Why is only 10 percent of a star's mass converted through fusion in the core?

Answer 1

When you fuse hydrogen to make helium, or fuse heavier elements to make iron, less than 1% of the mass is converted into energy. All the energy released comes from the binding energy of the helium nucleus; you've still got the same nucleons you started with, but because they're tightly bound in the helium nucleus their mass is slightly less. By comparison, if a particle and its antiparticle annihilate each other, 100% of their mass is converted into radiant energy, and if you allow matter to spiral into a black hole, you can extract up to 43% of its mass as electromagnetic radiation and gravity waves.

Answer 2

Essentially only 10% of the mass of the star is in core. This does not apply to all stars: low mass solar metallicity stars are fully convective...all the way down to their cores! The entire star is involved in fusion over time as gas from the envelope of the star gets carried down into the core. (I once estimated the main sequence lifetime of an M star to be something like 100 billion years! I don't remember exactly, except that is was MUCH longer than the estimated age of the universe.)

Also, some more massive more evolved stars experience periods of "dredging" where helium and carbon rich gas from core or maybe shells around the core make it to the surface. So called "carbon stars" have their high carbon content from this phenomenon. Presumably this means some hydrogen and helium from the envelope gets mixed into the area where fusion is sustained. This happens after the star leave the main sequence, however, so may not have a significant impact on life extension.

Answer 3

The material in the core of the star is under tremendous pressure due to the star's own gravity. At some distance away from the center, the pressure becomes too low to sustain fusion, so that material can't be converted.

Added clarification: The conversion is hydrogen to helium, not matter to energy.

Answer 4

because the rest of it is required to produce the pressures needed for fusion by gravitating towards the core, increasing its temperature and pressure by mass.

Answer 5

I fully agree with the above comment, and I would also like to add that it is actually 0.07Mc^2 ... 007, the nuclear "bond" energy... just a fun way to remember it =P

Answer 6

only antimatter can cause it to turn all its mass to energy.

Answer 7

anything more would be uncivilized