Questions and Answers about Neutron Stars

Note that many of these were sent to Cole Miller personally after reading his neutron star page, rather than all of the questions being from the Idaho high school students.

1. Are there neutron stars whose magnetic axis and rotating axis are the same, and if so what will happen?

Neutron stars are very hard to find since they are so small and not very bright. The easiest way to find them is when they emit beams of radiation as pulsars. Perhaps as you know, this happens when the rotation axis of the neutron star and the magnetic dipole axis are misaligned. If they were *exactly* parallel, then we wouldn't get the beams of radiation, and it would be very hard to see the neutron star.

Actually, the magnetic field of neutron stars has been mapped in a few cases, and it is much more complicated than a simple dipole. The two magnetic poles are not *exactly* on opposite sides of the star, but are kind of offset to one side. If most neutron stars are like this then it would be impossible for the rotation and magnetic axes to line up.

I guess the bottom line is that, no, we don't know of any neutron stars where the two axes line-up. That may be because it is incredibly unlikely for this to happen, or it may be because if it did the neutron star would not be a pulsar and so would be hard to find, or it may be because the magnetic axis is never a simple dipole and so can never be exactly lined-up with the rotation axis.

Cole Miller

2. Why does a neutron star have a magnetic field if it is composed of neutrons?

Excellent question! The answer is that a neutron star is not *entirely* composed of neutrons. It also contains some number of protons and electrons (probably about 10% each of the number of neutrons). It is those particles, which are electrically charged, that can produce currents and therefore sustain a magnetic field.

Cole Miller

3. Hello I am a Physics student living in the UK and I am currently doing a research project about neutron stars. I read through your online article about neutron stars and although I will admit I did find some of it a bit out of my depth I found it very informative. Part of the project we are doing involves us doing calculations on our research I was thinking maybe of doing maths on how much the star speeds up by, thinking of angular momentums from the incoming mass causing increased velocities as their radius from the centre of mass decreases but this has beaten my mathematical ability. I was also wondering if you could send me any further information on neutron stars for example what is the structure of a neutron star's core? What exactly is a "magnetar" and how are they formed? Also you said in your article that it might be possible for a white dwarf to accumulate enough matter to become a neutron star; So do you think it would be possible for a neutron star to accumulate enough mass to become a black hole?

Let's take those in reverse order. Yes, I do think it is possible for a neutron star to accumulate enough mass to become a black hole. Like a white dwarf, a neutron star has a maximum mass. That maximum is uncertain, but is probably around twice the mass of the Sun. Therefore, if a neutron star adds enough mass to get beyond that maximum, it will collapse, almost certainly to a black hole. This probably actually happens in many supernovae. The idea is the the core of a massive star collapses first to a neutron star, but if enough matter falls back then it can become a black hole.

A "magnetar" is a neutron star with an unusually strong magnetic field that might be 10^15 to 10^16 times the strength of the Earth's magnetic field (compared to 10^13 times the Earth's magnetic field for a typical young neutron star). No one has a good quantitative idea of how magnetic fields form in neutron stars. The maximum possible field strength is on the order of 10^19 times that of Earth (otherwise the magnetic fields contain enough energy to make the neutron star collapse to a black hole!). There is therefore plenty of room for such fields, but we don't know whether magnetars are simply the high magnetic field tail of normal neutron stars or whether they require a special formation history.

The content of a neutron star's core is quite a topical issue now. It could be "merely" neutrons (maybe 90% of the mass) plus some protons (10%) and electrons (an equal number to the protons, to maintain charge neutrality). It could include more exotic subatomic particles, including some with strange quarks. It could be things like kaon condensates. We're not really sure. A very precise measurement of the mass and radius of an individual neutron star would tell us a lot, because the content of the core affects the maximum possible mass and the relation between mass and radius for neutron stars. Mass measurements exist, but radius measurements are extremely tough. They are either imprecise, or highly dependent on models (i.e., not reliable), or usually both. This may change in the future as better X-ray spectroscopy becomes available.

