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ETpro's avatar

How big is your average black hole?

Asked by ETpro (34145 points ) October 14th, 2012

We speak of them being infinitely dense, but I take that to be a metaphor for something so incredibly dense that is beyond common understanding of the property, density. We speak of them being infinitely small, but that seems inconsistent with the visible event horizon and the size they exhibit when wandering ones create gravitational lensing of light from stars beyond themselves. So how many Earth diameters might a super-massive black hole be?

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9 Answers

wonderingwhy's avatar

All you can measure is its motion and the effect it has on the surrounding space. To that end the most massive black hole to date is NGC 4889 and weighs in at somewhere in the neighborhood of 21 billion Suns.

As far as infinitely dense, we have no knowledge of what lies beyond the event horizon, including if the laws of physics as we understand them hold up, which if they do can create a point of density (there’s your infinitely small) so great it begins to do very odd things to time and space; you might find this Discover article interesting.

PhiNotPi's avatar

We can speak of black holes according to either their mass or their Schwarzschild radius, which are directly related (and are proportional).

According to Wikipedia: “The Schwarzschild radius of the (currently hypothesized) supermassive black hole at our Galactic Center would be approximately 13.3 million kilometres.”

The formula for the Schwarzchild radius is r = 2Gm / c^2, where G is the gravitational constant, m is the mass, and c is the speed of light. By solving out the constants, you end up with
r ≈ m * 1.48×10^−27 m/kg

janbb's avatar

My average black hole is pretty dense and fairly large but I manage to climb out of it eventually.

gondwanalon's avatar

Black holes and the concept of singularities don’t compute well within my tiny cranium. I see them as a sort of a short circuit in space and time where the general laws of physics as we know them crumble.

The largest black holes contain billions of solar masses (mass of our star the sun). Steven Hawking talked about “mini” black holes. How small is “mini”?

Well, you may have heard about the event on June 30, 1908 at 7:14 A.M. where a colossal midair explosion occurred over Siberia that shook the entire planet, disturbed the magnetic field and flattened most of the trees in a 30 to 40 kilometer radius of the blast. Scientists still aren’t certain what caused it. But Steven Hawking proposed (“Black Holes” by Walter Sullivan, page 202) that a “mini” black hole wandering about the universe (a residue of the “Big Bang”) might have cause it. If Dr Hawking is correct then Black holes can be quite small.

Compare that with the super massive black holes located in most galaxies (up to billions of solar masses) and the average is likely to still be fairly darn huge. I’m sure that Dr. Hawkins could be more exact than that. HA!

Coloma's avatar

The size of the average human head. lol

On a serious note I am fascinated at the thought of our galaxy being absorbed by a random black hole. Poof!

ETpro's avatar

@gondwanalon I would guess that anything that’s been eating and not excreting for billions of years would have gained considerable weight by now.

@wonderingwhy Thanks for the link. I just can’t get enough on black holes.

@PhiNotPi I’m stuck using my wife’s Kindle Fire tonight. I seem to have killed my desktop in moving and this morning my laptop gave me the blue screen of death. It’s too troublesome look it up qwith this, but isn’t the Schwarzchild radius the measure of the event horizon?

@janbb Cute!

flutherother's avatar

I think @PhiNotPi has it right, that the size is proportional to the mass.

I think the size also depends upon your frame of reference. If you are falling into a black hole you will not be aware of the event horizon and the black hole will appear infinitely small.

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ETpro's avatar

@flutherother If I were falling into a black hole, I would probably not be aware of anything outside my own sheer terror!

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