Actually, under normal conditions, it takes 100million K to ignite hydrogen, but inside stars, the pressure and density of the materials allows the gas to ignite at much lower temperatures. The nuclei don't have to be so energetic to overcome the repulsion forces between the nuclei. So, given the pressure and density of the core region of the Sun, the nuclei find that they only have to be heated to around 15million K to be able to sustain the Sun in its present form.
BTW....30million K is near (it's a bit over, actually) the point where core reactions become dominated by the CNO cycle rather than p-p, as in the Sun. It occurs in stars just a tad heavier than the Sun...usually around 1.2M solar.
As a matter of fact, the lowest temp at which nuclear reactions for core burning hydrogen can be sustained is 7million K. The smallest body that can sustain such a temp is around 0.08M solar...or a very small M class star (M9).
The corona is a very thin gas that is heated via magnetic induction and sound. The articles you posted in your 3rd post here pretty much explain how it occurs. It's a combination of several processes which heats the coronal gases, not just one completely dominant process.
Here's a few things for you to read that'll explain it much better than me writing here.
|