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When a star forms where does the Gravity come from?

  1. May 14, 2012 #1
    Hey guys, in class today we learnt about the life cycle of a star and at the very first stage gravity pulls the helium or Hydrogen nuclei at such a speed that they fuse (nuclear fusion). As I understand it there is little gravity in space so where is this extra gravity coming from?

    Thanks for any help given. :)
  2. jcsd
  3. May 14, 2012 #2
    Well, the gravity-induced pressure at the center of stars is incredibly high compared to conditions seen anywhere else in space -- you're correct about that. But at the center of a star when fusion commences at the beginning of the main sequence, the density of matter is about 100,000 kg/m[itex]^3[/itex], much higher than the Earth's average density (even though the center of the Sun is a gas and the Earth is mainly solid), and the temperature is over 10 million Kelvin. At these temperatures and pressures, particles are moving fast enough and are close enough together to fuse hydrogen nuclei -- but this is all possible because of the immense amount of mass packed into a small region in the Sun. Interstellar space does not come anywhere close enough to these densities.

    But keep in mind that the Sun's gravity is still incredibly influential, even very far away from it; it's just that only in the center of the Sun is the gravitational force large enough to induce fusion.
  4. May 14, 2012 #3
    the gravity is always there because all the mass is there. All mass exerts an apparent gravitational force on all other mass. We, as in humans, only notice it when its a LOT of mass and is relatively densely packed. Such as the Earth. But your computer is exerting a force on you, and so is your chair, and so is everything.

    So when all the interstellar gas is chillin' out there in space, it's all pulling on each other. Usually it's sitting quite still because it is spread evenly, but if it gets disturbed, then you end up with a clump. This clump of stuff is seen by all the other matter at some radius from it as a single point object, and is attracted to it like that. Therefore it gets pulled towards it.

    The gravity doesn't come from anywhere, it's always there. It's just a matter of the Shell Theorem and how we "feel" gravity from masses.
  5. May 19, 2012 #4
    Wait a minute. I was wondering the same thing. Steely Dan & Shishkabob, you are both missing the point. The gas clouds aggregate UNDER the influence of gravity. Why is the gravity stronger at the focal point relative to the surrounding environment? Under the influence of this "gravity" the gas clouds aggregate. As the gravitational force continues to crush the core, the temperature increases until it ignites the gas and starts the process of fusion.
  6. May 20, 2012 #5
    imagine a bunch of particles chillin' in space. They are all roughly equally spaced apart. They all affect each other with the same gravitational force because they are all the same mass. Therefore, none of them go anywhere because there is no net force on any of them, so the chill. Stationary.

    Now imagine a big supernova nearby these particles. The supernova creates a shock wave and it hits this giant gas cloud of particles. When it does this, the particles are no longer equally space apart. They get pushed at each other, knocked around. There's a disturbance. After this disturbance, there may be a clump or two that are formed after the supernova shock wave passes by. Because the particles are suddenly more dense at this spot (because the shock wave pushed them around chaotically) the gravitational force due to this clump is now seen, by particles in its neighborhood, as an attractive spot.

    Now that there is a spot that is more attractive than any other spot, the particles begin to aggregate. Once the aggregation starts... there really isn't anything around to stop it. Another supernova shock wave would only serve to stir up the gas cloud even more. Things "snowball" from there.

    The initial state of calm and a net zero force on all particles is ruined by some outside force (supernova in this case). Since there is no longer a zero net force on all particles, they start to aggregate.
  7. May 20, 2012 #6


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    Here's a 21 second video

    it is based on computer simulation of what is called "structure formation"

    Stuff starts out almost perfectly uniform, a gas which is everywhere almost equal to its average density. Only some spots where accidentally it is slightly more dense or less dense than average.

    surrounding stuff starts falling towards the overdense regions---so they get denser
    and the underdense regions gradually get cleared out and become voids.

    You can watch it happen in the YouTube simulation

    what you are seeing is largescale structure formation, like galaxies and clusters of galaxies, not individual stars.

    but the general idea of stuff condensing works at smaller scale too.

    try googling "structure formation". It is a complicated subject and people study it a lot.

