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

Star Swallowed by Galactic Black Hole

  1. Jun 16, 2011 #1
    In an article in the NY Times http://www.nytimes.com/2011/06/21/science/space/21obhole.html" [Broken], it was written:

    “This is a singular event in the history of mankind,” Dr. Joshua Bloom, an associate professor of astronomy at UC Berkeley, said. “This black hole was otherwise sitting dormant, a star got too close, its gas got ripped apart and in doing so some of it got spit up.”

    My question is about objects getting too close to black holes. Does the point of no return vary with the mass of the object? Do more massive objects get caught in the death grip of a black hole further than a smaller object?
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jun 17, 2011 #2


    User Avatar
    Science Advisor
    Gold Member

    Here is the paper behind this new story:

    also as Report in Science: http://www.sciencemag.org/content/early/2011/06/15/science.1207150

    Right at the beginning of the paper they describe a disruption radius. If the star passes closer than this to the black hole, it will be disrupted and swallowed. They give the formula:

    R (MBH / M)^1/3

    R = radius of star
    MBH = mass of black hole (assumed > a million time M)
    M = mass of star.

    So yes, it depends on both mass and radius of star. However, a more massive star apparently needs to pass closer to be disrupted and absorbed.

    For concreteness, for a star like the sun, and black hole of 10^7 solar masses, the disruption radius is 5 times the event horizon radius.
    Last edited by a moderator: May 5, 2017
  4. Jun 17, 2011 #3
    I have another question:

    In the NY Times article, Bloom describes part of the swallowed star as having been "spit up." I presume this material spike fortuitously was pointed in the direction of earth, at which point the emission could be detected.

    If other stars were swallowed by this same black hole with a resulting spike pointed away from earth, then we would have no way of detecting such an event.

    Thus, while a (galactic) black hole may appear quiescent, it may be anything but. To detect an event, it really depends 1) if a spike occurred from a swallowed star and 2) if it was pointed in our direction.

    Is the above reasoning correct?
  5. Jun 17, 2011 #4


    User Avatar
    Science Advisor
    Gold Member

    Actually, with a trivial rearrangement, the formula for disruption radius is inversely proportional to the cube root of the density of the passing star. Thus, within the parameters and validity scope of this approximation, the more 'compact' a body, the closer it has to come to be disrupted.
  6. Jun 17, 2011 #5


    User Avatar
    Science Advisor
    Gold Member

    Actually, the described event is seen off axis of the jet (year long interaction of jet with matter away from the massive black hole). Events seen closer to on axis are brighter, with short peak luminosity.

    An active galaxy nucleus has high background luminosity from regular infall events, flashing very bright when a jet is oriented our way. Thus, a dormant black hole (that has cleared nearby material a long time ago) that sporadically captures a star is quite distinguishable - and very rare. Correction: Dormant black holes are not rare; infall of a star into one is rare - otherwise they wouldn't be dormant.
    Last edited: Jun 17, 2011
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook