Answer:
0.2 milisecond
Step-by-step explanation:
Let the mass of sun is M
mass of star = 2 M
initial radius of the star, R = radius of sun = 7 x 10^5 km
initial time period, T = 10 days
final radius, R' = 10 km
mass remains same.
As no external torque is acting on the star so its angular momentum remains constant.
So, Angular momentum before collapsing = Angular momentum after collapsing
I x ω = constant




T' = 2.04 x 10^-9 days
T' = 0.2 milisecond