Talk:Neutron star

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Early–October 2007

Introduction

I think the introduction has too much unnecessary jargon in it. I think it should be simplified and made more clear.17:25, 26 October 2008 (UTC) —Preceding unsigned comment added by 75.150.72.237 (talk)

Formation

The information on gravity and escape velocity don't belong in this section and since it is already included in the properties section it is also redundant. It should be deleted from this section
Furthermore, I think there should be, if possible, more detail put into the formation.Alexa7890 (talk) 19:00, 26 October 2008 (UTC)

This section is really short. Perhaps it would be a good idea to include the end of a neutron star and change it from just a "formation" section to a "formation and end" section.Alexa7890 (talk) 07:04, 27 October 2008 (UTC)

Properties

Is the information on the Equation of State correct? The citation that is given (#3) goes to the German page on neutron stars. I have come across articles that discuss the EoS for Neutron Stars which would imply that an EoS is known. Alexa7890 (talk) 13:16, 27 October 2008 (UTC)

No, the EOS is not known with ANY certainty. There are infact many competing modals. You should be cautious with neutron stars - they tend to publish "facts" about them decades before an issue is settled, or even properly explored. —Preceding unsigned comment added by 75.153.125.20 (talk) 01:42, 13 December 2008 (UTC)

The crust would appear black because all radiation is focused around the X-ray spectrum. Is this correct? Just because the radiation peaks in the X-ray doesnt mean it wint be radiating in the visible. Wont it look purple like other things that peak above the visible? Fig (talk) 12:37, 17 November 2008 (UTC)

Yes, that's wrong. When ideal black body is heated, maximum moves to shorter wavelengths, but partial luminosity grows in all spectral channel, there is no channels where it falls! IIRC, long wavelengt parts of spectrum (those far from peak) rise approx. proportionally T. As a result, bolometric luminosity rises proportionally to T^4.

Thus, neutron star is (unimaginably intense) white in visible light, because peak is so far away in X-rays, all colors of visible spectrum will have approximately same intensity. Hence, white. —Preceding unsigned comment added by 88.101.163.106 (talk) 01:10, 10 December 2008 (UTC)

The escape velocity is listed as 30% of speed of light and as 50% the speed of light, if it does vary that much it needs to be mentioned, as it is now it conflicts with the previous paragraph —Preceding unsigned comment added by Edman007 (talk • contribs) 16:59, 17 November 2008 (UTC)

Structure

The information of the density seems to belong in the properties section. The second paragraph needs to be considerably cleaned up. The "proceeding deeper" vocabulary isn't something that would be in an encyclopedia and the information should be made more clear. —Preceding unsigned comment added by Alexa7890 (talk • contribs) 23:14, 22 October 2008 (UTC)

Depth of The Crust

The crust is 1 meter or 1 mile thick? (section on structure). The text and the figure are contradicting themselves. 201.80.110.49 (talk) 05:31, 30 November 2007 (UTC)

The thing that is being referred to as 1 meter thick is the atmosphere, however from what I know this is incorrect. According to The Internet Encyclopedia of Science what can be called an atmosphere is maybe only a few micrometers thick. The figure is correct according to Universe Today and Space.com Alexa7890 (talk) 15:13, 22 October 2008 (UTC)

• Neither 1 meter nor a few micrometers are quite correct for the atmosphere: a typical depth scale in the neutron-star atmosphere is from a few millimeters to a few centimeters, depending on conditions (chemical composition, temperature, stellar mass and radius, magnetic field). 1 meter can be correct for the heat-blanketing envelope. For the entire crust (including the inner crust), the order of magnitude of 1 km is correct for typical neutron stars. Potekhin (talk) 08:12, 30 January 2009 (UTC)

Annd this was verified to be accurate by what means? —Preceding unsigned comment added by Trentc (talk • contribs) 03:24, 5 March 2009 (UTC)

Radius, volume, density in curved space

All the calculations regarding the volume and density of a neutron star that I have seen assume that the space inside a neutron star is flat. However, inside an object as massive as a neutron star, doesn't the curvature of space become significant? Wouldn't that mean that the internal volume is larger than the standard formula for a Euclidean sphere would suggest? I hope someone more familiar with General Relativity can answer these questions. Clement Cherlin 01:16, 16 November 2007 (UTC)

• All serious calculations of this kind are always done in frames of General Relativity. The flat spacetime may be considered as a simplification for general reader, because in many cases this approximation gives a correct order of magnitude for neutron stars. Potekhin (talk) 08:15, 30 January 2009 (UTC)

Magnetars categorised as rotation-powered neutron stars

Isn't a magnetar's power source it's magnetic field energy?

