For stars to lose their stability.

[Iroscato]

This is just a question that popped into my head, so I will see if I can ask it clearly.

Basically, how much weaker would gravity have to be for stars to no longer form? Is there a specific figure or percentage? Like if gravity had been 0.001% weaker, would that have done it? How narrow is the margin?

[Napoleon the Clown]

What do you mean? For stars to no longer be able to sustain fusion? Because any gravity is enough for matter to coalesce. If you mean sustain fusion, decreasing the gravitational constant would just mean that more mass would need to be added for it to be able to sustain fusion.

[Iroscato]

What do you mean? For stars to no longer be able to sustain fusion? Because any gravity is enough for matter to coalesce. If you mean sustain fusion, decreasing the gravitational constant would just mean that more mass would need to be added for it to be able to sustain fusion.

Umm, right, hadn't though of it that way. Bear in mind, the highest science qualification I rcieved was GCSE (16 years old), so I swim in murky waters
The most frustrating thing is that my great passion in life is science, but I don't know much about it.

OK, so say you took the sun itself, transported it somehow to another universe with gravity that was weaker by around 1-5%, what would happen?

[Simon_Jester]

After several thousand years, it would become slightly more orange-ish as the core cooled down a bit.

The really significant consequences would be for planetary orbits. Orbits that were circular or near-circular before, and stable, won't stay the same if you tweak the gravitational constant. They'll become considerably more elliptical, and possibly less stable.

[Iroscato]

After several thousand years, it would become slightly more orange-ish as the core cooled down a bit.

The really significant consequences would be for planetary orbits. Orbits that were circular or near-circular before, and stable, won't stay the same if you tweak the gravitational constant. They'll become considerably more elliptical, and possibly less stable.

Right, so a slight decrease in gravity would just lead to a slight decrease in temperature? Hmm, that's disappointing

[Napoleon the Clown]

Our sun currently has many times the mass needed to sustain fusion given its current composition, so you'd need to reduce the gravitational constant by a lot. I'm sure others would have actual calculations on hand to give something more than a vague answer. It is an interesting thought exercise, though.

[madd0ct0r]

you'd want to look at manipulating the forces involved in the fusion reaction - the strong and weak nuclear forces.

[Surlethe]

Here's how you find out the answer. First, you take Physics I and II. Then you take mechanics and statistical mechanics and thermodynamics. Then you take a two-semester astrophysics sequence.

Just kidding. You can do a rough approximation of this at home. You need to find the pressure and temperature at the center of a star of mass M (composed of an ideal hydrogen gas, so ignore all that bullshit about plasma and magnetic currents and convection and so forth). Then you need to relate the pressure and temperature to the rate of fusion.

Use the ideal gas law, the force law, and the virial theorem(?) to do the first. Then check to see how much mass you need to get fusion to start. This gives you a relationship between mass and G.

[Simon_Jester]

After several thousand years, it would become slightly more orange-ish as the core cooled down a bit.

The really significant consequences would be for planetary orbits. Orbits that were circular or near-circular before, and stable, won't stay the same if you tweak the gravitational constant. They'll become considerably more elliptical, and possibly less stable.

Right, so a slight decrease in gravity would just lead to a slight decrease in temperature? Hmm, that's disappointing

Well, that and planets careening out of their orbits into deranged and possibly unstable crack-ellipses, causing life on Earth to experience really nasty seasonal temperature variations and such.

At least, I think that would happen.

[starslayer]

Eh? Changing G won't affect the shape of orbits; that's controlled by the exponent on r. Orbits work nicely only (i.e., are stable and closed) if the force is an inverse-square law. If you change that exponent even a tiny bit, things start to go wonky. First, orbits precess much more readily and never form closed ellipses; as the change gets larger, orbits start to get very unstable, so the slightest tug either sends objects crashing into each other, or everything becomes unbound.

All messing with G does is change the strength of gravity relative to the other forces, so if you make G smaller, gravity gets weaker and orbits enlarge. They still play nice thanks to that 2 in r2.

