# Homework Help: White dwarf mass-radius relationship

1. Mar 8, 2013

### PeteWheatstraw

instead of typing it out, here is the problem

I know what to do, my math just isn't good enough to combine the equations properly. Been at it a few hours, brain hurts, please help!

2. Mar 8, 2013

### collinsmark

Hello PeteWheatstraw,

Welcome to Physics Forums!
As part of the forum rules, you need to show your work. If you get stuck, you need to show us where you're stuck. But you at least need to show what you've done so far, in order for help.

But for what it's worth (very general advice), start by substituting mass over volume for the density $\rho_c$, where the volume is the the volume of a sphere, $\frac{4}{3} \pi R^3$, and the mass is M.

After rearranging to isolate R, apart from the other variables, you'll end up with a bunch of numerical constants, many of which are under exponents. Break these numbers into their prime factors (e.g., 8 → 2·2·2), multiply out the exponents if necessary (e.g., 35/3 → [3·3·3·3·3]1/3) and regroup things so that you can get the numbers to be under the exponents that you want (e.g., [3·3·3·3·3]1/3 → 3·[3·3]1/3 → 3·32/3).

Last edited: Mar 8, 2013
3. Mar 9, 2013

### PeteWheatstraw

Thanks for the tips.

some how I end up with (I'd type it all out but it's late)...

2∏R/3G=1/M^-1/3

and I'm stuck, and I know its wrong in the 1st place...I'll try again in the morning. Really my issue is I have no idea what to do with the 5/3 exponent and when to do it.

Thanks again though.