# Maths/astronomy help!

#### emmaastronomy

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.

Last edited:

#### mr fantastic

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.

#### HallsofIvy

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.

#### emmaastronomy

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.

#### mr fantastic

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 $\displaystyle \frac{4}{3} \pi r^3$.

#### emmaastronomy

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?

#### mr fantastic

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.