[Surlethe]

Eh? Changing G won't affect the shape of orbits; that's controlled by the exponent on r. Orbits work nicely only (i.e., are stable and closed) if the force is an inverse-square law. If you change that exponent even a tiny bit, things start to go wonky. First, orbits precess much more readily and never form closed ellipses; as the change gets larger, orbits start to get very unstable, so the slightest tug either sends objects crashing into each other, or everything becomes unbound.

All messing with G does is change the strength of gravity relative to the other forces, so if you make G smaller, gravity gets weaker and orbits enlarge. They still play nice thanks to that 2 in r2.

I think he meant that if the change happened suddenly the planets' orbits would change from nearly circular to highly elliptical or unbound.

[Kuroneko]

Well, that and planets careening out of their orbits into deranged and possibly unstable crack-ellipses, causing life on Earth to experience really nasty seasonal temperature variations and such.

At least, I think that would happen.

Not at the scale the OP assumes. A 5.0% decrease of gravity of the Sun would make the Earth's orbital eccentricity about 0.23 with ~1.0 AU semi-minor axis and ~1.03 AU semi-major axis. I'm not sure that constitutes nasty; it might actually be better than the global warming we have now. For any circular orbit, the total energy is half the gravitational potential energy, so for a hypothetical instantaneous magical change, it would take about half the Sun's gravity to make orbits of the planets unbound, as most of them have nearly circular orbits.

Given a purely radial force, specific angular momentum l = r²(dθ/dt) is conserved, no matter how that force varies with time. The new eccentricity e will satisfy e²=1+2εl²/μ², where μ = GMSun and we can equate the specific energy ε to the effective potential:
[1] V = -μ/r + l²/2r²
since for an initially circular orbit, dr/dt = 0 means that all kinetic energy is in the angular momentum. This gives:
[2] e² = l²/(rμ) - 1 = r³(θ')²/μ - 1
Since initially e² ~ 0, the new orbit with e² ~ 1 (parabolic orbit) occurs at μnew = μold/2, as expected.

Use the ideal gas law, the force law, and the virial theorem(?) to do the first. Then check to see how much mass you need to get fusion to start. This gives you a relationship between mass and G.

The assumptions of the ideal gas law would be strongly violated at stellar cores. We'd better use a Fermi gas model... though rather than messing about with some complicated equation of state, we can still get a back-of-the-envelope result. Let's take a look at what might mean to change a dimensionful constant to get some back-of-the-envelope results. We can always take units of G = 1, in which case modifying gravity looks like changing all the masses.

We're trying to keep the all the physics the same as much as possible, so in particular the ratio of masses of all particles should stays the same. That means if we pick a particular particle of mass m, what we're really doing is modifying the dimensionless gravitational coupling β = Gm²/(ℏc) = (m/mPlanck)². For a young star, it makes sense to pick a proton, or some kind of averaged mass per nucleon for older ones. Let's say you have a ball of N nucleons and radius r. Adding a small number of nucleons decreases decreases the gravitational potential nucleon by G(Nm)m/r ~ βN/r ~ βN2/3). We can also vary the strengh of electromagnetism:
-- The number of nucleons in stars scales as (α/β)3/2,
where α is the electromagnetic coupling strength (fine structure constant, α ~ 1/137).

[NoXion]

My understanding is that stars are in some sort of equilibrium, with the outward pressure of stellar fusion being counteracted by gravitational contraction. My question is, would it be possible to make a star explode by temporarily suspending gravity within its volume?

[StarSword]

My understanding is that stars are in some sort of equilibrium, with the outward pressure of stellar fusion being counteracted by gravitational contraction. My question is, would it be possible to make a star explode by temporarily suspending gravity within its volume?

It works on Stargate. Don't know for sure if it works in real life.

[Eternal_Freedom]

It's called hydrostatic equilibirum. And yes, if you suspend gravity throughout the star's volume it will explode. Consider:

-The force of the nuclear reactions at the core are constantly pushing the matter of the star outwards
-The star remains intact
-Therefore something is holding it together

That something is the star's gravity. In other words, the forces on the mass of the star are balanced

Explosion outwards = Gravitational pull inwards

Remove that gravitational pull and there is nothing to balance the equation, the the star (certainly the outer layers) will expand rapidly outwards.

