Hi LJK

You do ask the big questions. The problem for big stars is that their mass is confining the core so much and driving core temperature and fusion reaction rates to ever greater highs. If you could suppress the core temperature slightly the star would contract and heat up slightly too. If you tried to suppress the fusion then the core contraction energy would ramp up. Very tricky. The best to hope for is to cause over-turn between the core and the non-fusing layers above, thus extending the star's life by ~ x 10. However eventually the hydrogen is gone and things start going sour again until helium ignition kicks in. Another possibility is to seed the star with proton-decay catalysts like Q-balls etc. Our Sun would last ~ 10 trillion years if it converted ~ 50% of its mass into energy. Hydrogen fusion gets a measly ~ 0.67% while the higher fusion reactions get progressively less and less, approaching the maximum ~ 0.9%. A true baryon-burning reaction would liberate almost the whole lot with the unused mass acting as a thermalising blanket - no one wants a hard gamma-ray source for a sun.

Big stars are wasteful of their energy but not so much of their mass - a star 8 times the mass of the Sun actually sheds all but about 1.4 solar masses - and that core's mostly useless iron - thus it isn't really such a waste, just inconvenient for us. Our Sun will take more fusion fuel to its grave than that star, but even it will return about ~45% of its (mostly unburnt) mass to the interstellar medium. A lot is burnt up during hydrogen shell burning as the helium core is contracting and heading for the helium flash. Then the post helium Main Sequence AGB phase (the brightest) gobbles up a fair amount too. The hydrogen Main Sequence uses just ~ 8-10%.

Red dwarfs are naturally efficient at fusion burning - the low mass RDs fully convect their hydrogen, thus burning ~ 50% before a radiative core develops and convection shuts down. Thus 0.2-0.25 solar mass RDs last ~ 1.5 trillion years. At that mass there's no helium burning phase but helium->carbon burning produces less energy anyway.

If two low mass helium dwarfs collided with enough energy they could kick into helium burning, but that lasts a mere ~ 0.5 billion years, the star radiating at ~ 35,000 K. Slightly heavier and the merged stars start burning carbon at a furious rate lasting a mere ~ million years and blazing at ~ 140,000 K. The final limit for a merged mass above ~ 1.44 solar masses is a naked supernova as the white-dwarf collapses into neutronium and rebounds, with no extended envelope to block the blast.

Adam