Stellar Core Collapse Speeds

Question

If gravity determines the speed of free falling objects, but stellar core collapses happen much faster than the speed of free falling objects, wouldn't that indicate that stellar core collapses are caused by something other than, or in addition to, gravity? Sorry, I'd like to amend question to ask if the speed at which collapsing matter is pulled to the core of a collapsing star is equal to the speed of free falling matter for a star that size?

Answer 1

Core collapse does occur on roughly a free fall timescale.

The free fall timescale is of order $(G\rho)^{-1/2}$, where $\rho$ is the density. Since the density is of order $10^{11-12}$ kg/m$^3$ in the electron-degenerate core at the point of collapse, this timescale is a second or less.

Since density increases inwards, the collapse timescale is shorter near the centre. Thus the collapse occurs inside-out.

The speed of collapse is of order the size of the core (a few thousand km) divided by the freefall timescale, so of order $10^4$ km/s.

The core has a mass of $\sim 1$ solar mass, so the gravitational acceleration is of order $GM/r^2\sim 10^4$ km/s$^2$. i.e. the whole core accelerates to speeds of $10^4$ km/s in a second. This is the acceleration at the "edge" of the core at the beginning of the collapse; it will be higher towards the centre where the densities are larger and will increase as the collapse progresses because the same mass will be enclosed within a decreasing volume of smaller radius.

Answer 2

Core collapse speeds cannot in any case exceed the speed of light. Information about the collapse is transmitted between different portions of the core at the speed of sound within the core material. This is fast, but not c.