, it would be because Lucas thought it better that way, and the reality of that environment was very much a secondary concern.
