Gas giants

[mojo (Terry)]

The other night, i was watching some doco on the planets. Which got me wondering about the gas giants. Specifically the hydrogen atmosphere.

This is probably a silly question and i'm sure there must be a simple answer, but if Saturn is about 75% hydrogen and Jupiter is about 90% hydrogen, you would think one spark and we would have ourselves a rather large bang?

[ballaratdragons (Ken)]

Terry,

I read somewhere that Jupiter is so close to being a Sun it's not funny! It has the mass & size. It is larger than some stars we can observe!!

And the article went on to say that a cataclismic happening could turn it into a star, and is not out of the question at any time in the future.

That'll make toast out of us

[Starkler (Geoff)]

You would need oxygen also I think to make a bang.

[mojo (Terry)]

Ah. Oxygen! There's an interesting element to a fire

And now that I think about it, there was the Shoemaker-Levy impact with Jupiter. That was a fairly big fire-lighter.

Although, Ken's got something. It is 90% hydrogen and it does have the prime ingredients to make up a star... I wonder what it would take to start a fusion reaction.

[ving (David)]

[silvinator]

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No seriously, that's all you would need

[[1ponders] (Paul)]

There have been a lot of theories Terry, but the one I'm most partial to is that while it does almost have the mass to for one of the smaller sub-stellar objects (N, L ? stars etc), there are too many impurities in its atmosphere to feasibly sustain any sort of fusion reaction. Yes the may be heaps of semi-liquid metalic hydrogen in the core but there is also a lot of ammonia and contaminants in the atmosphere that keep more from compressing into the core. But that's only one theory.

Many of the dim stars that are imaged (MNL etc) may not be undergoing fusion anyway, but are generating their heat from "gravitational" energy forcing the hydrogen atoms closer together. But its never quite enough to start fusion.


To the last few posters: Please keep the Astronomy Science sub-forum free of spam thank you

[silvinator]

Sorry Paul, but I should have expanded on my little comment. My apologies

What I was trying to say was supposed to be an answer to Terry's question about what would make Jupiter start a nuclear reaction. In order for Jupiter to start hydrogen fusion, all you would theoretically need would be to add more mass to it - ie. Hydrogen. A star can only form if it has a certain critical mass (not sure what it is but it would be more than Jupiter's mass obviously, and more than a brown dwarf). If a rotating mass of hydrogen is massive enough, it will contract and collapse, thus causing the hydrogen to ignite and then a star would form.

I hope that explains my previous post.

[Astroman (Andrew Wall)]

Well there are apparently sparks going on all the time in the atmosphere of Jupiter. Lightning flashes have apparently been observed. SO if it's not sparks to make it a star must be in the form of a chemical reaction.

[RAJAH235]

I think you'll find that it's the initial mass of the 'gas'/'dust' cloud. Jupiter was just too small to start with.... I too saw a doco on it, but a diff. doco where the question was discussed in some detail. Just not quite 'BIG' enough.
Also, come on you chemistry experts, doesn't being an 'Isotope'? H1, H2, have a bearing on the 'flammability'?? Was not one of my best subjects. L.

[Robby]

They have discovered many extra-solar planets out there way bigger than Jupiter, so as Paul said, mass is only part of the equation.
A spark or flame on Jupiter would not cause it to turn into a sun, even if the chemistry of the planet was similar to a sun. What is needed is a nuclear detonator (for want of a better word). Something to start the fusion of the hydrogen atoms into heavier elements and thus cause the release of energy due to the mass of the new elements being less of the mass of the elements that contributed to it creation. Ahh E=mc^2.
Something like that anyway!!! I'm no nuclear fisisist .

[[1ponders] (Paul)]

Here mojo, found this at:

http://www.faqs.org/faqs/astronomy/faq/part5/index.html

Quote:
Subject: E.10 Could Jupiter become a star?
Author: Erik Max Francis <max@alcyone.com>

A star is usually defined as a body whose core is hot enough and under
enough pressure to fuse light elements into heavier ones with a
significant release of energy. The most basic (and easiest, in terms of
the temperatures and pressures required) type of fusion involve the
fusion of four hydrogen nuclei into one helium-4 nucleus, with a
corresponding release of energy (in the form of high-frequency photons).
This reaction powers the most stable and long-lived class of stars, the
main sequence stars (like our Sun and nearly all of the stars in the
Sun's immediate vicinity).

Below certain threshold temperatures and pressures, the fusion reaction
is not self-sustaining and no longer provides a sufficient release of
energy to call said object a star. Theoretical calculations indicate
(and direct observations corroborate) that this limit lies somewhere
around 0.08 solar masses; a near-star below this limit is called a brown
dwarf.

By contrast, Jupiter, the largest planet in our solar system, is only
0.001 masses solar. This makes the smallest possible stars roughly 80
times more massive than Jupiter; that is, Jupiter would need something
like 80 times more mass to become even one of the smallest and feeblest
red dwarfs. Since there is nothing approaching 79 Jupiter masses of
hydrogen floating around anywhere in the solar system where it could be
added to Jupiter, there is no feasible way that Jupiter could become a
star.
End Quote:

[slice of heaven]

Full marks to Silvie then