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Conservation of Angluar Momentun question

  • #1

Homework Statement



When a star like our Sun no longer has any hydrogen or helium "fuel" for thermonuclear reactions in its core, it can collapse and become a white dwarf star. Often the star will "blow off" its outer layers and lose some mass before it collapses into the rapidly spinning, dense white dwarf. Suppose a star with mass 1.0 Msun , with a radius of 6.96×108 and rotating once every 25 days, becomes a white dwarf with a mass of 0.60 Msun and a rotation period of 131s.

What is the radius of this white dwarf? (You may assume the progenitor star and the white dwarf star are both spherical.)

Homework Equations



I'm assuming that there are no external torques so it's angular momentum is conserved.

So,
I1[tex]\omega[/tex]1=I2[tex]\omega[/tex]2

rotational inertia of a solid sphere I=(2/5)mr2

[tex]\omega[/tex]1=2.91*10-6 rad/s (from 1 rotation every 25 days)
[tex]\omega[/tex]2=0.04796 rad/s (from rotation period of 131s)


The Attempt at a Solution



After rearranging the angular momentum equation from above:

r22=(m1r12[tex]\omega[/tex]1)/(m2[tex]\omega[/tex]2)

r22=[(1.0 Msun)(6.96*108)2(2.91*10-6)]/[(0.6 Msun)(0.04796)]

so r2=6.99*106m



The problem I'm having is that when I try to enter in this answer into masteringphysics it tells me I'm wrong. I've checked over my work a few times but I can't see my error anywhere.

EDIT:
Looks like the same problem was posted here: https://www.physicsforums.com/showthread.php?t=255031

I guess I'll just have to confirm with the professor tomorrow.
 
Last edited:

Answers and Replies

  • #2
lightgrav
Homework Helper
1,248
30
no ... 0.4 solar mass was ejected , carrying off *their* portion of angular momentum.

so ... if that came from the outer Sun shell, its retained (effective) radius started at only
cuberoot(0.6) of what you were using.
 

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