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What is a black hole?

  1. Aug 10, 2015 #1
    I've been reading 'A brief history of time' and want to know more about how black holes are formed. Please help?
     
    Last edited: Aug 10, 2015
  2. jcsd
  3. Aug 10, 2015 #2

    e.bar.goum

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    There are some pretty great websites around that introduce black holes:
    http://www.universetoday.com/33454/how-do-black-holes-form/
    http://hubblesite.org/reference_desk/faq/category.php.cat=exotic
    https://en.wikipedia.org/wiki/Black_hole

    PF is really much better at answering specific questions, so I encourage you to do some reading, and come back with whatever questions you have!
     
  4. Aug 11, 2015 #3
    There are two main ways black holes form: direct collapse or a dying star. Just after the moment of creation, energy was spread more or less uniformly through space, but some places were slightly denser than other. As things got jostled around, some of these areas got so dense that they collapsed into a primordial black hole. It's thought that these probably were the anchors that created the galaxies. When a really really big star starts to finish it's fuel and sputter out, it's internal heat can't keep gravity at bay anymore and as the star collapses in on itself, the core gets so dense it forms a black hole.
     
  5. Aug 18, 2015 #4
    When the escape velocity of a star becomes extremely high then it starts consuming itself or starts degrading itself and a time comes when their is no existence of star at that place but there is a very high concentration of Gravitational force at that point and finally it starts attracting everything even light!! I hope you got the answer.
     
  6. Aug 18, 2015 #5
    This is not accurate. When stars get too big, they don't collapse, they explode: the nuclear forces overwhelm gravity at the radius of even the most densely packed star. Only the core gets crushed enough to turn into a black hole, and it's not simply gravity doing that. Gravity keeps the hole collapsed, but the momentum of the rest of the star falling onto the core is what crushes it into a hole.
     
  7. Aug 18, 2015 #6
    I simply want to say that when the escape velocity of that star exceeds the velocity of light then there is a formation of black hole takes place. I want to end my answer in simple language other wise it takes a lot of problem to the questioner. You are throwing light on the stages of black hole. The stellar nebula first becomes the massive star then in the red super giant then into supernova and then either in black hole or in neutron star. There many conflicts on theories of black hole and even on there existence. Everyone wants to explain the fundamentals of black hole in his own way. Researches are still going on but still we are unaware of a extremely vast part of universe...
     
    Last edited: Aug 18, 2015
  8. Aug 18, 2015 #7

    phinds

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    I think your attempt to simplify the explanation of "what is a black hole" is just making it more confusing. You make it sound like magic, whereas it is a well understood physical process and no, there are not "millions of conflicting theories"
     
  9. Aug 18, 2015 #8

    Chronos

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    Any region of space where the escape velocity = c, is a black hole by definition.
     
  10. Aug 19, 2015 #9
    Isn't this the definition of the event horizon? Isn't a black hole escape velocity >= c?

    There are not many conflicting theories, as far as I know there are three: collapse stellar core, direct collapse of primordial black holes, and merger of two preexisting black holes. There are conflicting theories about what's inside the hole, but not how it comes to be. And their existence is certainly known, there is a monstrous one in the middle of our galaxy, we can see stars flying around it.
     
  11. Aug 19, 2015 #10

    SteamKing

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    This description isn't accurate, either. :frown:

    When a massive star reaches the end of its life, its core is no longer capable of using nuclear fusion to create enough energy to support itself against gravity wanting to collapse it further. When fusion ceases, gravity wins the battle and collapse occurs rather suddenly. As the collapsing core gets denser and denser, eventually a point is reached in some stars where the core cannot collapse further, and it stops abruptly. The envelope of the star, having rushed in to fill the void left by the collapse of the core, strikes the surface and rebounds. The shock wave created by the rebound is sufficient to blow the star's envelope into space, leaving the neutron star core behind:

    https://en.wikipedia.org/wiki/Type_II_supernova

    If the core is sufficiently massive, when collapse occurs, there will be no rebound, and a black hole will be created:

    https://en.wikipedia.org/wiki/Black_hole

    Sometimes, even if a dead star leaves a neutron star behind, this star can collect enough additional mass from surrounding gas to collapse into a black hole.

    It's also possible for two neutron stars to collide and, because their combined mass is so great, only a black hole is left behind.
     
  12. Aug 19, 2015 #11
    That works with stars in the 10 to ~130 solar mass range, or stellar remnants with a combined mass greater than ~3 solar masses (such as the collision of two neutron stars). However, once you get stars that are greater than ~130 solar masses the temperature, pressure and gamma-rays from the collapsing core it is sufficient to create electron-positron pairs, which annihilate each other. The thermal energy released by the electron-positron pairs overcomes the gravitational binding energy of the core and the core also explodes, leaving nothing behind. So like neutron stars, which have a range between 1.44 and ~3 solar masses, black holes also appear to have a minimum and maximum range with regard to stellar remnants. Obviously another mechanism, other than a stellar core collapse, is required to produce black holes greater than ~39 solar masses.
     
  13. Aug 19, 2015 #12

    phinds

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    Since supermassive's seem to all be at the heart of galaxies, it seems very reasonable that they grew from scratch by accreting new material from the very dense area at the center of the galaxy. First enough to create a BH and then more until they had cleared out a large area around them (small fraction of the galaxy's diameter but a good sized volume in absolute terms).

    I guess there would have had to have been a star there at some point along the way, but then just more and more feeding.
     
  14. Sep 25, 2015 #13
    A black hole IS actually just a 'hole' in the fabric of the universe and is black as no light escapes it. Creation of a black hole is more complex depending on the mass of the star of other stuff.
     
  15. Sep 26, 2015 #14

    davenn

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    do you have a reference for that ummm interesting comment ?
     
  16. Sep 26, 2015 #15
    Well, It will be easier to imagine gravity with the universe as a fabric. I read it somewhere,couldn't remember.
     
  17. Sep 26, 2015 #16
    How come popular literature writes "nothing can escape, not even light". Light does travel at c, doesn't it? Or is, perhaps, my conception of c at fault here, then?
     
  18. Sep 26, 2015 #17
    He meant >c
    nuuskur you are also right too.
     
  19. Sep 26, 2015 #18

    davenn

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    no he meant c meaning light DOESNT escape


    you really need to stop and think and maybe do some reading before you keep making these regular oddball comments throughout many of your posts

    Dave
     
  20. Sep 26, 2015 #19
    So if i reach earth's escape velocity exactly, i will not get out of earth? exclude friction.
     
  21. Sep 26, 2015 #20

    Drakkith

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    It is not. There is no such thing as a 'fabric of the universe'.
     
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