Science Questions

Why aren't planets compressed by gravity like stars are?

Sun, 6th Sep 2009

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Question

Stephanie asked:

As I understand it the nuclear fusion of stars stops gravity from compressing them until they cannot be compressed anymore because of the force of the neutrons repelling each other and when stars run out of fusible materials they are compressed to this point. What keeps the planets from being compressed by gravity as the stars are when they run out of fusible materials?

Answer

Dave -   Well, yeah.  Star, it’s not actually the fusion which is holding the stars up directly.  It’s actually their temperature.  If you had a gas, the hotter it is the more pressure it will exert, the harder it would push out.  So stars are basically supported, they’re basically made out of very, very hot gas - plasma that are supported by their temperature.  So if a star gets hotter it will expand, star cools down it will shrink.  A planet doesn’t have to be supported like that, planets are made out of solid, lumps of things they’re basically supported by the repulsion between atoms and molecules, in the same way as the table is supported or you’re supported.  So they are not big enough for the need the temperature to support them and basically just molecules and atoms are strong enough.

Chris -   Because planets like Jupiter are just around the threshold of what we call brown dwarfs, aren’t they, they’re failed stars are not quite big enough to squeeze themselves enough to trigger a fusion to actually get going.

Dave -   Yes, small stars it can also be supported just by this molecular strength basically.

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Stephanie asked the Naked Scientists: As I understand it the nuclear fusion of stars stops gravity from compressing them until they cannot be compressed anymore because of the force of the neutrons repelling each other; when stars run out of fusible materials they are compressed to this point. So what keeps the planets from being compressed by gravity as the stars are when they run out of fusible materials? What do you think? Stephanie, Sun, 6th Sep 2009

A planet's mass is much lower than a star's mass. lightarrow, Sun, 6th Sep 2009

. . . . .  so the internal gravitational attraction is not enough to cause crushing. The atoms are strong enough to withstand the force in planets.

(added for clarity, lightarrow - OK?) lyner, Sun, 6th Sep 2009

Ok. I was lazy, didn't want to add anything else  lightarrow, Sun, 6th Sep 2009



But, for some stars, humangeous super nova explosion compress the centre of the star...

...i think... wanhafizi, Mon, 7th Sep 2009

The radiation implosion of a super-nova that results in a black hole isn't really too different from the radiation implosion that is used to start the fusion process in fission-fusion and fission-fusion-fission bombs.

I believe that much of the initial mass of a pre-super-nova star is blown away by the explosion and the mass of the resulting neutron star or BH is far less than the star's original mass i.e. although our sun could form a BH if it were 1.4 times larger, the resulting BH would have less than 1.4 solar masses.

Not sure about this though, off hand. LeeE, Mon, 7th Sep 2009

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