# Volume Vs Mass - Kepler 7b

by Ian J.
Tags: kepler, kepler-7b, mass, polystyrene, volume
 Mentor P: 9,614 The core density is relatively high (and pressure and temperature are really high), the density in the outer regions (>99.99% of the volume) is low, giving a low average density.
P: 10
 Quote by mfb That's not the order of effects - gravity compresses stuff until there is an equilibrium between pressure gradient and gravitational forces.
Yes, I understand that. I did not mean to imply that gravity is determined by volume. Mass and density determine the gravitational effects, and that in turn determines the volume.

If there is insufficient mass, there will be no fusion, because gravity will not be able to compress the material sufficiently. However, if you could introduce more mass, then the effects of gravity increases, compressing the gas (reducing the volume) in the process, and thus increasing the density of the material.

 Quote by mfb Without gravity there would be no planets or stars at all.
Agreed.

 Quote by mfb How would that object be stable? It would just expand until it is a regular planet.
A ~13 Jupiter mass planet with a density of 2.156 g/cm3 would have a radius (not volume, I am paying closer attention) only twice that of Jupiter. Why would that be unstable? I do not understand.
P: 10
 Quote by mfb The core density is relatively high (and pressure and temperature are really high), the density in the outer regions (>99.99% of the volume) is low, giving a low average density.
I was under the impression that density was calculated by dividing the mass of an object by its volume. While I do understand that it is merely a mean average density, I was not aware that density was also determined by the distance one was from the center of an object.

Could you please point me to someplace were I might find the equations for calculating this graduating density?

Thanks.
Mentor
P: 9,614
 Quote by |Glitch| If there is insufficient mass, there will be no fusion, because gravity will not be able to compress the material sufficiently. However, if you could introduce more mass, then the effects of gravity increases, compressing the gas (reducing the volume) in the process, and thus increasing the density of the material.
That's what happens at ~13 Jupiter masses.

 A ~13 Jupiter mass planet with a density of 2.156 g/cm3 would have a radius (not volume, I am paying closer attention) only twice that of Jupiter. Why would that be unstable? I do not understand.
Why do you think such an object would exist? It would have a volume similar to Jupiter, slowly fusioning deuterium.

 I was not aware that density was also determined by the distance one was from the center of an object.
It is not "determined" by that, but density can depend on the position in the object. For stars, the density is mainly a function of the radius and decreases with increasing radius.
This is given by the hydrostatic equilibrium.

 Related Discussions General Astronomy 1 General Astronomy 1 General Physics 7 Astrophysics 3 Calculus & Beyond Homework 1