Cole Miller

4. How can gravitational redshift CHANGE gravitational mass, or should I say, changes equivalence principle?? You say baryonic mass is different than gravitational mass in a neutron star. I thought gravitational redshift only refers to the change in observed frequency of radiation; how does that change the mass?

The equivalence principle isn't changed. The effect has to do with two ways you might measure the mass of an object (say, a neutron star). First, you might imagine taking all of the particles in the neutron star separately, weighing them, then adding up all the mass. That's the baryonic mass (so-called because it is the baryons, i.e., the protons and neutrons, that comprise most of the mass). The other way to estimate the mass is to have a satellite orbit the neutron star at a great distance, then measure the mass by using Kepler's laws. That's the gravitational mass.

The baryonic mass isn't affected by gravitational redshift (you still have the same number of particles you did before). However, the gravitational mass is. Think of it this way. According to Einstein's theory, energy can be considered a form of mass, therefore energy gravitates. The gravitational energy is negative (because you would have to add energy to the system to disperse all the particles to a great distance). Therefore, the effect of the gravitational energy is to decrease the gravitational mass. For most objects this effect is significant, but for a neutron star it can amount to 20% of the total mass, so it makes a difference.

Cole Miller

5. I was surprised to learn, some time ago, that neurtron stars are very hot. I had assumed that being small they must have cooled rapidly and become dark and dead bodies since no thermomuclear reactions take place within their cores. How long would it take a neutron star to cool to say room temperature? Is such a thing possible? How would such a cold neutron star look? I always imagined that it would have a metallic glint.

You're right that neutron stars just cool off after their birth, but since they start hot (a trillion degrees!) there is a lot of heat to get rid of. After several billion years they are still at many thousands of degrees, and it would take a large multiple of the age of the universe to get to room temperature (exact values depend on unknowns about the neutron star; for example, it matters how much energy they have lost in neutrinos). The surface of such a star would probably be made of hydrogen in long chains of atoms. At room temperature, the emission would be almost all in the infrared, so the star would appear completely dark. If you were to shine a light on the neutron star, its appearance would depend on whether it had maintained its strong magnetic field. If so, the light would not be absorbed until fairly deep in the atmosphere, and when it was radiated it would again be in the infrared, so it would appear black. If the magnetic field had decayed away, the star might have a metallic glint; I'm not sure, because it's difficut to project the optical look of such an object.

Cole Miller

6. Assuming a cool neutron star that had long ago captured all loose matter in its vicinity, it would surely be a most dangerous stealth object to any interstellar traveler. Would there be any possible way to detect its presence short of approaching it closely enough to be in danger of capture? In other words, would there be any marker for its presence at astronomical distances?

If a cool neutron star did capture, say a stray asteroid, would the force of impact cause the star to light up or would it be so minor as to produce no great effect?

Is there any conceivable way that matter could leave the surface of a neutron star and be launched into space? If it collided with an asteroid traveling at 90% of the speed of light (granting the possibility of such speed) would the force of such impact dislodge some material and accelerate it beyond the star's gravitational field? If neutron material were dislodged from a neutron star, would it retain its density or would it immediately expand into normal matter?

The neutron star starts off hot because of the heat generated when the core of the pre-supernova star collapses into a neutron star. The star does give off gamma rays at that point (as well as neutrinos, early on). The star does shrink; the heat puffs it out, but it settles down later. The cooling is fast initially (seconds) but slows down as time wears on. Relativity doesn't change the shape of neutrons, because a fundamental principle of relativity is that in the rest frame of anything (i.e., the frame in which that thing isn't moving), everything appears to be normal (that may be a bit cryptic, but check out some books on relativity if you want more details). The high density, however, may change the shape of nucleons. Finally, although most of the bulk of a neutron star is neutrons, there are also protons and electrons present, and near the surface where the density is low there are atoms. Admittedly these atoms are distorted by the strong magnetic field, but they are there.

