The Density of Matter in a Black Hole

Question: A 10 solar mass object would have 10 solar masses enclosed in a radius of 30 km.   The density of a spherical object scales as M/R3.  Relative to the 10 solar mass black hole, how many times denser would a black hole the mass of the Earth be?  How many times less dense would a black hole of 1 million solar masses be?  – Paul

Answer: As my colleague Cole Miller points out in his description of the properties of black holes and neutron stars, unlike ordinary things (e.g., rocks), which have a size roughly proportional to the cube root of their mass, black holes have radii proportional to their mass.  Taking the event horizon of a black hole as the definition of its outer boundary, the event horizon of a nonrotating black hole the mass of our Sun would have a radius of about 3 kilometers.  This implies that the more massive the black hole is the denser it is, meaning that larger black holes are not very dense.  For example, a one-billion solar mass black hole, which is the type of black hole that is thought to exist at the center of some galaxies (like ours), has an average density just twenty times the density of air.

Jeff Mangum

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In What Direction Does the Outflow from a Quasar Go?

Question: Thank you for answering! Is the supposed spinning the reason all the body’s caught in the pull stay in a synchronized belt around it? If so, where does a quasar emit? “North and south?” Or anywhere it pleases? I know very little known, but these questions are the reason I can’t sleep at night.  – Nathan

Answer: Quasars emit their outflows which are suggested to coincide with the rotation axis of an embedded black hole.  This direction is preferred as it represents the direction where the angular momentum is lowest, thus allowing the outflow material to propagate away from the black hole.

Jeff Mangum

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Can a Neutron Star Become a Black Hole?

Question: Can you help with this new question could a man made neutron star be made to then collapse it into a black hole, but I am not sure if it is answerable.  A neutron generator can generate to 108 neutrons per second.  A 1 microampere ion beam accelerated at 200 kV to a titanium-tritium target can generate up to 108 neutrons per second. The neutron yield is mostly determined by the accelerating voltage and the ion current level.  If you were going to attempt to create a man made neutron star using a neutron generator in space, because this is the only place where you could do this experiment.  First using the neutron generator which can make 108 neutrons per second, you would start by putting all the neutrons into one place.  As you generate, and put all the neutrons into one place you would probably have a microscopic sphere of neutron matter after a while.  So here is the problem, I do not know the exact point in spherical mass where neutrons become stable in a neutron star, because its the pressure from gravity that is compacting them, and stopping them from decaying back to protons.  If you knew the exact point in spherical mass where neutrons become stable with gravity, you could calculate the number of neutron generators you would need, with the time that the neutrons would decay back into protons.  The other thing I do not know is does the exact point in spherical mass in a neutron star where neutrons are stable,  would that exact point in a spherical mass of neutrons have the same strength gravity as a medium sized star to crush the neutrons into place, and keep the neutrons stable, as you were putting them together with the neutron generator.  Making a sphere the size of a mile may not be enough because you would have to compact the neutrons together closely in the same way a medium sized star does to keep the neutrons stable from decaying.  So does a mile, or more of a sphere of non-compacted neutrons be enough to create a gravitational field as strong as a medium sized star, or does the sphere of neutrons need to be compacted together more to create stronger gravity to hold the neutrons into place.  So to put it as simple as possible is putting neutrons together in a sphere a mile or more in spherical mass going to create enough gravity to compact the neutrons into place to keep the neutrons stable from decaying into protons.  Getting more material to complete this man neutron star to collapse it into a black hole would not really be a problem if you parked it next to a giant star, because the gravity would draw in the material from the giant star and add it to its own mass to make it larger, and I read somewhere the neutron star would not burn so close to a giants star, to absorb its material, and add it to its own mass.  Can you help with any of the questions.  – Nicholas

Answer: I think that your question is whether a neutron star can become a black hole.  Neutron stars are thought to have masses between the Chandreskhar limit of 1.39 solar masses to about 3 solar masses.  If a neutron star gathers more mass and gets to the point where its mass reaches about 10 solar masses, its mass will overcome the neutron degeneracy pressure that supports it against gravity and collapse to become a neutron star.

Jeff Mangum

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Can Time Dilation Near a Black Hole Explain the Orbits of Stars Near the Black Hole?

