A star the size of our Sun runs out of nuclear fuel and, without losing mass, collapses to a white dwarf star the size of our Earth. The radius of our Sun is 6.96×108m , the radius of Earth is 6.37×106m .

Question

A star the size of our Sun runs out of nuclear fuel and, without losing mass, collapses to a white dwarf star the size of our Earth. The radius of our Sun is 6.96×108m , the radius of Earth is 6.37×106m .
If the star initially rotates at the same rate as our Sun, which is once every 25 days, determine the rotation rate of the white dwarf.

bobpursley · Answer

angular momentum remains the same.

momentinertia*w=finalmomentinertia*wf

2/5(mri^2)wi=2/5(mrf^2)wf

wf=wi*(ri/rf)^2
wf=2pi/25 *(6.96E8/6.37E6)^2 rad/day

wf=3000 rad/day or 477 rotations/day

check the math.

Helpful · Answer

mastering physics requires the answer to be in rad/sec so the answer ends up being .035 rad/s