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B Why do stars experience gravity more than its mass?

  1. Jun 11, 2016 #1
    I will try to be as literal as I can.

    I was researching about the black hole, according to NASA and much other organisation a black hole form when a star collapses under its own gravity. "Stellar black holes form when the center of a very massive star collapses in upon itself". Gravity is directly proportional to mass, considering star as the sun in this situation. Why would it face such a gravity that is enough for to implode itself? Whereas Earth doesn't implode under its own gravity, given the current arrangement of the solar system .

    This raises another question, what if the earth is in a vacuum provided enough space , will it implode? Also does sun's implosion has anything to do with the gravity that it influences on other planets?
     
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  3. Jun 11, 2016 #2

    phinds

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    Black holes are created when a large enough mass gets concentrated in a small enough volume. Pretty much ANYTHING is enough mass to create a black hole, but only if you can get it condensed down to a small enough size. Right now the sun's gravity is balanced by the internal pressure of fusion so it cannot collapse to a small enough size to become a black hole. This is all explained in great detail in a large number of internet entries on the formation of black holes.
     
  4. Jun 11, 2016 #3
    Different YouTubers and references have different opinions, so I don't know what to choose. So you mean to say when matter gets dense to a certain point it becomes a black hole?

    That's nice to know but I'm afraid my actual question isn't answered to my satisfactory level. I want to know, why is sun imploding in the first place? Other objects do have gravity which is proportional to their mass. So, why don't they experience "implosion" but sun do?



    P.S. You guys are AWESOME!! and thanks for all your answers you've provided me with before
     
  5. Jun 11, 2016 #4

    phinds

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    Suns implode when they run out of fuel for the fusion process which then is no longer there to balance gravity. There are no differences of opinion on this as far as I am aware; it is a well understood process and should be explained without confusion on any reputable science page.
     
  6. Jun 11, 2016 #5
    All things with mass have gravity. Not all things have enough mass in a small enough area to collapse into a black hole under their own gravity, even in the absence of any other force.

    Stars have significant internal forces which overcome gravity. Light, which is perhaps the first thing people associate with the sun, is one source of pressure pushing outward from the Sun. Gravity, being the weakest known force in the universe, is not able to collapse a star into a black hole until the mix of elements fueling its fusion begins to change in favor of heavier elements with lower total energy released.

    Why doesn't a person collapse into a black hole? Because we don't have enough mass in a small enough area. Why doesn't the sun collapse? Because its fusion reactions release too much energy as outward pressure. Why do some stars become black holes? Because they burn ever heavier elements that increase their density while also releasing less energy as outward pressure.
     
  7. Jun 12, 2016 #6
    As the fuel of a star gets consumed the outward pressure created by the fusion is overcome by gravity. Gravity is not only caused by mass but by the energy momentum tensor which includes other factors like pressure. As the star collapses under gravity the pressure increases and thus so does gravity.

    As for the second part of your question, the earth is in a vacuum.
     
  8. Jun 13, 2016 #7
    I've heard even in a vacuum, there are still some forces which are on the act. Further, yes Earth is in a vacuum but it is under the gravitational influence of other bodies. Thanks for your kind reply.

    In short, if an object is very dense, it can collapse into a black hole? No matter how less it's mass is, given its volume is even lesser to a certain degree?.
    If that's true, then all the stars which collapse into a black hole are very dense during their lifetime as a star. True?

    Basically, stars are very dense and therefore they experience more gravity than their own mass ? (It must be true for all the stars since this is the reason this entire process of black hole is happening). So, all the stars should be a potential black hole?

    If all the statements above are true, then why would sun face inward gravitational force at all because it does not have that much density to become a black hole?


    Or is it that getting imploded by self-gravity and turning into black hole due to high density of body are two different things but connected?
     
  9. Jun 13, 2016 #8

    jbriggs444

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    Yes. If you squeeze any object tightly enough so that it becomes small enough, it can collapse to a black hole.

    They must become dense in order to collapse, yes. However, they need not be dense for their entire lifetime.

    This is not entirely correct. Stars are not all very dense. The density required to form a black hole is not fixed. It is lower for very massive objects and is higher for less massive objects. Not all stars will evolve to a density high enough to form a black hole.

    All objects have an inward gravitational force because of their own mass. You and I have inward gravitational force because of our own mass. That inward force is very tiny for us. The strength of our bodies is vastly greater than this force, so we do not implode.
     