[Steel]

It's called hydrostatic equilibirum. And yes, if you suspend gravity throughout the star's volume it will explode. Consider:

-The force of the nuclear reactions at the core are constantly pushing the matter of the star outwards
-The star remains intact
-Therefore something is holding it together

That something is the star's gravity. In other words, the forces on the mass of the star are balanced

Explosion outwards = Gravitational pull inwards

Remove that gravitational pull and there is nothing to balance the equation, the the star (certainly the outer layers) will expand rapidly outwards.

Unfortunately from such a simple analysis you can't really predict an explosion, as you don't have any idea how dynamic an equilibrium it is in.

It could even be that the effect of removing gravity is that the star very slowly drifts apart over billions of years, and in fact only becomes of low enough density to die at the time it would have run through its fuel or even later as the fusion was proceeding at a slower rate with the lower density!

Of course that is unlikely to be the case, but you can't just make a general statement on the exact effects without some quantification, and indeed the effect of reducing gravity could easily be the opposite to what you expect. Hence astrophysics.

[Eternal_Freedom]

That's all true, but NoXion didn't ask about reducing gravity so much as completely nullifying it within a star. At the very least you'll then have a lot of ionised plasma in a dense area. All that plasma wil immediately repulse the other plasma around it. So just from electrostatic forces alone the star will dissipate.

You are of course correctt hat it may not be fast enough to constitute an explosion, but it will certainly be a very bad thing for the star in question.

More to the point, if we completely nullified the gravity within a star, wouldn't that also remove the star's gravitational pull on other bodies in the solar system? Of course it would take time for the effects to propogate but even a vague geuss tells me that would be a bad thing.

[NoXion]

Crap, I forgot to account for that. If gravitation propagates at lightspeed (I'm not sure why I'm assuming this, I think I read it somewhere), then even if the amount of time needed to suspend gravity long enough to cause destablisation of the stellar body is relatively instaneous, it will have some sort of effect, I would think. I'm guessing here, but I reckon that everything orbiting the Sun will straighten their trajectories as the break in gravity passes through them, with the effect of widening their orbital distances (at least for more circular orbits, I have no idea what would happen to the more elliptical orbits like those of comets for example). So in addition to the Sun either exploding or becoming cooler, the planets are all effectively pushed away from the Sun. Bloody hell.

[Kuroneko]

At the very least you'll then have a lot of ionised plasma in a dense area. All that plasma wil immediately repulse the other plasma around it. So just from electrostatic forces alone the star will dissipate.

The plasma consists of ionized particles, but it is neutral. But the high temperature/thermal speed means that changes will propagate fairly quickly... though the upper ends of Steel's examples are incredible, he's correct in that a better model is needed to estimate the timescale.

More to the point, if we completely nullified the gravity within a star, wouldn't that also remove the star's gravitational pull on other bodies in the solar system? Of course it would take time for the effects to propogate but even a vague geuss tells me that would be a bad thing.

See my post above--it takes only halving gravity to make the planets unbound. Beyond that, and you get some hyperbolic escape orbit, which becomes a line in the limit of no gravity at all. Though nullifying gravity within a star is not necessarily mean nullifying its effects outside of the star.

If gravitation propagates at lightspeed (I'm not sure why I'm assuming this, I think I read it somewhere), then even if the amount of time needed to suspend gravity long enough to cause destablisation of the stellar body is relatively instaneous, it will have some sort of effect, I would think.

The hypothetical scenario is already practically magic. That gravitational waves travel at lightspeed is a prediction of our leading theory of gravity, GTR... the same theory that would be directly falsified by the Sun no longer gravitating. So it's senseless to apply it here, and no reason to believe that its other predictions are applicable. Though an instantaneous drop in gravity is not inconsistent with Newtonian gravitation.