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Why aren't stars in a star cluster attracted to each other?

  1. Oct 27, 2011 #1
    If a globular cluster is a dense(relatively speaking) collection of stars, why don't the stars attract each other gravitationally? Why don't they all collide to form a mega-star?
  2. jcsd
  3. Oct 27, 2011 #2
    They do attract each other gravitationally. They don't all crash into one another for the same reason the Earth doesn't crash into the sun.
  4. Oct 27, 2011 #3
    If they all collided and merged it wouldn't result in a mega star. It would be a black hole that would be created.
  5. Oct 28, 2011 #4
    It is not that I'm a expert on astronomy. But isn't that the same as saying the cluster is rotating?
  6. Oct 31, 2011 #5
    I'm not an expert either, but I was playing with someone's orbital simulation software once where you could define any number of planets and stars and set their initial velocities, etc., and you then see how they all interact. I noticed that it is very difficult to get interstellar objects to collide. They have to hit practically dead-on. Otherwise they just whip around each other.

    Obviously, if that were to happen in real life there would be severe damage to both bodies in terms of atmospheric and other disturbances. But literal collisions are hard to do even when done on purpose.
  7. Nov 1, 2011 #6
    ^ this is true

    Even when 2 galaxies merge, there are only a few of the billons of stars that actually collide, otherwise they just swing around each other.
  8. Nov 1, 2011 #7


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    Of course. The stars in the cluster are moving around each other, all orbiting the barycenter of the cluster.
  9. Nov 1, 2011 #8
    That doesn't mean that the cluster is rotating because the orbital angular momentums doesn't have the same direction. Star clusters usually have no total angular momentum. In addition the trajectory of the stars doesn't have to be orbits.
  10. Nov 1, 2011 #9


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    Is it being suggesting that stars in a globular cluster might be oscillating like bees in a hive?

  11. Nov 1, 2011 #10
    That's a nice analogy but the motion of the stars is determined by the common gravitational field and not by aerodynamics.
  12. Nov 1, 2011 #11


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    I'm not saying every star will be moving in the same direction, only that they will be moving around in orbits. If the trajectory isn't in an orbital then the star will escape the cluster. Unless I'm misunderstanding the way orbit is being used here.
  13. Nov 1, 2011 #12
    How about a chaotic path due to many close flybys?
  14. Nov 1, 2011 #13
    As was pointed out earlier, cluster stars seldom collide with each other. Let's try to quantify this result.

    Collision rate = (number density of targets) * (collision cross section) * (velocity)

    For stars with radius r separated by distance a and traversing their separation distance in time T, the rate becomes

    (1/T) * (r/a)2

    One has to be careful of gravitational effects, since the stars will be pulled together before they collide. This changes the cross section from pi*r2 to

    pi*r*(r + 2(G*M)/(v2))

    for mass M and velocity at infinity v. Even then, it's easy to show that collisions are rare.
  15. Nov 1, 2011 #14


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    Isn't that an orbit still? It's still bound to the cluster.
  16. Nov 1, 2011 #15
    One can estimate the likelihood of close flybys per orbit.

    The average orbit speed ~ sqrt(G*M/A)
    M = total mass, A = total size
    For N stars, they are related to individual mass m and separation a as
    M ~ N*m
    A ~ N1/3*a

    To be deflected significantly, a star must pass within about G*m/v2 of another star, and that is about A*(m/M) or A/N.

    A star's mean free path, its travel distance between such deflections, is
    1/((stars' number density) * (deflection cross section))

    Working it out, 1/((N/A3) * (A/N)2) or N*A.

    So a star will make on average about N orbits between sizable deflections.
  17. Nov 1, 2011 #16


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    Maybe they're not the right type.
  18. Nov 1, 2011 #17


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    *Groan* Terrible joke!
  19. Nov 1, 2011 #18
    You're using a different definition of the word orbit than I'm used to.
  20. Nov 1, 2011 #19


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    It's a good question.

    I am not sure if orbits are really stable in a cluster where the orbits have a wide distribution of planes.

    For any given star, as long as it does not manage to acquire enough velocity to actually escape the cluster (and it's entirely possible some do) it could happily drift around on an highly unstable path.
  21. Nov 1, 2011 #20


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    Yup, maybe! I found this in wiki:
    "The results of N-body simulations have shown that the stars can follow unusual paths through the cluster, often forming loops and often falling more directly toward the core than would a single star orbiting a central mass. In addition, due to interactions with other stars that result in an increase in velocity, some of the stars gain sufficient energy to escape the cluster."

    Respectfully submitted,
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