Expert: Tom Whiting - 12/30/2007

Question
Hi Tom,
  I know our Sun burns Hydrogen, and that Hydrogen is a very flammable gas. My question is, if Jupiter, (and possibly other planets)is mostly Hydrogen by mass, is it flammable? Could you theoretically strike a match and start it burning? Would it then be a small star? Thanks for listening to my astronomical fantasies !

Answer
Hi Mark,
Yes, they are indeed, fantasies, so I'll set you straight
on why all that doesn't happen, and can't happen, even though you are correct about hydrogen gas being flammable.

True, Jupiter is mostly hydrogen gas (H2), but there is no
oxygen present, so the "match" goes out, unless your
"match" is made of solid oxygen.  Recall that "burning"
is simply rapid oxidation, that is, combining hydrogen gas
and oxygen gas to form water plus heat. (Examples on Earth
are wood, gasoline, candles, etc). But there is no
free oxygen present on Jupiter, or any of the gas giant
planets, so that's why Jupiter does not "ignite" and burn.  And, even if it did, that ain't the way a star works anyway, see below.  (Even if you got Jupiter's core
hot enough, there is not enough mass present to maintain
a fusion reaction in it's core...it's several masses shy
of becoming a little red dwarf star.)

Our sun is not "burning" hydrogen even though astronomers
tend to mistakenly use that word...it's actually FUSING hydrogen, not the gas H2, but rather, bare hydrogen nuclei (single protons--into helium nuclei - 2 protons and
2 neutrons) down in it's high temperature--super high
pressure--core, to produce energy.  This isn't the normal
hydrogen "gas" that you and I generally think of, but rather a very hot, (like 25,000,000 degrees F!) fluid-like plasma of pure hydrogen nuclei under unimaginable pressure, the electrons having long fled out of there, as they cannot exist in orbit around the protons as there is too much heat energy present.  Our sun uses, what is called
the "proton-proton chain" fusion reaction, whereas bigger
stars use the "CNO cycle" fusion reaction down in their
cores, which is faster and more efficient. (You can Google
those if you are interested in seeing the actual reactions
which occur in several steps -- plus to understand them,
you need a basic physics/chemistry background).

Now here's the interesting part...the sun converts 400 million tons of hydrogen nuclei to 396 million tons of helium nuclei...the resulting 4 million tons mass going
directly to energy using Einstein's E = mc squared formula,
EVERY SECOND.  But even then, a star the size of the sun
has an expected total lifespan of nearly 10 billion years.
Not to worry, as it's only at the halfway point in its life
so we have several billion years of life left.
(But eventually, we will have to get off this 8000 mile
diameter rock we live on because the sun will eventually
run out of available hydrogen nuclei down in it's core).

Most of the energy produced down in the core is gamma ray radiation, so if we could magically be transported to the
sun's core, it would be totally dark! (No visible light). The gamma ray radiation works it's way out to the surface, being shifted down into the other forms of energy; visible light, UV radiation, heat, radio...but it takes nearly a million years to work it's way out to the "surface" or top photosphere layer of the sun. Then it departs the photosphere at light speed, and takes only 500 seconds to get to the Earth's orbital distance.
So, just some interesting facts I thought you'd like to
know.
And hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA   

FOLLOW UP:
Hi Mark,
thanks for the very nice comments....
but to be honest, everything I told you is pretty much
common knowledge to us crazy people that are nuts about
astronomy, and astronomical observing.  And since I taught
it for 13 years, I had to memorize all those fine details.
So feel free to ask any question again.
Clear skies,
Tom Whiting
Erie, PA