    At the level of star formation there are a lot of different effects to take into account. Supernova explosions have been mention. Anything that disturbs a cloud of gas will cause regions of overdensity that can become the nucleus of clumping.
    Two gas clouds falling together and colliding can send shocks thru each cloud and start the process. Dark matter plays a role. Radiation pressure plays a (generally disruptive) role.

    the main idea is simple: stuff falls together and curdles into clumps. nothing is ever perfectly uniform and even very slight non-unformities can become the seeds of structure.
    but the actual practice of running computer simulations that model star formation can be complex because of all the different effects.
    Last edited by a moderator: Sep 25, 2014
  8. Jun 6, 2012 #7
    it comes from bend in space caused by accumulation of mass...?
  9. Jun 7, 2012 #8
    Imagine 10 people standing on a trampoline. If they stand at random points on the trampoline, it will only give way say about a foot. Now, if they all get together at the center and stand on top of one another, the total mass of the occupants is the same, but the affect on the trampoline is different. When all the mass is compacted at one point, the trampoline will deflect downwards significantly more.

    When gas clouds are young and less dense, they experience gravitation from a bunch of sources relatively far off. But as the cloud condenses and all respective particles get closer together, they all start to experience the affects of the gravity from all the same other particles, just from closer. This is where you get the 'more gravity' from.
  10. Jun 13, 2012 #9
    Heads up for a great explanation. It's something I've never really been able to explain in a easy-to-grasp way (that being to others AND myself actually).
  11. Aug 8, 2012 #10

    I love physics but not competent and I have a question on Gravity. Gravity always has mystified me. As different forces such as nucleostatic/ electrostatic forces etc.., exist with in the atom, so is it possible that only these forces are creating gravity in an object of large mass such as planet/satellite possess?

    Is it possible to create a computer model to simulate the constituents and mass of the earth with properties similar to it and test the generation of gravity? I mean to visibly demonstrate and understand.

  12. Aug 8, 2012 #11
  13. Aug 8, 2012 #12


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    Nope. Gravity is due to energy and mass, so while it does take these forces into account for things like energy levels of atoms and such, you must also look at the rest mass of these particles too.

    Not to *test* General Relativity, but you could simulate the Earth and it's gravity. The complexity depends on how accurate you want to be. If you REALLY want to get accurate you are going to need to take into account a LOT of stuff and do huge amounts of calculations. The equations for GR are NOT easy to calculate, even for computers.

    I'd recommend just using newtonian gravity for any simulations that don't require amazingly precise accuracy, as it works just fine for almost everything we do. Even sending rovers to Mars and probes to Jupiter only requires newtonian mechanics.
  14. Aug 8, 2012 #13
    Thank you Drakkith for the response and thanks for the simple explanations. I find it pretty difficult to understand by reading tons of technical data and hence avoid going to google and look up for complex theories.

    Few my other questions regarding your previous answers: 1) Let's assume a large block of stone which has great mass, I assume it is capable of having some gravity. My question in this regard is a how a massive constant object like lying at one place in this case is having any other energy (apart from the energy which is already present in the atoms)?

    2) Is it possible to view this gravitational energy by some means like IR rays or some other technology or at least in the future will it be possible with some device to view it optically?

    Thank you for your time.
  15. Aug 8, 2012 #14


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    Gravity is a force (or can considered to be one for our purposes). It does not require energy to function. Remember that energy is the ability to do work. This generally requires an object be accelerated in some manner, using a force. None of the fundamental forces of nature require an expenditure of energy to function. In fact, THEY are reason we have energy in the firs tplace.

    Energy is not a physical object that can be seen and felt. It is simply a way to describe a system of objects and how they may interact. A moving object has kinetic energy. This simply means that it can do work on something if the right circumstances arise. (Such as the object impacting another object)
  16. Aug 8, 2012 #15
    Thank you for quick responses. I will spend some time recollecting my school physics. Physics always fascinated, though I was not smart to grasp some of the concepts and moved on to other field, but I still love that subject.
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