You could easily detect a neutron star at a distance by its gravity, which is that of about one and a half Suns. This would become evident long before there was any danger. If something did hit the star, then indeed there would be a huge release of energy (about 30 times the energy per mass of a hydrogen bomb!). Yes, if an asteroid somehow traveled at 90% of the speed of light relative to the neutron star, there is in principle enough energy to kick some matter out into space (whether it actually happened would depend on some details). A chunk of the star would explode and change from mainly neutrons into normal matter, because it's the high density and strong gravity that allows the stuff to stay in that state!

Cole Miller

7. In the neutron star page at, you state that the mass of a neutron star is about 20 % lower than its actual baryonic mass due to the gravitational redshift. I've seen this phrase used in regards to optical effects but never in regard to a mass reduction.

Is the effect you're describing here associated with the relativistic lengthening of the radius that occurs when the escape velocity approaches c? In other words, as the star collapses to something approaching a black hole, I know that the true radius can no longer be described as the circumference divided by 2*pi. Does this lengthening of the radius result in an apparent lose of mass (since gravitational is inversely proportional to the square of the radius? Does this effect account for the missing 20%?

It's not the change in the radius per se, although all those effects have a common origin. The point is that gravitational binding energy is negative. That means, for example, that to take a ball from the surface of the Earth and put it to a large distance away requires an input of energy. Therefore, the total energy of the ball+Earth is less than it would be if you weighed the ball and Earth separately. Stated another way, the total mass of the ball+Earth (as measured by a distant orbiting satellite) is less than the total mass of the ball plus the total mass of the Earth, weighed separately.

For a neutron star this effect is really large, so that the total mass of the NS is 20% less than the total mass of all its constituents, weighed separately. That's the effect.

Cole Miller

8. Hi! I just read your interesting article on your website. What I would like to know is, how do you observe those midgets? Is it through radio telescopes, and if so, how can they pinpoints such a minuscule region? Or is everything just indirect conclusions and deduction? Optically those tiny powerhouses are obviously unobservable. What amazes me very much is the knowledge of the internal structure of neutron stars. How can one know so much with so much detail?

As you point out, neutron stars are much too small and distant to be resolved by any telescope. However, that's also true for almost all stars. Like with main sequence stars, neutron stars can be observed for their spectrum (in many wavelengths, including radio, X-ray, gamma-ray, and even optical in a few cases). If they are pulsars, the high regularity of their pulsations presents a standard against which minute changes can be measured. That is, we can measure not only the spin period, but its change, which tells us about its magnetic field and in some cases (e.g., when there is a "glitch", or sudden change in the rotation) might give clues to their interior structure. The temperature of the surface can sometimes be estimated from X-ray emission, and this also gives indirect clues towards the interior structure; if real exotica were common it turns out that we'd expect cool neutron stars.

Much of the knowledge about the interior structure of neutron stars comes not from astronomical observations but from nuclear physics on Earth. That is, we know from experiments how matter will behave under certain extreme conditions of density, so we can figure out how this would apply to neutron stars. However, beyond nuclear density we don't have good laboratory data, so that's where observations of neutron stars themselves might feed back into nuclear physics.

The overall answer to your question is that, indeed, indirect conclusions and deduction are what we need to use for neutron stars. But that's not special to just these objects; in astronomy we almost always have to draw our conclusions indirectly, for the simple reason that we can't go out into space and experiment on stars, so we have to make do with the information we can gather.

Cole Miller

9. I have a question about the picture showing the accretion geometry of a neutron star. The picture is Accreting-Neutron-Star.jpg from the Marshall Space Flight Center on nstar.html. It shows a zoom in on the neutron star, just showing the large and familiar accretion disk on the edges of the picture. Well-illustrated and closer to the neutron star are the magnetic field lines (I assume just the really strong ones are shown, much like how no one draws the looping return paths on the outside of a solenoid). Those field lines look warped compared to, say, the shape of the Earth's magnetic field lines. What causes the warpage? Is it due to the plasma flow from the accretion disk? Why does the plasma from the jet flow where it is shown?

