Getting a grip on a strongly magnetized neutron star’s geometry

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Questions and Answers about Neutron Stars

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

1. Are in that location neutron stars whose magnetic centrality and rotating centrality are the same, and if and then what will happen?

Neutron stars are very hard to notice since they are so small and not very bright. The easiest way to notice them is when they emit beams of radiation as pulsars. Mayhap as you know, this happens when the rotation centrality of the neutron star and the magnetic dipole axis are misaligned. If they were *exactly* parallel, then we wouldn’t get the beams of radiations, 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 unproblematic 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 similar this then information technology would be impossible for the rotation and magnetic axes to line up.

I guess the lesser line is that, no, we don’t know of whatsoever 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 non be a pulsar and so would be hard to find, or it may be because the magnetic centrality is never a simple dipole and and then tin 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?

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

Cole Miller

3. Hi I am a Physics educatee living in the UK and I am currently doing a research projection nearly neutron stars. I read through your online article nigh neutron stars and although I will admit I did notice some of information technology a bit out of my depth I found it very informative. Role of the project we are doing involves us doing calculations on our enquiry I was thinking peradventure 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 only this has browbeaten my mathematical ability. I was also wondering if you could send me any further information on neutron stars for case what is the construction of a neutron star’s core? What exactly is a “magnetar” and how are they formed? As well you lot said in your article that information technology might be possible for a white dwarf to accumulate enough thing 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’due south accept those in contrary order. Aye, I practise think it is possible for a neutron star to accumulate enough mass to become a black hole. Similar a white dwarf, a neutron star has a maximum mass. That maximum is uncertain, only is probably around twice the mass of the Sun. Therefore, if a neutron star adds plenty mass to become across that maximum, it will plummet, near 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 affair falls dorsum then it tin can become a black hole.

A “magnetar” is a neutron star with an unusually potent magnetic field that might be 10^fifteen to 10^16 times the forcefulness of the Earth’s magnetic field (compared to 10^xiii times the Earth’south magnetic field for a typical young neutron star). No ane has a good quantitative thought of how magnetic fields form in neutron stars. The maximum possible field strength is on the order of 10^xix times that of Earth (otherwise the magnetic fields comprise enough energy to make the neutron star collapse to a black pigsty!). In that location 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 cadre is quite a topical issue at present. It could be “merely” neutrons (maybe ninety% of the mass) plus some protons (10%) and electrons (an equal number to the protons, to maintain accuse neutrality). It could include more exotic subatomic particles, including some with foreign quarks. Information technology could exist things like kaon condensates. We’re non actually 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, just radius measurements are extremely tough. They are either imprecise, or highly dependent on models (i.e., not reliable), or normally both. This may alter in the hereafter every bit ameliorate 10-ray spectroscopy becomes bachelor.

Cole Miller

4. How can gravitational redshift CHANGE gravitational mass, or should I say, changes equivalence principle?? Y’all say baryonic mass is different than gravitational mass in a neutron star. I idea 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 issue has to practice with two means you might measure out the mass of an object (say, a neutron star). Outset, you might imagine taking all of the particles in the neutron star separately, weighing them, so adding up all the mass. That’s the baryonic mass (so-called because it is the baryons, i.e., the protons and neutrons, that contain most of the mass). The other way to estimate the mass is to have a satellite orbit the neutron star at a nifty distance, and so mensurate the mass by using Kepler’s laws. That’southward the gravitational mass.

The baryonic mass isn’t affected by gravitational redshift (you nonetheless have the same number of particles yous did earlier). Nevertheless, the gravitational mass is. Think of it this fashion. According to Einstein’s theory, free energy tin can exist considered a form of mass, therefore energy gravitates. The gravitational energy is negative (because you would accept to add together energy to the system to disperse all the particles to a not bad distance). Therefore, the effect of the gravitational energy is to subtract the gravitational mass. For most objects this effect is significant, but for a neutron star it can corporeality to 20% of the total mass, so it makes a difference.

Cole Miller

five. I was surprised to learn, some fourth dimension ago, that neurtron stars are very hot. I had assumed that being pocket-size they must have cooled apace and get nighttime and dead bodies since no thermomuclear reactions have identify 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 ever imagined that it would have a metallic glint.

Y’all’re right that neutron stars just absurd off afterward their birth, merely since they start hot (a trillion degrees!) there is a lot of rut to get rid of. Later on several billion years they are even so at many thousands of degrees, and it would take a large multiple of the age of the universe to become to room temperature (exact values depend on unknowns nearly the neutron star; for example, it matters how much free energy they accept 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 exist well-nigh all in the infrared, then the star would announced completely dark. If you were to smooth a calorie-free on the neutron star, its appearance would depend on whether information technology had maintained its strong magnetic field. If so, the light would not be absorbed until adequately deep in the atmosphere, and when it was radiated it would again be in the infrared, so information technology would appear black. If the magnetic field had rust-covered away, the star might have a metallic glint; I’m not sure, because it’s difficut to projection the optical wait 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 exist any possible style to detect its presence short of approaching it closely plenty to be in danger of capture? In other words, would in that location be whatsoever marker for its presence at astronomical distances?


