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In all the descriptions of the stellar life cycle it seems as though helium doesn't start being fused until all (most?) of the hydrogen is gone.

Is this true? Why is this?

It seems counter intuitive. Consider a fire consisting of gasoline (hydrogen), and wood (helium). Sure the gasoline would burn up quicker, but the wood would still be consumed while the initial gasoline was being consumed.

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The analogy is facile. Helium fuses at a temperature ($10^8\ \text{K}$) roughly ten times higher than hydrogen ($10^7\ \text{K}$), so a better analogy would be alcohol and thermite. That higher temperature is achieved only by massive gravitational contraction after hydrogen fusion [EDIT: in the core] is exhausted.

EDIT:

To expand, different mass stars undergo radically different life cycles, so "hydrogen fusion is exhausted" means different things for different stars. In all cases, fusion occurs only in a tiny core region.

For the lightest stars, convection (think rapidly boiling water) churns the entire star, so all of their hydrogen will eventually fuse. This will take much longer than the age of the universe, but even in the distant future, they will never compress enough to generate helium-fusing temperatures.

For heavier stars, including the Sun, convection only mixes the core region, so exhausting hydrogen fusion only means fusing all the hydrogen in the core. The tricky wrinkle is that after the exhaustion of core hydrogen, after the core collapses and heats, and after helium fusion starts, a thin shell of previously-pristine hydrogen surrounding the fusing helium can, itself, commence fusing as well. Hence the first, clarifying edit above.

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  • $\begingroup$ The last paragraph of your edit gets it backwards. Shell hydrogen fusion starts before Core helium fusion does. $\endgroup$
    – Dan Is Fiddling By Firelight
    Commented Feb 15, 2012 at 20:43
  • $\begingroup$ A minor technical point (doesn't affect conclusions): the Sun's core is not convective. Off the top of my head, I don't think stellar cores become convective until energy is chiefly produced by the CNO cycle and I think that happens around $1.5\,\text{M}_\odot$. $\endgroup$
    – Warrick
    Commented Feb 16, 2012 at 13:18
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    $\begingroup$ I would add that most nuclear burning stages are self-regulating. That is, once fusion sets in, increases in temperature result in increases in energy output, expanding the star and cooling things down. Thus as long as a region is burning hydrogen, it will be kept near the minimum hydrogen-burning temperature and thus will not burn any helium. $\endgroup$
    – user10851
    Commented Feb 17, 2013 at 3:37
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Temperature equates to the speed the nuclei are travelling at. Since Helium nuclei need to collide with greater energy to fuse this can only occur at a higher temperature. As a gas gets hotter it expands and so becomes less dense, reducing the amount of energy generated by fusion.

With a mixture of Hydrogen and Helium the energy generated by Hydrogen fusion keeps the gas from becoming dense enough to fuse Helium, and once the energy from Hydrogen fusion falls enough, the gas can shrink enough to start fusing Helium.

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  • $\begingroup$ Why then does a star expand to a red giant once it enters the helium phase if the gas must compress in order to attain enough heat to fuse the helium? (I think I know the answer I just think the answer would be more complete with this bit. Maybe this should be another question). $\endgroup$
    – deft_code
    Commented Jan 18, 2012 at 3:29
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    $\begingroup$ the core compresses and gets hotter, in doing so it starts to put out a lot of extra energy, that has to go somewhere so it goes into making the envelope larger. Richard Pogge explains this in some detail in his Astronomy 162 class which you can download from his website as a podcast astronomy.ohio-state.edu/~pogge/Ast162 $\endgroup$
    – Zachary K
    Commented Jan 18, 2012 at 4:59
  • $\begingroup$ @ZacharyK: Although it's often claimed, that reason is not the reason a star's envelope expands when it's core collapses. (I haven't listened to Pogge's lectures, so I'm not sure exactly what he said.) There's no reason the energy has to be trapped in the envelope. In fact, one can construct models stars that are made of pure helium. After exhausting their core helium, the core contracts just like a normal star, but the envelope doesn't expand. Something else must causing the expansion... but it's actually still not known what! $\endgroup$
    – Warrick
    Commented Jan 19, 2012 at 9:07
  • $\begingroup$ @Warrick: I suspect that it's an opacity issue. If the envelope traps enough energy, you get the expansion, but if it's too transparent you get the helium star. The difference between the two could require very fine tuning of the microphysics, which just isn't there yet in modern simulations. $\endgroup$
    – Andrew
    Commented Jan 20, 2012 at 3:27
  • $\begingroup$ @Andrew: I can't expound much detail in a comment but basically, the question of why stars expand into giants is open. That is, no-one can look at a pre-giant stellar model and say "this will become a giant" or "this won't". The microphysics in modern simulations is pretty good. Even if you restrict the opacity to electron scattering, a star will still become a giant (just not quite as big). It's suspected that gianthood is related to the presence of a nuclear burning shell and a sharp density gradient around the core. i.e. if you suppress the density gradient, some stars don't become giants. $\endgroup$
    – Warrick
    Commented Jan 20, 2012 at 9:40
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Actually hydrogen makes up the majority of the mass of a star throughout its entire life, even during helium fusion. Basically, things proceed like so:

Core hydrogen fusion produces the energy that powers the star for most of its lifetime.

At a certain point, all of the hydrogen in the core has been burned. Note that hydrogen is fusing in a shell around the helium core.

The helium core, since it isn't fusing -> not producing any outward force collapses inwards. As it collapses, it grows hotter and hotter, but there still isn't any helium fusion. The heat produced by this collapsing core actually causes the shell hydrogen fusion to increase like crazy. This extra hydrogen fusion causes the star to become a red giant.

At a certain point, the helium will become so hot and dense from its continued collapse, that it will finally begin to burn (~10^8 K as Andrew said.) This causes the helium core to expand, and the hydrogen fusion becomes confined to the core of the helium region. This actually cools the hydrogen shell by quite a bit, causing hydrogen shell fusion to decrease tremendously. This causes the entire core-shell output to drop, and the entire star becomes somewhat smaller as it collapses inwards. Now the star is running almost entirely on helium fusion.

This process is known as "helium flash."

But at the end, you are left with a start that is nearly entirely made of cooler hydrogen, with a small and extremely hot ball of helium at its core.

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