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Why do neutron stars suck the gas from their companions?

  1. May 21, 2010 #1


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    I always here about neutron stars and black holes sucking the gs from their companions. However, a star loses mass before it becomes a neutron star so I don't understand why it afects its neighbor more when it is a neutron star than when it was a regular star.
  2. jcsd
  3. May 21, 2010 #2
    not sure I understand your question, but a neutron star has indeed a stronger gravitational field than a regular star. A typical neutron star is about twice the mass of the sun and has a radius of only 12 km. That would result in a very strong gravitational field that electrons can no longer orbit around the nucleus of the atom ant they combine with the proton to form neutrons (hence the name neutron star). This gravity results in an escape velocity of about one third the speed of light and if you were to fall from a height of one meter at the surface of a neutron star you will reach it at a speed of 2000 km PER SECOND. It should be obvious now why a neutron star has much more gravity than a regular one.

    Having that said,I should clarify that a neutron does not "suck" mass from every companion star. you can have a regular star orbiting a neutron star or even a black hole peacefully without being sucked in. It's a question of how far the regular star is.
  4. May 21, 2010 #3


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    Maybe it is just a bias in the images shown us by astronomers, but I hear and see a lot about neutron stars and black holes sucking matter from other stars, but I almost never hear or see anything about a regular star sucking matter from another regular star.
  5. May 21, 2010 #4
    there is one famous scenario where a white dwarf can suck some mass from a neighboring star which would result in a type 1a supernova. but of course you wouldn't recognize a white dwarf as a "regular star"

    the thing is regular stars are very huge in size that in order for them to suck material from other stars they have to come very close to each other that they would practically collapse after a very short time (not very sure of this though)
  6. May 21, 2010 #5
    As far as the neutron star goes hosman described it very accurately. Another way they can siphon material from their companion is via their large and strong magnetic field which will attract some of the particles to be funneled onto te neutron star.

    As far as your second question, it is not so much of a bias as much as it is a rareity. Normal main sequence stars generally do not siphon from one another, since they are generally equal in size and mass. Neutron stars and black holes aren't the only objects which draw material from a companion. White dwarf stars, the small dense leftover core from a star, when in a binary system will draw material from the larger star, resulting in a type 1a supernova. If one was to generalize this phenomena, it would be due to the differing gravitational potentials of the two companions, which is why it is less likely for main sequence stars found in binary systems. Hope that helps.

  7. May 21, 2010 #6
    If you mean that a regular star is the star on the main sequence, I also have never found something related to this. We usually see a neutron star, or BH or a white dwarf sucking matter from the other star, because they had a faster evolution compared to their companions, thus, when their companions become a Red Giant they suck the RG matter, due to the fact that on the RG phase the radius of the star expands a lot the gravitational attraction is weaker.
  8. May 21, 2010 #7


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    So there is nothing about becoming a black hole or a neutron that causes the siphoning?
    Does the loss of energy radiating from the star have an affect, since it isn't creating an outward pressure?
  9. May 21, 2010 #8


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    Last edited by a moderator: Apr 25, 2017
  10. May 21, 2010 #9
    Lord, binary stars do this sometimes, and it can be the cause of a (NON SUPER) Nova.
  11. May 22, 2010 #10
    Hi all,

    To start with: at the same distance from the center of the neutron star (as long as you are outside the object), the gravitational field of a neutron star is not any stronger than that of its predecessor. At the surface: yes, but that's not what the donor star cares about.

    The fact that it 'sucks' gas of its neighbor, is the neighbor's own fault. Because the other star evolves too, it grows. For the outskirts of the donor star, the gas is attracted about as much by its own star, as by the neutron star (or white dwarf, or black hole, a compact object anyway). Then it may be transferred to the neutron star.

    This is easier to see, but the bias thing talked about above I think is a little bit overdoing it.
  12. May 22, 2010 #11


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    You're right, my perceptions have more to do with my own biases than those of astronomers or of the universe.

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