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Astronomy/Black Holes and Neutron Stars


QUESTION: Neglecting differences in mass, what are the observable differences between black holes and neutron stars?

ANSWER: For stellar-mass black holes, there aren't many differences. The normal way of observing a neutron star or stellar-mass black hole is when an aging binary companion dumps large amounts of mass onto/into the "dead" star. Under those circumstances the material tends to spiral into the black hole, or onto the neutron star, in a so-called accretion disk located in the orbital plane of the two stars. As material spirals toward the event horizon of the black hole or the surface of the neutron star it is compressed and heated by the material behind it, which is doing the same thing, but is blocked by the material in front of it. This generates a lot of heat and radiation, but the material in the accretion disk is too thick to allow the radiation to escape in the plane of the disk, so it (and often large amounts of mass) are ejected toward the two poles of the accretion disk plane. Stellar-mass black holes can heat the material to higher temperatures and produce brighter ejection plumes than neutron stars, but the observation of such an object wouldn't necessarily allow us to tell whether its source is a black hole or a neutron star.

There is at least one theoretical difference, in that gamma-ray bursters (objects that briefly emit huge amounts of gamma radiation in only one or two directions) are thought to be caused by the collision and merger of a pair of orbiting neutron stars, whereas black holes are thought to be incapable of producing that particular phenomenon. (An interesting sidelight of these supposed collisions is that they are theoretically responsible for the creation of most of the gold in the Universe, because every other current theory that tries to explain why gold is as abundant as it is falls far short of the mark.)

Now if we leave the realm of stellar masses (and therefore, neutron stars as well), super-massive black holes, which can have masses of tens of millions or billions of solar masses, can strongly affect the motion of stars in their neighborhood, and in fact the black hole at the center of our Milky Way Galaxy has had its mass measured by the exceptionally rapid "quasi-orbital" motions of stars that lie relatively close to it. There is also the possibility, with such very massive black holes, of having far larger masses falling into them and/or their accretion disks, in which case the radiation emitted is so great that it creates "polar" ejection plumes as large as galaxies (which have been observed in many galaxies, presumably proving the existence of super-massive black holes near the center of those galaxies). If the radiation and mass ejected is large enough, it may produce what we call a quasar, but there aren't any quasars close enough to us to study their properties in great detail. All we can say is that are a result of immense amounts of radiation being produced inside extremely small regions, and the only way we can think of to explain such events is to have huge amounts of material try to fall into a super-massive black hole, and producing a super-polar-ejection event.

There is also a theoretical possibility that super-massive black holes could bend the light of more distant objects in a relatively extreme way, which is often depicted in artistic representations which are almost certainly not at all the same as what we would see if we were actually close enough to a black hole to see the results of that distortion. But the closest massive black hole is the one in the center of our galaxy, and we haven't observed any distortion of light by it, nor have we seen any evidence of the bending of light by any other black hole. We have seen distortion of light by very massive objects (namely by whole galaxies, and even more impressively by massive clusters of galaxies), but never by black holes, no matter how massive.

So you can see that there are some things that suggest that this event here or that one there was caused by a black hole or neutron star, but in the realm of stellar masses, it is difficult or impossible to say which was the cause. It is only when you get to super-massive black holes that you begin to see things that can only be explained by black holes, and since neutron stars cannot have masses or more than 2 or 3 solar masses, they do not enter into that part of the discussion at all.

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QUESTION: I believe that my next question has already been answered in the first answer but I'll ask it just to be sure.  

Is the explanation of the relativistic jets produced by neutron stars and the explanation of the relativistic jets produced by black holes the exact same explanation (that is, are they explained by the exact same process for both objects or are there any differences pertaining to either one)?

(Sorry for the delay in answering this. Yesterday was my wife's birthday and we were out and/or busy all day, so I didn't receive your question until well after midnight, and was too tired to give it a proper response at that time.)

The basic process of accretion disk radiation is the same. The primary constraint on the energy produced by the accretion disk is how fast material from the source of the material is falling onto the disk. For very massive black holes (greater than the mass limit for neutron stars, which cannot be more than 2 or 3 solar masses), and particularly for supermassive black holes, the distance within which material can be pulled toward the black hole is greater, so larger masses could be trapped in the accretion disk, and greater amounts of radiation produced. That is why extremely massive black holes can blow material out of galaxies, whereas low-mass black holes can only spew out far smaller amounts of radiation and mass. But within the range of masses where the object could be either a black hole or a neutron star, the basic process should be almost indistinguishable.

I can imagine circumstances in which the fact that neutron stars have extremely intense magnetic fields, which can affect the behavior of the infalling material (particularly if the field is not aligned with the rotational axis, causing intermittent radiation similar to that of a lighthouse, as in the case of so-called pulsars), there would be additional ways of observing their existence (which I forgot to mention the other day). But otherwise, accretion-disk radiation from neutron stars should look about the same as the similar radiation from equal-mass black holes.

Incidentally, the compression of the infalling gas can generate so much heat and radiation that as much as 70% of its mass can be turned into energy, per Einstein's E = m c-squared formula. This is about a hundred times more energy than can be generated by the most energy-efficient nuclear reaction, the conversion of hydrogen into helium in the core of a normal star. That is why quasars are so bright. Dumping a solar mass of gas into a super-massive black hole can produce 100 times as much energy as the Sun would radiate in its entire 10+ billion year lifetime, or about as much energy as a trillion ordinary Suns.


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Courtney Seligman


I can answer almost any question about astronomy and related sciences, such as physics and geology. I will not answer questions about astrology and similar pseudo-scientific rubbish.


I have been a professor of astronomy for over 40 years, and am working on an online text/encyclopedia of astronomy, and an online catalog of NGC/IC objects.

Astronomical Journal, Publications of the Astronomical Society of the Pacific (too long ago to be really relevant, but you could search for Courtney Seligman on Google Scholar)

I received a BA in astronomy and physics and a MA in astronomy, both from UCLA. I was working on my doctoral dissertation when I started teaching, and discovered that I preferred teaching to research.

Awards and Honors
(too long ago to be relevant, but Phi Beta Kappa and Sigma Xi still keep trying to get me to become a paying member)

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