Maths/astronomy help!

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April 27th 2009, 05:08 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.
April 27th 2009, 05:55 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.
April 27th 2009, 05:56 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.
April 27th 2009, 10:04 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.
April 27th 2009, 02:20 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 $\frac{4}{3} \pi r^3$ .
April 30th 2009, 01:58 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 $\frac{4}{3} \pi r^3$ .

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

Is that even right? And how could i get a fraction out of that without figures?
April 30th 2009, 03:01 AM
mr fantastic

Quote:

Originally Posted by emmaastronomy

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