If a cool neutron star did capture, say a stray asteroid, would the force of touch on cause the star to light up or would it exist so pocket-sized as to produce no great effect?


Is there any conceivable way that affair 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 strength of such impact dislodge some cloth and accelerate information technology beyond the star’southward gravitational field? If neutron material were dislodged from a neutron star, would it retain its density or would information technology immediately expand into normal matter?

The neutron star starts off hot because of the rut generated when the core of the pre-supernova star collapses into a neutron star. The star does requite off gamma rays at that signal (also as neutrinos, early). The star does compress; the heat puffs it out, but it settles downwardly 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 flake cryptic, but bank 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 majority 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 past the strong magnetic field, simply they are there.

You could easily detect a neutron star at a distance past its gravity, which is that of almost ane and a half Suns. This would become axiomatic long before there was any danger. If something did hit the star, then indeed at that place would exist a huge release of energy (about 30 times the energy per mass of a hydrogen bomb!). Aye, if an asteroid somehow traveled at 90% of the speed of low-cal relative to the neutron star, there is in principle enough energy to boot some thing out into space (whether it really happened would depend on some details). A chunk of the star would explode and change from mainly neutrons into normal matter, because it’south the high density and strong gravity that allows the stuff to stay in that country!

Cole Miller

vii. In the neutron star page at http://www.astro.umd.edu/~miller/nstar.html, you lot 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 merely 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 budgeted a black hole, I know that the true radius can no longer be described equally the circumference divided past 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 outcome account for the missing xx%?

It’southward not the alter in the radius per se, although all those effects take a mutual origin. The point is that gravitational binding energy is negative. That means, for instance, that to have a ball from the surface of the World and put it to a large altitude away requires an input of energy. Therefore, the total energy of the ball+Earth is less than it would be if yous weighed the ball and World separately. Stated some other style, the full mass of the ball+Earth (equally 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’southward the effect.

Cole Miller

eight. Hi! I but 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 but indirect conclusions and deduction? Optically those tiny powerhouses are obviously unobservable. What amazes me very much is the knowledge of the internal construction of neutron stars. How tin can i know so much with and then much particular?

As you signal out, neutron stars are much too small and afar to be resolved by any telescope. However, that’due south also true for near 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 tin be measured. That is, we can measure not just the spin period, only its change, which tells the states about its magnetic field and in some cases (eastward.g., when there is a “glitch”, or sudden modify in the rotation) might requite 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 nosotros’d expect cool neutron stars.

Much of the knowledge about the interior construction of neutron stars comes non from astronomical observations but from nuclear physics on Earth. That is, we know from experiments how matter will bear under certain farthermost weather condition of density, so we tin figure out how this would apply to neutron stars. Still, across nuclear density we don’t have practiced laboratory data, so that’s where observations of neutron stars themselves might feed back into nuclear physics.

The overall reply to your question is that, indeed, indirect conclusions and deduction are what we need to use for neutron stars. But that’s non special to just these objects; in astronomy we almost always accept to draw our conclusions indirectly, for the simple reason that we tin 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 nigh the picture showing the accretion geometry of a neutron star. The picture is Accreting-Neutron-Star.jpg from the Marshall Space Flying Eye on nstar.html. It shows a zoom in on the neutron star, simply showing the large and familiar accretion disk on the edges of the flick. 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 i draws the looping return paths on the outside of a solenoid). Those field lines look warped compared to, say, the shape of the Earth’south magnetic field lines. What causes the warpage? Is information technology due to the plasma flow from the accretion disk? Why does the plasma from the jet menses where information technology is shown?

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

If yous were to await at field lines, the result would be that close to the star, the field lines would wait only like a standard dipole field (assuming that higher multipoles didn’t contribute). Very far away, though, in the midplane of the deejay the deejay plasma would penetrate into the field, pregnant that the field lines would bend closer to the star in the midplane of the disk than they would higher upwardly.

When the plasma couples strongly to the field, information technology can be thought of every bit “beads on a wire” moving along the field. When the field has a geometry such that the chaplet can slide downwardly 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 consequence is that if the field is weak enough or the plasma period is strong enough, matter falls onto the star, only 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 one thousand thousand. Each of them likely weighs around 1.v solar masses, so the total mass is probably around a billion solar masses.

