Collapse the star had a mass of 2 times the mass of the sun and a radius the same as the sun. before the collapse it rotates once every 10 days. after the collapse it has not lost any mass but now has a radius of only 10 km. what is its rotation rate in rev/s after the collapse

0.2 milisecond

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

the working equation for the angular size is the product of the physical size and 360 degrees divided by the product of two pi and distance from earth. the minimum size would be 0.559 degrees while the maximum size would be 0.49 degrees.