Why do we say a solar-like star has run out of fuel when it has used only 10 percent of its total hydrogen?

Answer 1

In order for the hydrogen to fuse into helium, it needs the tremendous heat and pressure at the core of the star. The outer 90% of the hydrogen never gets compressed enough so it never gets used.

Answer 2

Only the hydrogen at the core of a star can be fused to helium. Much of the hydrogen in a star is in the outer layers, and it doesn't participate in the core fusion.

When the core runs out of hydrogen, the star contracts (due to a decrease in the core temperature) and this causes the core to heat up again and start fusing helium.

Answer 3

The second answerer is pretty much right. Stars like our sun are not fully convective, so the hydrogen outside the core never makes it into the core.

Very small stars, red dwarves, can be fully convective though, allowing them to consume almost all of their hydrogen before they move off of the main sequence, which is one reason why red dwarves last so freakin long.

Answer 4

Just where did you get "we' from? I never, ever said that!!