Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

N-star vs. BH

  1. Jul 28, 2004 #1


    User Avatar
    Homework Helper

    What is the observational difference between a neutron star and a black hole? Is the evidence conclusive, or is it based on a strong reasonability argument?
  2. jcsd
  3. Jul 28, 2004 #2


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    part of the answer is obvious and you know already---hope you will be patient if I mention it, just to narrow down the question

    some of the BH astronomers observe are too massive to be neutron stars

    the black hole at center of our galaxy has mass of several million solar
    as measured by watching a star whip around it----results and analysis are online, it was ESO that observed it, but I dont have the links handy

    the narrowed-down question is then
    what are the observational difference between a n-star and a stellar mass BH?

    Basically I'm going to wait and hear what the more knowledgeable people say---but I will speculate that you cant always tell the difference or be sure which you are looking at. What would let you tell the difference is if there was a lot of stuff falling into the hole----so the accretion disk is very hot and bright in X-ray... there would be an X-ray temperature and brightness too great for a neutron start to produce.

    Another case would be of a pulsar, from the magnetic field frozen in the n-star, and the poles of the magnetic field not aligned with the axis of rotation. I cant imagine how a BH could imitate a pulsar signal.

    what i think it may come down to is that you can distinguish between n-star and BH in certain cases but not always

    apologies for rushing ahead with speculation---very curious to hear what
    the authoritative word on this is
  4. Jul 28, 2004 #3


    User Avatar
    Science Advisor

    Conclusive evidence of almost anything regarding a black hole is a near impossibility. Black holes cannot be observed by their very nature. If we find an object we are able to observe, it is not black hole. Therefore, since no black hole has ever been directly "observed", they're very existence is (and ever shall be) inferred by reason. That being said, some observations lead to no other "reasonable" conclusion than the existence of a black hole. At least, none that we know of.
  5. Jul 28, 2004 #4


    User Avatar
    Science Advisor
    Gold Member

    Pulsars are the smoking gun evidence of neutron stars. However, not all neutron stars are pulsars. Only those whose rotational axis is properly aligned with our line of sight give detectable pulses. Neutron stars in binary systems are detectable by their mass. The mass will be above the electron degeneracy [Chandrasekhar] limit [about 1.44 solar masses] but less than the neutron degeneracy limit [about 3 solar masses]. So if you find a star orbiting an invisible companion, the companion is probably a neutron star if it has a mass between 1.44 and 3 solar masses. If it is over 3 solar masses, like Cygnus X-1 [which is about 10 solar masses], it most likely is a black hole.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook