I always hear the narrator of documentaries say that a star explodes because it ran out of fuel.
Usually things explode when they have too much fuel, not when they run out of fuel. Please explain...
I always hear the narrator of documentaries say that a star explodes because it ran out of fuel.
Usually things explode when they have too much fuel, not when they run out of fuel. Please explain...
Short answer:
A tiny fraction of the gravitational potential energy released by the very rapid collapse of the inert iron core gets transferred to the outer layers and this is sufficient to power the observed explosion.
In more detail:
Consider the energetics of an idealised model star. It has a "core" of mass $M$ and initial radius $R_0$ and an outer envelope of mass $m$ and radius $r$.
Now suppose the core collapses to a much smaller radius $R \ll R_0$ on such a short timescale that it decouples from the envelope. The amount of gravitational potential energy released will be $\sim GM^2/R$.
A fraction of this released energy can be transferred to the envelope in the form of outward moving shocks and radiation. If the transferred energy exceeds the gravitational binding energy of the envelope $\sim Gm^2/r$ then the envelope can be blown into space.
In an exploding star (a type II core collapse supernovae) $R_0\sim 10^4$ km, $R\sim 10$ km and $r \sim 10^8$ km. The core mass is $M \sim 1.2M_{\odot}$ and the envelope mass is $m \sim 10M_{\odot}$. The dense core is mostly made of iron and supported by electron degeneracy pressure. The star is said to have "run out of fuel" because fusion reactions with iron nuclei do not release significant amounts of energy.
The collapse is triggered because nuclear burning continues around the core and so the core mass is gradually increased and as it does so it gradually shrinks (a peculiarity of structures supported by degeneracy pressure), the density increases and then an instability is introduced either by electron capture reactions or photodisintegration of iron nuclei. Either way, electrons (which are what is providing the support for the core) are mopped up by protons to form neutrons and the core collapses on a free fall timescale of $\sim 1$ s!
The collapse is halted by the strong nuclear force and neutron degeneracy pressure. The core bounces; a shock wave travels outwards; most of the gravitational energy is stored in neutrinos and a fraction of this is transferred to the shock before the neutrinos escape, driving away the outer envelope. An excellent descriptive account of this and the previous paragraph can be read in Woosley & Janka (2005).
Putting in some numbers. $$GM^2/R = 4\times 10^{46}\ {\rm J}$$ $$Gm^2/r = 3\times 10^{44}\ {\rm J}$$
So one only needs to transfer of order 1% of the collapsing core's released potential energy to the envelope in order to drive the supernova explosion. This is actually not yet understood in detail, though somehow supernovae find a way to do it.
A key point is that the rapid collapse takes place only in the core of the star. If the entire star collapsed as one, then most of the gravitational potential energy would escape as radiation and neutrinos and there would be insufficient energy even to reverse the collapse. In the core collapse model, most (90%+) of the released gravitational energy is lost as neutrinos, but what remains is still easily sufficient to unbind the uncollapsed envelope. The collapsed core remains bound and becomes either a neutron star or black hole.
A second way to cause a star (a white dwarf) to explode is a thermonuclear reaction. If the carbon and oxygen can be ignited in nuclear fusion reactions then enough energy is released to exceed the gravitational binding energy of the white dwarf. These are type Ia supernovae.
To give an answer in more simple turns. (Yes very simplified, but it should introduce the basic concept).
A Star "burns" by nuclear fusion between lighter elements such as Hydrogen turning to Helium. The heat and energy of that burning constantly pushes on the matter inside the star holding it up. The fusing hydrogen generates enough energy to stop it from being able to collapse down into the center.
As the star starts running out of fuel that "fire" gets colder, and the pushing out gets weaker.
Eventually the push isn't enough to keep the star apart and it all rushes back together. That collapse releases a huge amount of energy which causes the explosion.