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Two simple questions regarding black holes

  1. May 11, 2007 #1
    Which are all the methods by which black holes are detected?
    How do we get to know how massive a black hole
  2. jcsd
  3. May 11, 2007 #2
    Ted Bunn made a nice http://cosmology.berkeley.edu/Education/BHfaq.html#q7" about black holes, check the section "Is there any evidence that black holes exist?".

    There seems to be ample evidence that some objects are above a critical mass/density to be considered black holes. But the fact that these objects exist is obviously no proof of the correctness of general relativity with regards to black holes, but if we assume that general relativity is correct they must be black holes.
    Last edited by a moderator: Apr 22, 2017
  4. May 17, 2007 #3
    Black holes that are not eating somthing are virtually invisible. We can only detect them by their gravitationel effect on nearby stars. Black holes that are feeding on a nearby star is another matter. The gas from the star will
    go round the black hole in a spirale while heating. This is called an accration disk. This super-hot gas emits a lot of visible light and a lot of radiation.
  5. Aug 30, 2007 #4
    As the black hole "feeds" on a nearby star does the black hole become more massive? Theoretically, how many stars could it devour?
  6. Aug 30, 2007 #5


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    Classically there is no limit - black holes at the centre of galaxies are very large, the one in the middle of our own milky way is at least 3.5M times the mass of the sun.
    It doesn't get all this mass from swallowing stars, but also dust, gas and anything else around it.
  7. Aug 31, 2007 #6
    Wow! 3.5 million times the mass of the sun is pretty great indeed! To take things a bit further, would it be theoretically possible for a black hole to swallow the entire universe? It seems that size of the black hole is irrelavant since even a small one (Tolman-Oppenheimer-Volkoff limit) could grow without limit building an ever increasing gravitational field. Luckily, there is a lot of space in between celestial bodies. What else is there to stop total engulfment?
  8. Sep 17, 2007 #7
    All of them.
  9. Sep 18, 2007 #8
    Theoretically, how many stars could it devour?

    NO in the real world
    Theoretically even, chances are billions to 1,

    and thats just in a galixcy
    some things are just way tooooooo far apart
  10. Sep 19, 2007 #9
    Well that's two different questions. How many could it devour? As far as I know, there's not really an upper limit to the size of a black hole. You might want to look at this, for example (which seems a little fishy to me, with "wait but what about entropy?" being my first question...maybe an interesting undergrad research topic for me ;)). How many is it likely to devour is dependent on star density, star size, time, and probably initial black hole size. It's also evaporating at some rate. You have galactic black holes that have a rather monstrous amount of matter, and you can merge those (rather more efficient than hunting down individual stars, if we're going for mass).

    I remember reading something about a black hole going through a periodic absorption and discharge in some sort of dense region, causing the region to take on a long-term periodic emission on a larger scale than you'd usually get. Can't find it right now, but really interesting.
  11. Mar 19, 2008 #10
    x/distance between the two objects squared= the amount of force at which gravity pulls the two(or more) objects(calculated in Newtons). This means a black hole with the mass of 4.5 the mass of the sun's core the size of a fork will have MUCH more gravital pull as a black hole with the same mass and the size of the earth.
  12. Mar 19, 2008 #11
    Has it already happened that a black hole ate an other black hole?
  13. Mar 19, 2008 #12
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