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Space Science

Milky Way's Black Hole Wasn't Always Such a Wimp 83

Posted by Soulskill
from the you-wouldn't-like-me-when-i'm-angry dept.
scibri writes "Sagittarius A*, the dormant supermassive black hole that lies at the center of our galaxy, was much more active not that long ago. Astronomers using the Fermi Gamma-ray Space Telescope have picked up some faint gamma-ray signals that suggest Sagittarius A* was emitting a pair of powerful gamma-ray jets like other galactic black holes as recently as 20,000 years ago (arXiv paper). If our black hole was more active in the past, it could explain why Sagittarius A* seems to be growing about 1,000 times too slowly for it to have reached its current mass of about four million solar masses since the Galaxy formed about 13.2 billion years ago."
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Milky Way's Black Hole Wasn't Always Such a Wimp

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  • by buchner.johannes (1139593) on Wednesday May 30, 2012 @03:48PM (#40160235) Homepage Journal

    What makes a black hole dormant? Lack of gamma ray jets... ?

    Lack of gas and dust streaming in. The disk + torus the infalling gas produces while accreting produces all the radiation we see from black holes in active galactic nuclei (AGN). Another side effect are the jets that you can see in radio frequencies (although not in all AGN.

    There is actually a gas cloud falling in in these decades, so we might see our black hole light up. http://www.nature.com/nature/journal/v481/n7379/abs/nature10652.html [nature.com]

  • by Colonel Korn (1258968) on Wednesday May 30, 2012 @03:49PM (#40160257)

    The singularity itself? A teaspoon of singularities would have infinite weight. Maybe you mean everything inside the event horizon? In that case calculate the Schwarzschild radius (2Gm/c^2) of 4 million solar masses, then get the density [4 million solar masses /(4/3 pi r^3)] and multiply by the volume of a teaspoon. I think the density of everything inside the event horizon for that big of a black hole is actually pretty low.

  • by buchner.johannes (1139593) on Wednesday May 30, 2012 @04:04PM (#40160401) Homepage Journal

    I'd like to know where this black hole came from. Was there some random star floating through space, which died, and then it started gobbling up everything? Including our galaxy (which will eventually fall in). Or maybe the superblackhole was a previous galaxy from ~25 billion years ago that fell into itself?

    To my knowledge it is currently unknown how those massive black holes (millions of solar masses) form originally. We know they form very early in the universe (1Gyrs after the big bang, our universe is ~14Gyrs old). Do they come from many stars? Were stars in those early times extremely massive? Is there some way of growing black holes very fast?
    Those are open questions in Astrophysics ... you are welcome to join in :)

    We know that merging galaxies should combine their black holes but also grow them (more gas infall) -- but nobody knows how two black holes merge ( https://en.wikipedia.org/wiki/Binary_black_hole#The_final-parsec_problem [wikipedia.org] ).

  • by sanosuke001 (640243) on Wednesday May 30, 2012 @04:17PM (#40160549)
    4x10^6 solar masses = 7.95568 × 10^36 kilograms
    Schwarzchild radius = 1.1804758431349163 x 10^10 meters
    4/3 pi r^3 = 6.89064573 × 10^30 m3


    mass / volume = 1.19612658 × 10^-27 kg / m3
    1 tsp = 5.0 × 10^-6 m3
    1 tsp of Sagittarius A* = 5.9806329 × 10-33 kg
  • by FrootLoops (1817694) on Wednesday May 30, 2012 @04:47PM (#40160905)

    Your mass/volume ratio is way off, though the other three are correct. It should be...

    mass / volume = 1.155 * 10^6 kg/m^3
    1 tsp = 4.929 * 10^-6 m^3
    1 tsp of Sagittarius A* = 5.693 kg

    So, it's pretty heavy, but eg. neutron stars are far, far heavier. This black hole is far denser than the sun, which has about 6.94 g per tsp [wolframalpha.com].

  • by osu-neko (2604) on Wednesday May 30, 2012 @04:49PM (#40160915)

    > The singularity itself? A teaspoon of singularities would have infinite weight.

    No, it wouldn't. Black holes have a finite weight.

    Right, but an infinite number of singularities will fit in a teaspoon (or any volume, for that matter).

  • Re:quieted by mass (Score:2, Informative)

    by Anonymous Coward on Thursday May 31, 2012 @03:58AM (#40164425)

    The Schwarzchild radius is proportional to the mass M. The tidal effect is proportional to the derivative of the gravitational field, which is proportional to M / R^3. Setting R to be the Schwarzchild radius, to measure the tidal effect at this point, we find that it is proportional to 1 / M^2. So the more massive a black hole is, the smaller the tidal effect at its event horizon - and for a sufficient black hole mass, the tidal effect must be insufficient to break up a star.

    Calculating the mass at which this happens is left as an exercise for the reader. ;-)

I've never been canoeing before, but I imagine there must be just a few simple heuristics you have to remember... Yes, don't fall out, and don't hit rocks.

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