The plasma flow from the disk does indeed cause warpage of the field lines. This also happens with the Earth, because of the charged particles in the solar wind. For an accretion disk, you can think of it this way. The energy density in the disk scales with radius r as r^(-2.5), whereas the energy density in the magnetic field scales as r^(-6). Therefore, very far away (large r), the disk energy density dominates. This means that the magnetic field of the neutron star has little effect; indeed, it is dragged with the rotation of the disk. However, close in (small r), the magnetic field energy density dominates. Therefore, plasma from the disk couples strongly to the stellar magnetic field.

If you were to look at field lines, the result would be that close to the star, the field lines would look just like a standard dipole field (assuming that higher multipoles didn't contribute). Very far away, though, in the midplane of the disk the disk plasma would penetrate into the field, meaning that the field lines would bend closer to the star in the midplane of the disk than they would higher up.

When the plasma couples strongly to the field, it can be thought of as "beads on a wire" moving along the field. When the field has a geometry such that the beads can slide downward towards the star, they will. However, remember that the field lines rotate with the star. Therefore, when the plasma couples to the field it is forced to rotate at the stellar spin frequency. If this is greater than the local orbital frequency, the plasma is forced *out*; if it is less than the local orbital frequency, the plasma can fall in and accrete onto the star. The net result is that if the field is weak enough or the plasma flow is strong enough, matter falls onto the star, but the geometry forces the plasma near the magnetic poles.

Cole Miller

10. What is the estimate of total mass of all neutron stars in milky way?

The estimates of the number of neutron stars in the Milky Way are uncertain, but a round number is probably several hundred million. Each of them likely weighs around 1.5 solar masses, so the total mass is probably around a billion solar masses.

Cole Miller

11. Do you suppose that if a GRB were to occur, say, 1000 light-years from the earth, the gamma rays would kill life on the planet?

If a gamma-ray burst were to occur 1000 light years from Earth and pointed directly at us, then for a few seconds it would appear much brighter than the Sun. I suspect that the result would be that the side facing the gamma-ray burst would be blasted pretty strongly, probably also creating horrendous winds in the rest of the planet. My guess is that sea life would survive on the far side of the Earth, so the planet wouldn't be completely sterilized, but I wouldn't want to be around then!

Cole Miller

12. One unanswered question I have is whether a neutron star can accrete enough material to pass through successive states of density, i.e. quark star and on to a black hole collapse? Are there other states?

In principle if you fed enough matter to a neutron star it would collapse to a black hole. We don't know if there is an intermediate state; some people think it is possible, but the evidence is inconclusive. In practice, it is not clear whether systems exist that transfer enough mass for a collapse to a black hole to happen, but there is enough evidence for neutron stars near their maximum mass that I think it likely that these collapses do occur on occasion. It also isn't obvious what the observational signature would be; maybe it's a spectacular explosion, but maybe it just goes quietly!

Cole Miller

13. The recent (yesterday?) article about neutron stars that periodically blow up by undergoing fusion the accreted hydrogen on their surface, begs the question about whether this process increases the net mass of the neutron star or not, or if there is some repetitive explosion process without ultimate collapse?

The mass does increase; at least 95% of the accreted mass does stay on.

Cole Miller

14. First, concerning magnetars: when their magnetic field causes them to slow down, do they become pulsars for a time before becoming a regular neutron star or do they go straight from a magnatar to a neutron star? Second, I have heard some references that claim neutron stars that are not part of a multiple star system and are not accreting matter sometimes explode as supernovas. Is this true or do they just exist as a stable silent lump of neutrons with a solid crust of iron nuclei forever?

Thanks for your interest. Magnetars probably do have a period of activity as radio pulsars; it is, however, difficult to see them, because they spin down rapidly and as part of that probably have very narrow opening angles for their radio emission (so that we would be likely to miss them). As far as isolated neutron stars go, they just sit and cool off. The only possible loophole to that would be if a neutron star were born spinning rapidly and with a high enough mass that it would collapse into a black hole if it were rotating more slowly (rotation supports the star against gravity, so a rotating NS could have a higher mass). In that case, one could imagine the star slowing down and eventually collapsing, but that would probably be quiet and certainly wouldn't release enough energy to cause the star to explode.

