a) A hydrogen star is one where hydrogen is still the primary fuel of a star. Yes our sun is one - it is still converting hydrogen into helium, in its core.
I’ll go into the stellar life cycle here - it’ll make things easier to explain.
We begin with a protostar, which is gas that is collapsing in under gravity. As the gas collapses, hydrogen burning (this is hydrogen fusion) begins, and once the protostar achieves hydrostatic equilibrium, the protostar joins the main sequence. This is an age zero main sequence star. This name allows us to distingush between a newly formed star, in which all the elements are distributed evenly through the star, and one which has been burning for a while, in which there will be a greater concentration of helium in the core, than at t=0. For instance, the sun’s core contains more helium than hydrogen (because of nucleosynthesis), but there is enough hydrogen in the core to ensure our sun remains in its core hyrdogen burning phase for about 5x10[sup]9[/sup] years.
As the number of atomic nuclei in a star’s core decreases with time (because it takes 4 H atoms to make one He atom), the pressure in the star’s core decreases, so gravity allows the star to collapse, which makes the star hotter. This means that the H in the outer layers of the star is heated, and we get H burning in the outer-lying regions as well. This hasn’t happened to the sun yet - in about 5x10[sup]9[/sup] years!
Once the H in the core is depleted, H burning will only continue in the oyter layers - this is the shell H burning phase. Initially, this only happens in the hottest region immediately outside the core, but then spreads out to the outer-lying regions. This shell burning heats up the core, and causes the core to contract. Because He is heavier than H, the He formed in the shells drops down to the core, which makes the core heavier, which causes the core to contract more.
Eventually, the star will become a red giant, and since their surface gravity is fairly weak, much of the gas in the shell H burning layers is ‘blown off’. There will still however be a thin shell of H burning gas surrounding a denser core of almost pure He. In the core of red-giants the temperature has risen to allow He fusion to occur. We do not have He fusion in the sun’s core at the moment because temperatures are too low. - temperatures of 100 million K are needed.
There are two stages to He burning. Firstly, two He atoms combine to form a short-lived, unstable atom of beryllium (Be). Then, if a third He nucleus strikes the Be atom before the Be atom decays (which is entirely plausible in the dense core of a star), a stable carbon isotope is formed (carbon 12), and energy is released. This is called the triple alpha process
So, to answer your second question (I think I’ve answered your first one above), the He is stored in the core of the star. However, the star cannot use He as a fuel until the temperature is high enough, as opposed to low enough, for two He atoms to overcome the electrostatic repulsive force (remember the He is two protons and two neutrons, and so both are positive, and will, under normal circumstances, repel each other) and fuse.
Does this help?
Also, a good starting point is “Universe” by Roger Freedman and William Kaufmann III.
