Why does gravity increase when a star dies?

[HenryKhais]

I was curious as to why when a star discontinues fusion its gravitational pull grows stronger?

My guess - When a star is actively burning, the fusion is somehow warping space and/or suppressing the gravitational pull. When the star stop fusion, this force is no longer present and the star feels the true gravity caused by its mass, thus collapsing on itself and forming a black hole.

If possible, please keep the answer simple.

Thanks :)

 

[ModusPwnd]

Does it? I don't think so...

Do you mean why the gravity is able to pull the density higher? I think that is because the fusion stops so the star can collapse in on itself. The fusion balances the gravity. Fusion stops - gravity can pull the star in on itself. The gravity is the same though.

Its not what you might think off hand, but when a star collapses into a black hole it doesn't necessarily exert any more gravitational pull. The gravitational pull is a function of its mass. If its mass goes down via some kind of supernova explosion then its gravitational pull will decrease. Then if it sucks mass back in, its gravitational pull will increase.

 

[Drakkith]

The star's overall gravity does not increase, the star just gets denser. Since gravity is stronger the closer objects are, when the star shrinks gravity is now stronger between all parts of it since they are now closer together. If the Sun were to collapse, the gravity exerted on itself would be greater since it is much denser and smaller, but the orbits of the planet would not change because they did not get any closer to the Sun.

The core of the star resists collapsing thanks to the outward pressure generated by the temperature of the gas/plasma. During the star's main sequence phase, fusion serves to replace the lost energy radiated away from the core, keeping the temperature of the core stable. Otherwise it would would go through a continual process of shrinking, heating up, losing energy, shrinking, and heating up more.

This may seem a bit counter-intuitive, but the core is a volume of hot gas under pressure. It resists the inward pressure thanks to the outwards force it gets from being hot. If energy is radiated away from the core, then it drops in temperature and loses a bit of outward force and the inward pressure compresses it, causing it to shrink. Since compressing a gas causes it to heat up, the core ends up with a slightly smaller volume and a slightly higher temperature than before. The temperature is higher than before because the star shrinks and the inward pressure due to gravity increases (because the star is now denser), requiring the core to be at a higher temperature to exert enough force to counteract this higher pressure. This is a continual process, the core undergoes all phases of this process simultaneously. The net result is that, without a way to replace this lost energy, the core gradually shrinks and grows hotter.

During main sequence, the energy lost from the core is replaced by fusion, so the temperature never falls and the core doesn't shrink. (Technically it does, but this takes place over a few billion years instead of a few million and is due to gradual loss of hydrogen fuel in the inner parts of the core) Once the fuel runs out the core undergoes a slow collapse like I explained above until the temperature is high enough to fuse helium and other elements (if the star is massive enough).

 

[enorbet]

@HenryKhais - Your "guess" is actually pretty close. Your title and the assumption that gravity increases is the only thing that's wrong. It might be easier to start at the beginning rather than jumping right to Black Holes. Consider a Neutron Star instead, or even our own Sol. Whether a star is destined to form one or the other, or something else, is completely a factor of mass.

Our star, Sol, one solar mass, will never become either a black hole or a neutron star. It will become a Red Giant and then finally blow off the outer layers into a planetary nebula and shrink to a White Dwarf which will ultimately cool into a Black Dwarf. The gravity will be less than Sol as we know it because some 45% of it's mass will have been shed but the 55% remaining will occupy roughly .001 (1/1000th) the original stellar volume. That much gravity in that little space is nothing short of intense but it isn't greater than what the Sun had at one solar mass.

If you enjoy thinking of these events on these scales you might like http://faculty.wcas.northwestern.edu/~infocom/The Website/evolution.html a solid overview without being overly complex or talking down... a nice balance for most people.