Question: Regarding galaxy rotation.  Although the supermassive black hole at the center of a Galaxy probably isn’t strong enough to speed up the rotation of its more distant stars so as to make the velocity curve level, nevertheless the time-dilation effect of such a black hole will make the orbital rotation speeds of the inner stars appear slower than it actually is as seen from the point of view of an observer outside the galaxy in question. Couldn’t this be a more reasonable explanation rather than hypothesizing ‘dark matter’?  – Tom

Answer: I don’t think that time dilation will affect the observed speed of stars near a black hole as observed from a point far from the black hole.  Time dilation affects what local observers measure, which means that the time measured by an observer at a star near a black hole will see time running slower than the time measured by the far-away observer.

Jeff Mangum

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What Happens to a Black Hole That Radiates to “Nothing”?

Question: Just to clarify, Hawking’s radiations takes place simultaneously to the black hole growing? and, then becomes dominant once the black hole stops growing?  Further, having depleted with a final outburst/explosion leaving behind ” nothing ” – to what level is this “nothingness” defined? No mass,no energy,no waves, no “quantum fluctuations”? dark matter, no dark energy? It seems to be that it is becoming increasingly difficult to define ” nothing “?!  – Simon

Answer: As you have said, “Hawking Radiation” is the theoretical process by which black holes that spin lose energy (and, therefore, mass).  For black holes that lose more mass than they gain (i.e. those that do not collect a lot of mass due to accretion), in theory a black hole can lose all of its mass and “vanish”.  This means that all matter associated with the black hole has been emitted as energy.

Jeff Mangum

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Do Black Holes Move Through Spacetime?

Question: Does a black hole move through spacetime or does it draw spacetime to “it” giving the illusion its moving?  – Nigel

Answer: Black holes move through spacetime just like any other object in the universe.  It is true, though, that black holes “warp” spacetime more than other objects due to their extremely intense gravitational field.

Jeff Mangum

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How Can Jets of Matter Escape a Black Hole?

Question: I don’t understand this:
I read somewhere (and this isnt a quote but its close enough) that

….the gravitational pull of a black hole is so great that nothing can escape from it – not even light ( thats what makes them black).

I also read that its the general scientific concensus that nothing can go faster than light ( or else some law is getting broken or something like that)… doesn’t this mean that the stuff in the jets escaping out of black holes is going faster than light can go ,cos light cant get out?  – Nigel


Answer: The matter that we observe as jets emanating from a black hole are not actually coming from the black hole itself.  The jets are composed of matter which is escaping from the accretion disk which surrounds the black hole.  Although the mechanism by which the jets are produced is not completely understood, the process likely involves the acceleration of matter near the poles of the black hole and an interaction with the tangled magnetic field in the region near the poles of the accretion disk.  The material in the jets is measured to be travelling at less than the speed of light.


Jeff Mangum

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A Black Hole That Comes Back to Life?

Question: My partner was driving  home yesterday night ( 29 june 2015) listening  to the radio, they were talking about a black hole that they realised had came back to life again.  Would you know specific black hole they talking about?  Or what constellation its near to.  Like i said my partner was hearing  it on the radio on the 29 june 2015.  – Pete

Answer: I suspect that the story you heard was about V404 Cygni, which is a black hole and star system in our galaxy, in the direction of the constellation Cygnus, which has recently “flared”, or produced an outburst of energy at xray wavelengths.  Although the specific mechanism by which black holes flare is not entirely understood, it likely has something to do with the amount of matter being fed to the black hole (by, in this case, the nearby star that is gravitationally coupled to the black hole).


Jeff Mangum

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What Lies Beyond the Known Universe?

Question: Do dimensions really exists? [Also,] are the black holes really or possibly portals leading to different dimensions (like a whole new universe or a totally different universe)?  [Also,] right now I read in the blog about like the diameter of universe being 96 billion light years (in other sense the universe being confined within these 96 billion light years diameter) so what lies beyond that diameter?  – Pravesh

Answer: One can speculate or theorize as to what exists beyond that which we can measure, but that is, strictly speaking, not science.  What scientists do is make observations of the universe and apply the laws of physics to those observations to interpret what they observe.  As there are no actual observations which can tell us what lies beyond a black hole or the edge of the universe, we can only speculate or theorize as to what might lie beyond what we observe.  To my knowledge there are no theoretical predictions which are based on physics which predict what lies beyond black holes or the edge of the universe, which leaves us only speculation, which is not science.  So, a scientist would answer that there is no information to tell us what lies beyond a black hole or the edge of the universe.

Jeff Mangum


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Can a Black Hole Become Unstable and Produce a “Small Bang”?

Question: Can a black hole become unstable and cause a “small bang”? Creating parallel universes? – Raoule

Answer: I don’t believe that there are any reliable theoretical calculations that produce this series of events.  In fact, I don’t believe that “unstable” black holes lead to explosions of any kind.

Jeff Mangum

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