# Maths/astronomy help!

• Apr 27th 2009, 04:08 AM
emmaastronomy
Maths/astronomy help!
For a star of constant density, calculate the fraction of the total mass inside a core of radius R/4, if the radius of the star is R.

I have no idea.

Thanks for any help.
• Apr 27th 2009, 04:55 AM
mr fantastic
Quote:

Originally Posted by emmaastronomy
For a star of constant density, calculate the fraction of the radius R/4, if the radius of the star is R.

I have no idea.

Thanks for any help.

This question is incomplete. Please re-check it and then post the complete question.
• Apr 27th 2009, 04:56 AM
HallsofIvy
Quote:

Originally Posted by emmaastronomy
For a star of constant density, calculate the fraction of the radius R/4, if the radius of the star is R.

I have no idea.

Thanks for any help.

Sorry, but I have no idea what "calculate the fraction of the radius R/4" means. Since you mention density, do you mean calculate the mass out to radius R/4 or the fraction of the total mass that is? Do you know the formula for volume of a cube? Mass is just density times volume.
• Apr 27th 2009, 09:04 AM
emmaastronomy
Quote:

Originally Posted by HallsofIvy
Sorry, but I have no idea what "calculate the fraction of the radius R/4" means. Since you mention density, do you mean calculate the mass out to radius R/4 or the fraction of the total mass that is? Do you know the formula for volume of a cube? Mass is just density times volume.

Yes you are right. It should read For a star of constant density, calculate the fraction of the total mass inside the core of radius R/4, if the radius of the star is R.
• Apr 27th 2009, 01:20 PM
mr fantastic
Quote:

Originally Posted by emmaastronomy
Yes you are right. It should read For a star of constant density, calculate the fraction of the total mass inside the core of radius R/4, if the radius of the star is R.

Start with the facts that mass = density times volume and that the volume of a sphere is $\displaystyle \frac{4}{3} \pi r^3$.
• Apr 30th 2009, 12:58 AM
emmaastronomy
Quote:

Originally Posted by mr fantastic
Start with the facts that mass = density times volume and that the volume of a sphere is $\displaystyle \frac{4}{3} \pi r^3$.

So to find the mass of the core = (density x $\displaystyle \frac{4}{3} \pi r^3$ ) / 4

Is that even right? And how could i get a fraction out of that without figures?
• Apr 30th 2009, 02:01 AM
mr fantastic
Quote:

Originally Posted by emmaastronomy
So to find the mass of the core = (density x $\displaystyle \frac{4}{3} \pi r^3$ ) / 4

Is that even right? And how could i get a fraction out of that without figures?

You have to substitute r = R/4 to get the mass of the core.

You have to substitute r = R to get the total mass of the star.

To get the required percentage, you have to calculate (core mass)/(total mass) times 100.