  10. Jun 13, 2016 #9

    sophiecentaur

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    It's all to do with the actual amount of mass that a star has in the first place. Gravitational forces are very very weak compared with the electric and nuclear forces that keep matter in the shape we are familiar with. A low mass star is no different from a lump of rock (or our own bodies) and it will just cool down and shrink until the gravitational forces are balanced by the electric forces between and nuclear forces within the atoms in it (that define the volume taken up by each atom). Such old stars will be very hard to spot because they will not be emitting significant amounts of light or IR when they are 'brown' hot.
    However, if a star starts off with enough mass and runs out of enough internal nuclear energy to 'pump it up', the pressure at its centre due to heat and the gravitational attraction of all its mass can be great enough to overcome the normal atomic forces and the material at the centre will become crushed till its density is higher than normal matter. This will cause the whole star to shrink a bit, which will increase the crushing effect and you will have a runaway effect. The density will increase and increase and the diameter will decrease and decrease until the gravitational attraction at the surface is high enough even to prevent light getting away - hence the term "black" hole. Conventional Physics laws will not apply at this stage, inside the hole but, outside, objects will be subjected to its gravity and will be attracted and probably go into an endless orbit around something they cannot see. Their orbits will be similar to orbits around a large star of the same mass that hasn't actually formed a black hole yet. Our Galaxy has a super massive black hole in the centre which keeps all the stars orbiting around it. We are not all falling inwards just because there's a black hole in there.
     
  11. Jun 13, 2016 #10

    phinds

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    My understanding is that this is not correct. The mass of the SMBH at the center of the milky way is a very small portion of the total mass of the galaxy and the orbits would be little affected (except for the VERY close-in stars) if it were not there. Now whether or not the galaxy would have formed in the first place without it is still not understood.
     
    Last edited: Jun 13, 2016
  12. Jun 13, 2016 #11

    mfb

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    You cannot compare "gravity" to "mass", that like comparing the number of apples to the color of a banana.

    All objects have internal forces from gravity, pulling them together. The only question is where does this force come from.
    • Planets and smaller objects don't have a large mass, their gravitational forces are not very strong - they compress the solid matter in their interior until that solid matter pushes back sufficiently to stop further compression.
    • Stars have more mass, they get compressed until the interior is hot and dense enough to start fusion. This heats up the interior even more, and gives it a really high pressure. That pressure is sufficient to stop a further collapse.
    • At some point stars run out of fuel for fusion. With nothing to keep up the pressure, they get smaller again, with three possible results for increasing masses:
      - A white dwarf, basically an ultra-compact version of solid matter.
      - A neutron star, even more compact, where more exotic quantum-mechanical effects provide the necessary pressure
      - A black hole, if the star remnant has so much mass that nothing can provide sufficient pressure to stop the collapse
     
  13. Jun 13, 2016 #12

    sophiecentaur

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    I stand corrected. But my point was rather that a Mass will have the same effect on orbiting objects, whether or not it's a black hole. They don't just get sucked in, willy nilly.
     
  14. Jun 15, 2016 #13
    I think the color of the banana is directly proportional to the number of apples? I meant Gravity is directly proportional to mass , true?. But according to you they aren't related.
    So, in the case of planets they don't have a large mass, therefore, they would face proportionally less gravity. Applying this concept, every object would be facing equal gravity proportional to their mass right? So why the special case with Stars?.

    Even if star loses mass it would loose gravity proportional to its mass. So nothing would change.

    I'm confused, I think you should start by defining what exactly gravity is, and whether gravity of an object is proportional to it's mass or not. Or is it something complicated?
     
  15. Jun 15, 2016 #14

    sophiecentaur

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    It took a couple of thousand years to get this but Newton came up with a perfectly good expression for the force on a unit mass, due to another mass at distance r. We now use the equation to describe the Field:
    F = GM/r2
    Why bother launching into an attempted verbal description when this bit of Maths says it all? Failure to use this concise description seems to have caused an amazing amount of confusion.
     
    Last edited: Jun 15, 2016
  16. Jun 15, 2016 #15

    mfb

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    You can have a mass of 5 kg, or 10 apples, but how do you assign a single number to the gravitational interaction of some complex object? It just does not work. The gravitational force on an object depends on its position, and on the mass distribution of the object that attracts it.
    That does not make sense. I don't know where exactly this misconception comes from, but as long as you don't give up the idea that you can assign a single number to the gravitational acceleration you won't make progress.
    There is nothing special about stars, they are just objects with a lot of mass.
     
  17. Jun 15, 2016 #16

    Bandersnatch

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  18. Jun 15, 2016 #17

    sophiecentaur

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    No I didn't. I wrote "the force on a Unit mass". Note my edit for clarity, where I insert the word "field".
     
  19. Jun 15, 2016 #18

    A.T.

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    The gravitational field has the dimensionality of acceleration, so it's better to use "a" or "g" not "F":
    g = GM/r2
     
  20. Jun 15, 2016 #19

    sophiecentaur

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    I can use any letter I want as long as I define it - which i did. My point was that verbal wrangling about something so straightforward as an equation with just four variable terms in it gets us nowhere.
     
  21. Jun 16, 2016 #20

    mfb

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    The force between two point-masses is not the full story, however. To derive things like the interior pressure you also need the shell theorem, and then an integration over the density profile, and another integration over the pressure increase as function of radius. To make it worse, the pressure then feeds back to the density in real objects.
     
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