Cole Miller

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

If a gamma-ray burst were to occur 1000 light years from Earth and pointed straight at us, and so 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 flare-up would be blasted pretty strongly, probably also creating horrendous winds in the balance 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 tin can accrete enough material to laissez passer through successive states of density, i.eastward. quark star and on to a black hole collapse? Are there other states?

In principle if you fed plenty matter to a neutron star it would collapse to a black pigsty. We don’t know if there is an intermediate state; some people think information technology is possible, but the evidence is inconclusive. In practise, information technology is non articulate whether systems be that transfer enough mass for a plummet to a black hole to happen, but there is enough evidence for neutron stars near their maximum mass that I think it probable that these collapses exercise occur on occasion. It too isn’t obvious what the observational signature would be; maybe it’southward a spectacular explosion, merely maybe information technology only goes quietly!

Cole Miller

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

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

Cole Miller

xiv. Starting time, concerning magnetars: when their magnetic field causes them to tiresome downwardly, practice they get pulsars for a time before becoming a regular neutron star or practise they go straight from a magnatar to a neutron star? Second, I have heard some references that claim neutron stars that are non office of a multiple star arrangement and are not accreting affair sometimes explode as supernovas. Is this truthful or do they just exist every bit a stable silent lump of neutrons with a solid crust of iron nuclei forever?

Thanks for your interest. Magnetars probably practice have a menstruation of activity every bit radio pulsars; it is, nonetheless, hard to encounter them, because they spin downwardly apace and every bit 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 built-in spinning quickly and with a high plenty mass that it would collapse into a blackness hole if it were rotating more slowly (rotation supports the star against gravity, then a rotating NS could accept a higher mass). In that case, 1 could imagine the star slowing down and eventually collapsing, but that would probably be serenity and certainly wouldn’t release enough free energy to cause the star to explode.

Cole Miller

15. I am a school (physics) teacher trying to understand a petty more most the end of the life of stars. I am confused by what I’ve read in books and on the spider web, especially virtually the masses of neutron stars and black holes. Some sources refer to masses of ane.five solar masses (largest mass for white dwarf being the end point) and iii.0 solar masses (smallest mass for a black pigsty). Your website http://world wide web.astro.umd.edu/~miller/nstar.html mentions 15 and 30 solar masses.


I really 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 well-nigh 10 ten the Sun’s mass.


Can y’all help me to understand this?


Are all these masses that are much larger than iii 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?

Lamentable 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 concluding mass of the neutron star or blackness hole that results from that star. Suppose a star begins its life at 20 times the mass of the Dominicus. It is a very vivid star; then much and then, in fact, that the radiation it emits throughout its life drives a lot of mass abroad from the star. This is especially truthful during the behemothic phase. Then, when the central core collapses and produces a supernova, even more mass is driven away. The upshot is that the cardinal cadre (a neutron star in this case) ends upwards with only about ane.5 to 2 times the mass of the Sun.

For a star that begins with twoscore times the mass of the Sun it’s the aforementioned affair, except that manifestly plenty 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

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

Thanks for your question! The bones 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 almost ane/400 of the supernova’s kinetic free energy is absorbed past the companion. The binding energy of a star (i.e., the energy required to completely destroy information technology) is at least 1/100 of the kinetic energy of a supernova. This means that, yes, the outer layers might be diddled off, but most of the star volition survive only fine. Mind yous, I wouldn’t desire 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 practice they have on space?

A magnetar is a type of neutron star with an especially strong magnetic field. Let me give you lot an idea of the scale we’re talking about, though. The strength of the World’due south magnetic field at the pole is about 0.5 Gauss. The strength of a refrigerator magnet at its surface is about 100 Gauss. The force of the magnetic field in an MRI auto is nearly ten,000 Gauss. This barely scratches the surface, though: the magnetic field at the surface of a typical immature neutron star is nigh a *trillion* Gauss, which is and so strong that the very atoms are stretched out into cylinders along the field lines. Magnetars have fields nearly 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 (considering 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 swell-granddaddy of all earthquakes. This produces a outburst of gamma rays, and the brightest of the bursts are so powerful that they can produce aurorae on Earth from 25,000 low-cal years abroad! Not something you’d similar to go near…

How they are created is a mystery. In fact, in general, we don’t have practiced knowledge of how magnetic fields are produced in near objects. In fact, a joke attributed to Sir Martin Rees (an eminent astronomer in England) is that if y’all slumber though an astronomy talk and want to inquire an intelligent-sounding question, only say “what well-nigh 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 plummet and turbulence produces an extra-stiff magnetic field. The honest answer, though, is that we actually don’t know for certain.

Proficient questions!

Cole Miller

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Source: https://www.astro.umd.edu/~miller/teaching/questions/neutron.html