See http://solomon.as.utexas.edu/~duncan/magnetar.html#New_Kind_Of_Star

there is alot of argument over this. It seems like it is it's magnetic field - but the origin of the field itself is open to discussion. It's like saying your TV is electric powered - ignoreing the coal plant on the other end. Also, most neutron stars are rotation powered - the high field stars seem to be an exception, but we really don't know with any certainty. —Preceding unsigned comment added by 75.153.125.20 (talk) 01:44, 13 December 2008 (UTC)

89.48.108.46 (talk) 16:22, 11 December 2007 (UTC)

• This issue is not definitely solved yet. Most popular hypothesis is that the source is the magnetic field energy, but there are alternative models. Potekhin (talk) 08:17, 30 January 2009 (UTC)

SOLAR MASS

SOME ISSUES HERE WITH THE SOLAR MASS OF THE NEUTRON STAR AND THE SUN. IF IT IS 1.35 SOLAR MASSES, THEN IT WOULD NOT BE SMALLER THAN THE SUN —Preceding unsigned comment added by 155.214.128.4 (talk) 15:38, 26 February 2008 (UTC)

Yes, it can be smaller, if it is denser. And neutron star are very dense. --84.10.180.181 (talk) 14:36, 23 March 2008 (UTC)

What happens to them eventually?

How does a neutron star end it's life, what happens to it? And how? It doesn't have fuel like a regular star, and it's gravity holds it together, how long can they stay that way? The snare (talk) 05:51, 19 August 2008 (UTC)

You don't know much about gravity do you? Gravity is the reason its smaller then earth... Gravity is crushing it. Thats why it has such high pressure. Ulitmatly it would porably either be crushed or the nuetrons would escape.--Jakezing (talk) 13:19, 27 October 2008 (UTC)

Jakezing, I asked my astronomy professor about this and she said that a neutron star will remain mostly static, although they will cool down a bit Alexa7890 (talk) 02:26, 28 October 2008 (UTC)

He may be a professor but that dosn't make him right.--Jakezing (talk) 22:20, 28 October 2008 (UTC)

Who is youtr daddy and what does he do? Who are you and what are your credentials?. Reeks of pungent arrogance. -220.255.7.249 (talk) 11:27, 24 December 2008 (UTC)

A neutron star will end it's life quietly - the professor is correct. At least for most stars. And yes, that doesn't make him necessarily correct, but noone is ever necessarily correct. Vacuous statement. —Preceding unsigned comment added by 75.153.125.20 (talk) 01:45, 13 December 2008 (UTC)

I'm still a little confused, so it will cool down, but then what? Break apart and dissipate somehow? And how will it do that? The snare (talk) 03:20, 24 January 2009 (UTC)

Why do you think something else must happen? Assuming the neutron star is isolated and no external matter is falling in, it would be pretty safe to say that nothing will happen. Now, it's possible that after an extremely long amount of time (something like a trillion trillion trillion times the age of the universe) pending confirmation that baryonic matter can decay into non-baryonic matter (never been observed), the neutrons might decay into lighter particles (mesons and leptons) and eventually the star would evaporate. That scenario must be considered speculative. Dauto (talk) 05:12, 29 January 2009 (UTC)

So, you're saying neutron stars are eternal as far as we know? When there is nothing but photons left in the universe, there will also be neutron stars literally forever? Also, don't neutrons become protons, at least when they are alone and not in a nucleus? The snare (talk) 02:22, 2 February 2009 (UTC)

That's right. Neutrons left alone become protons in 15 minutes, but neutrons in a neutron star are stable. Unless all baryonic matter converts in nonbaryonic matter (hypothetical possibility as mentioned by Dauto), an isolated neutron star will be eternal. But if a neutron star accretes matter, it may eventually collapse into a black hole. Potekhin (talk) 05:28, 2 February 2009 (UTC)

Don't atoms (normal ones, so a deuterium atom in this example- just so we have one neutron) eventually break down and dissipate? They don't last forever, so I've been told, they aren't perpetual motion machines, don't know about neutron stars though. The snare (talk) 03:14, 16 April 2009 (UTC)

Deuterium is stable, and does indeed last forever. Tritium is the unstable isotope of hydrogen. It beta-decays, transforming one of its neutrons into a proton, and continues as helium-3, which lasts forever. Matter changes forms, but doesn't generally disappear (barring exotic processes like proton decay, which hasn't been observed). The key concept is that the amount of mass (or energy) involved remains constant. --Christopher Thomas (talk) 21:52, 27 May 2009 (UTC)

Don't forget about the gravity of the thing. It's really strong and makes it hard for matter to escape, so the example with deuterium doesn't really apply here. And yes, neutron star isn't a perpetual motion machine, it emits a lot of energy during it's lifetime - that's why it cools down. Regarding the topic, it's really hard to say how does the star end it's life because we can't see the really old ones - they're too cool and therefore emit too little energy to be observed. In theory they can live forever or collapse into a black hole as said by Potekhin or maybe they change into a basket full of oranges ;), we will probably never be sure of that. --Siberie (talk) 04:55, 24 May 2009 (UTC)

Question

How is it possible for a neutron star to be very hot. Atoms and molecules are to be in motion for the flow of charge of heat while there is no charge on neutron star. Myktk (talk) 15:36, 21 October 2008 (UTC) Khattak