Cole Miller

15. I am a school (physics) teacher trying to understand a little more about the end of the life of stars. I am confused by what I've read in books and on the web, particularly about the masses of neutron stars and black holes. Some sources refer to masses of 1.5 solar masses (largest mass for white dwarf being the end point) and 3.0 solar masses (smallest mass for a black hole). Your website mentions 15 and 30 solar masses.

I actually started researching because a text book the students use says black holes result from stars with masses 4 times the Sun's and supernovas are caused by the collapse of (blue giant) stars with masses of about 10 x the Sun's mass.

Can you help me to understand this?

Are all these masses that are much larger than 3 solar masses masses of objects before lots of material is lost (in the supernova explosion) and the smaller masses the masses of the core that remains?

Sorry about the confusion! The distinction to be made is the *initial* mass of the star (as in, the mass it has just when it starts thermonuclear fusion), versus the final mass of the neutron star or black hole that results from that star. Suppose a star begins its life at 20 times the mass of the Sun. It is a very bright star; so much so, in fact, that the radiation it emits throughout its life drives a lot of mass away from the star. This is especially true during the giant phase. Then, when the central core collapses and produces a supernova, even more mass is driven away. The result is that the central core (a neutron star in this case) ends up with only about 1.5 to 2 times the mass of the Sun.

For a star that begins with 40 times the mass of the Sun it's the same thing, except that apparently enough matter falls back during the supernova that the central core goes beyond what a neutron star can support and becomes a black hole.

Cole Miller

16. How could a star in a binary survive its companion exploding in a supernova?

Thanks for your question! The basic reason that a star in a binary could survive its companion exploding in a supernova is that a surprisingly small fraction of the energy of the supernova is actually absorbed by the companion. Even for a very close binary, the binary separation would be a good 10 times their size. This means that only about 1/400 of the supernova's kinetic energy is absorbed by the companion. The binding energy of a star (i.e., the energy required to completely destroy it) is at least 1/100 of the kinetic energy of a supernova. This means that, yes, the outer layers might be blown off, but most of the star will survive just fine. Mind you, I wouldn't want to be around when it happened... :)

Cole Miller

17. The question I have for you is, what are magnetars, how are they created and what kind of affects do they have on space?

A magnetar is a type of neutron star with an especially strong magnetic field. Let me give you an idea of the scale we're talking about, though. The strength of the Earth's magnetic field at the pole is about 0.5 Gauss. The strength of a refrigerator magnet at its surface is about 100 Gauss. The strength of the magnetic field in an MRI machine is about 10,000 Gauss. This barely scratches the surface, though: the magnetic field at the surface of a typical young neutron star is about a *trillion* Gauss, which is so strong that the very atoms are stretched out into cylinders along the field lines. Magnetars have fields about a thousand times stronger yet, and at that level really weird things happen microscopically (photons can split in two, for example).

The effect they have is that the field "wants" to come together (because this would reduce its energy), and the crust of even a neutron star can't resist this force forever. As the field pulls, then, the crust breaks sometimes, giving the great-granddaddy of all earthquakes. This produces a burst of gamma rays, and the brightest of the bursts are so powerful that they can produce aurorae on Earth from 25,000 light years away! Not something you'd like to get near...

How they are created is a mystery. In fact, in general, we don't have good knowledge of how magnetic fields are produced in most objects. In fact, a joke attributed to Sir Martin Rees (an eminent astronomer in England) is that if you sleep though an astronomy talk and want to ask an intelligent-sounding question, just say "what about magnetic fields?" Neutron stars are produced when a massive star dies and its core collapses, and the best current guess is that something about the collapse and turbulence produces an extra-strong magnetic field. The honest answer, though, is that we really don't know for sure.

Good questions!

Cole Miller

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