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  1. #1
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    Change in Gravity

    When a star goes through itís life cycle does the amount of gravity it exerts change or is it always constant? Our sun is fairly stable right now but when it turns into a red giant and then into a white dwarf, does itís gravitational field change strength? Youíd think that when it goes super nova itís gravitational pull would become weaker because it has expelled lots of material.
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    Weill a star radiates away energy, so it depends where you draw its boundary, it gets lighter. If it goes supernova and explodes, the mass is ejected along with energy, which is why we see it, so the mass will fly away in all directions but if you draw a boundary around that, it will still be at a net gravity centre. Stars also emit massive particles so they are losing mass. Then they attract mass too from the gas cloud around them. They do not shrink to nothing, at some point their gravity pressure will not be enough to keep the fusion core going. So the star lights up when the gravity attraction of its gas cloud creates enough pressure. Its gravity value may include a gas halo.
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    Quote Originally Posted by profloater View Post
    Weill a star radiates away energy, so it depends where you draw its boundary, it gets lighter. If it goes supernova and explodes, the mass is ejected along with energy, which is why we see it, so the mass will fly away in all directions but if you draw a boundary around that, it will still be at a net gravity centre. Stars also emit massive particles so they are losing mass. Then they attract mass too from the gas cloud around them. They do not shrink to nothing, at some point their gravity pressure will not be enough to keep the fusion core going. So the star lights up when the gravity attraction of its gas cloud creates enough pressure. Its gravity value may include a gas halo.
    I was thinking the sun boundary being where the solar wind starts. Can solar wind also include solar mass ejections or is that a different beast. Correct me if I’m wrong, that means the solar wind is actually causing the sun to loose mass which means it also looses some of its gravitational pull as well. But as a solar system the overall gravitational loss is nothing. Seems like going super nova would cause a lot of the matter to actually escape the gravitational pull of the star. How else will matter from one solar system get “used” by another?
    I know that I know nothing, so I question everything. - Socrates/Descartes

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    First of all, the sun is not nearly massive enough to go supernova. But it will expand into a red giant, expel most of its mass and then collapse into a white dwarf.

    An object's gravitational pull is a function of its mass, so as the sun ages it will slowly lose mass through radiation and solar wind, though it's a very small amount percentage wise until it reaches the later red giant stages where it blows off its outer layers. The final white dwarf remnant will be a fraction of the original Sun's mass and as such its gravitational influence at a given distance will be a lot less than now. The planets' orbits will move away from the Sun as this happens, and as a result even if the sun expands beyond 1 AU, the Earth may not be engulfed if it strays far enough away.

    If you take the solar system as a whole, its overall mass won't change much if at all as the sun dies, as the total mass will still be there as ejected gas and dust, just not all within the sun. An alien race in a space ship measuring the solar system's total gravitational pull at say 1 parsec won't notice much of a change, if at all, until the material reaches that point. Most of the mass loss will be from photons and neutrinos leaving the solar system at the speed of light. Over time the ejected material will gradually disperse.

    But at closer (planetary) distances, there will be a change, and the planets' orbits will change as a result.

    In the case of a larger star that explodes as a supernova, it's basically the same principle, only bigger and (a lot) faster. Most of the mass will be in the remnant nebula, which will eventually dissipate. Some will be in the remnant core (neutron star or black hole). As the ejected material passes an orbiting planet (if the explosion doesn't destroy it), the star's gravitational influence on the planet will quickly change and the planet's orbit will be disrupted. I wonder if in some cases planets can be thrown from a solar system in a supernova explosion. I don't know the answer to that.

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    Quote Originally Posted by kpatz View Post
    ...The final white dwarf remnant will be a fraction of the original Sun's mass and as such its gravitational influence at a given distance will be a lot less than now. The planets' orbits will move away from the Sun as this happens, and as a result even if the sun expands beyond 1 AU, the Earth may not be engulfed if it strays far enough away....
    Thanks for that excellent rundown, kpatz. I hadn't before heard the bolded part above -- a rather existential question regarding dear Earth itself! But of course the widening orbit makes sense. The only question being the timescale that the sun casts off its mass far enough away such that it loses some of its gravitational grip on the Earth.

    So I found it interesting that you mentioned, with respect to the solar system region as a whole, "Most of the mass loss will be from photons and neutrinos leaving the solar system at the speed of light." I can attest that the Sun is indeed pumping out massive amounts of photons continually! I imagine the solar wind would add a significant part of that loss [eta: over time]. So if such be the case for a large part of the timeline of the mass loss, the widening orbit would then be gradual, which is a good thing I think, since it means Earth's orbit is slowly but surely getting out of the way of the Sun's later, less hospitable stages. I wonder if such widening of an orbit would necessarily be accompanied by a slow increase in the ellipticity of the orbit....

    Or is there a more "sudden," huge mass loss in the Sun's late stages that might be "too late" to widen Earth's orbit enough, so the planet gets enveloped and scoured by the expanding Sun?
    Last edited by Cougar; 2021-Apr-21 at 12:03 AM. Reason: bracketed text
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    Quote Originally Posted by Cougar View Post
    Or is there a more "sudden," huge mass loss in the Sun's late stages that might be "too late" to widen Earth's orbit enough, so the planet gets enveloped and scoured by the expanding Sun?
    Won't Earth get hit with whatever the Sun expels anyway? It's scoured inside or scoured outside, from what little I understand.

    Certainly there's no way it'll be a livable environment by then anyway. It won't be the Earth we know now.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  7. #7
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    This paragraph confuses me, particularly the last sentence

    Quote Originally Posted by kpatz View Post
    If you take the solar system as a whole, its overall mass won't change much if at all as the sun dies, as the total mass will still be there as ejected gas and dust, just not all within the sun. An alien race in a space ship measuring the solar system's total gravitational pull at say 1 parsec won't notice much of a change, if at all, until the material reaches that point. Most of the mass loss will be from photons and neutrinos leaving the solar system at the speed of light. Over time the ejected material will gradually disperse.
    The reason for my confusion is that first you say the over all the solar mass won't change. Then in the last sentence you say ejected material will disperse. Maybe I'm just reading it wrong but to me disperse means leave, spread out and go away from.
    I know that I know nothing, so I question everything. - Socrates/Descartes

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    Quote Originally Posted by DaCaptain View Post
    The reason for my confusion is that first you say the over all the solar mass won't change. Then in the last sentence you say ejected material will disperse. Maybe I'm just reading it wrong but to me disperse means leave, spread out and go away from.
    While the sun is on the main sequence it loses relatively little mass, from fusing hydrogen to helium and losing mass as light, and solar wind. It’s pretty impressive in absolute terms, but isn’t much mass relative to the sun’s total mass, since it is so very massive compared to everything else in the solar system. There will be little change for billions of years. Then in around five or six billion years the sun will go off the main sequence and become a red giant. It will expand dramatically with a much higher luminosity, and the outer layers will become very thin. Without getting into too much detail, it will reach a point where it will lose large amounts of mass as solar wind. Eventually, the remaining white dwarf will be significantly less massive than the sun now.

    That mass from the extreme solar wind will disperse over time, though for a short time (thousands of years) it will form what is called a planetary nebula:

    https://en.wikipedia.org/wiki/Planetary_nebula

    One other thing to note, the mass of the sun that goes beyond the Earth’s orbit might as well not exist as far as the sun’s gravitational attraction on Earth goes. On the other hand, a distant hypothetical observer measuring the mass of the solar system won’t detect a change until the lost mass goes beyond them as well. This involves the shell theorem:

    https://en.wikipedia.org/wiki/Shell_theorem

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