# Rate of star collpse

It is believed that our sun will become a red giant with a diameter of about 1 AU. When the fusion slows down, gravity will collapse the sun. Since the energy release from the fusion diminishes slowly (maybe hundreds of years for a significant change of 1%).

Let's assume that only a small fraction of the sun (10%) is needed to collapse (to 1/1000 the volume) to produce a nova. Would it still be possible for a star to collapse in a few hour (instead of decades)?

• This is not how the Sun will end its life. – Rob Jeffries Dec 20 '14 at 13:56

When the star is almost out of fuel, it cannot burn fuel as effectively as before, so, the core compresses a little, increasing the temperature, which increases the efficiency and rate of fusion, which makes the 'running out of fuel' part faster leading to a positive feedback process (compression $\rightarrow$ faster exhaustion $\rightarrow$ more compression). This gradually keeps 'collapsing' and burning fuel faster till a point where it cannot counter the collapse using an increased fusion rate due to lack of sufficient fuel (this might still take a few million years, maybe, but it keeps collapsing faster in a runaway fashion till that point). Beyond this point, the collapse happens at the dynamical timescale till it hits degeneracy (or ignition temperature of the previously inert core), which leads to puffing, novae and supernovae based on the mass of the core and its composition.
Well, I'm not sure exactly what you mean by "collapse" (if it takes $10^9$ years to die, the last few hours satisfy what your asking about). However, maybe some explanation would help: