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

Gravity of Blackholes

  1. Jul 30, 2015 #1
    Do blackholes have gravity less than their mother star?.if no then why not,though during formation of blackholes
    outer layer of star explodes out in space and inner core crush by its own gravity this way its mass decrease..So If i consider Gravity a force,by newton law of gravitation Decrease in mass should leads to decrease in gravity..
    Last edited: Jul 30, 2015
  2. jcsd
  3. Jul 30, 2015 #2


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    The volumn of a black hole's event horizon is much less than the star that it originates from. Check out the section on Gravitational Collapse to understand how it occurs.
  4. Jul 30, 2015 #3


    User Avatar
    Gold Member

    The reference to Newton's law of gravity in the same post as black holes is flawed thinking. Black holes are a phenomena that is a natural consequence of General Relativity (which, to oversimplify, is just a fancy name for the modern laws of gravity), which is impossible in Newtonian gravity.

    Also, while Black Hole are more gravitationally intense at the event horizon than a star is at any point at its surface or within it, once a Black Hole is formed its event horizon can expand as it acquires more mass that gets sucked into it. While the initial mass per volume within the event horizon of a Black Hole when it forms is just slightly greater than the mass per volume of a neutron star or atomic nucleus, this does not continue to hold true as the Black Hole acquires mass and its event horizon expands.

    The huge black holes at the center of galaxies, for example, have far less mass per volume within their event horizon, than a star does. The larger the Black Hole, the less mass per volume within its event horizon it has, a very counterintuitive reality. If the mass of a large black hole were spread evenly over the space within its event horizon, there would not be enough gravitational pressure to form a black hole.
  5. Jul 30, 2015 #4


    User Avatar
    Science Advisor
    Gold Member

    Density is not much of a limiting factor on gravitational collapse. The average density of molecular gas clouds in star forming regions is on the order of a 10^6 molecules per cc^3 - shich is sufficient density for gravitational collapse into a star. By comparison a laboratory grade vacuum has a density of about 10^10 molecules per cc^3.
  6. Jul 30, 2015 #5


    User Avatar
    Science Advisor

    Essentially, yes, the resulting black hole has less gravity at the same distance. The gravitational attraction around a spherically-symmetric body is purely determined by the amount of mass inside a given radius. Because a large portion of the matter that makes up the star is ejected when it collapses into a black hole, the mass that ends up within the black hole is only a fraction of the total star's mass, so the overall gravitational pull is less.

    But a black hole is also vastly smaller, so that if you're near the event horizon, the gravitational pull will be far greater than it would have been at the surface of the star.
    Last edited: Jul 30, 2015
  7. Jul 31, 2015 #6
    Thanks for clearing my confusion.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Gravity of Blackholes
  1. Supermassive Blackholes (Replies: 12)

  2. Stable Blackhole? (Replies: 3)

  3. Blackholes and galaxies (Replies: 15)

  4. Parts of Blackholes (Replies: 1)