Calling the sun a big burning ball of gas is one of my bugbears of teaching. It is not gas. You will find that all the large planets in our solar system are gas. The mass and pressure become so much that nothing can remain a solid. In fact, there are supergas planets that have no solar system at all and they simply float along in space. No one is quite sure how they formed, but there they are. BUT, they are not stars. Read on to find out why.
Have you seen the BBC show ‘Wonders of the Universe’?
Here’s a bit about stars: http://www.youtube.com/watch?v=n_KyYFYNvpI
The real issue isn’t that ‘hydrogen is a flammable gas’ because that is very ‘Telluscentric’ view. The hydrogen of the sun is more than burning and it is NOT gas.(sorry to correct everyone but it is a very fundamental point to understand.) It is in a PLASMA state. http://en.wikipedia.org/wiki/State_of_matter
It has to do with the amazing mass and gravity associated with having that much of anything in one spot in space. Stars are so big that they have surpassed being gas and have are in states of plasma. That goes for all stars, regardless of the elements present in them.
I hope that helps, and if I raised more questions than answered, just ask them and I will try to answer.
My 7 year old is really having fun with this little program. Is shows the scale of the Universe and might help you get your head around how big (and small) things are.
http://www.onemorelevel.com/game/scale_of_the_universe_2012