I think it has to do with compressing the core of a star into something smaller then earth and having all of it be neutrons, Pressure and heat go hand in hand and neutron stars are like black holes just less gravity..--Jakezing (talk) 13:18, 27 October 2008 (UTC)

heat and charge are two very different things, temperature is the average speed its molecules/particles are moving at, see Heat and Temperature, charge is in basically from charged particles (protons/electrons) see Electric_charge, with a lot of the high temperature things the heat can separate the electrons from the nucleus, leaving many charges particles (ions) which allow the plasma to become "charged" which is what your thinking of see Plasma_(physics), with a neutron star the pressures become so great that the electrons combine with the protons (a + and - charge equals a 0 charge), and if you have no charged particles you have no charge as a whole (see Degenerate_matter#Neutron_degeneracy) Edman007 (talk) 06:01, 24 November 2008 (UTC)

The above poster claiming that charge and heat are different is correct. But to answer your question more fully - Temperature is related to the ratio of a change in entropy to a change in energy. A very small change in entropy here requires a massive change in energy, because the star has such high density. The result is that the temperature is very high. To the poster who claimed that neutron stars are like black holes - thats really not a fair comparison. Black holes violate in principle every law of physics. People like Hawking, Wheeler, and Unruh have spent their lives figureing out how our laws of thermodynamics can exist next to black holes - forget working INSIDE them! —Preceding unsigned comment added by 75.153.125.20 (talk) 01:49, 13 December 2008 (UTC)

It's not clear to me what the original poster meant with that question. He mentions the fact that neutrons are not charged particles but does not explain why he thinks that the presence of charged particles should be required in order for something to be very hot. He might want to better explain his position. He seems to believe that only charged particles can have a temperature. That's simply not true. Dauto (talk) 05:21, 29 January 2009 (UTC)

I've wondered this too. Heat is determined by how fast the electrons are moving, but since a neutron star is all neutrons and no electrons, how can it have heat? The snare (talk) 03:10, 16 April 2009 (UTC)

Heat is determined by the energy of the moving particles. Boltzmann's constant says that on average a particle will have 1.38×10⁻²³ Joules/Kelvin . This is true for electrons in most metals because they act as a gas. It is true for molecules in a gas and nuclei and electrons in a plasma. Neutrons will jostle around in a neutron star just like molecules in a gas. At 1×10⁶ K , their mean velocity will be about 128 km/s which is ~c/2000. They are likely superconductors of both heat and electricity. The later may explain the huge magnetic fields in neutron stars. Trojancowboy (talk) 02:37, 28 May 2009 (UTC)

More Questions

"Outside the nucleus, free neutrons are unstable and have a mean lifetime of 885.7±0.8 s (about 15 minutes), decaying by emission of a negative electron and antineutrino to become a proton:[6]" Source: http://en.wikipedia.org/wiki/Neutron So its life time should not be more than 15 minutes.

Also, if surface gravity of neutron star increases 7x10^14 every meter in one second then is this figure higher than speed of light? 96.52.178.55 (talk) 17:00, 31 May 2009 (UTC)Khattak

I think you mean "15 minutes", not "15 seconds". The key concept here is that particles are stable when decaying would cost them energy, and are unstable when decaying would produce extra energy. In vacuum, or in an environment like Earth's, neutrons are unstable, because a proton plus an electron has lower energy (rest mass plus minimum kinetic energy) than the original neutron. Within the neutron star, however, matter is packed very densely (dense enough to be degenerate matter). In degenerate matter, all particles have to occupy different energy levels (they aren't allowed to have the same energy, angular momentum, and quantum spin as another particle). The energy levels allowed for electrons under these conditions are much higher than the energy levels allowed for neutrons, so neutrons decaying (into proton-plus-electron pairs) would require extra energy. The neutron is the lower-energy state under these conditions, so inside a neutron star, neutrons are stable.

With regards to surface gravity, the important thing to realize is that gravitational acceleration and escape velocity are very different things. Gravitational acceleration tells you how fast you're picking up speed when falling towards an object, and escape velocity tells you how fast you have to leave the surface in order to escape the object. When you're right near the surface of a neutron star, it'll pull you towards it very quickly. However, the neutron star is small compared to other celestial bodies, so the distance over which you speed up this quickly is relatively small. As a result, the speed needed to escape is still less than the speed of light.

I hope this answers your questions. --Christopher Thomas (talk) 07:37, 1 June 2009 (UTC)

Surface gravity

The value of 2×10¹² g is far too high. If approximated by Newton's Law of gravity a 2 solar mass neutron star with 10 km radius would have about 2.7×10¹² m/s² = 2.7×10¹¹ g. A 3 solar mass black hole would have about 5×10¹¹ g (at the Schwarzschild radius of 9 km}}. Although one would have to use the relativistic equations for a correct result the Newtonion equation should at least give the correct order of magnitude. I have therefore corrected the value in the properties section; the range of 2×10¹¹ to 2×10¹² g given a few lines above remains as a matter of further check (with relativistic formulae, if possible).--SiriusB (talk) 15:02, 26 December 2008 (UTC)

• Yes, relativistic formulae give corrections of a few tens percent at most, so the order of magnitude is the same as with the Newton's Law of gravity. For typical neutron stars the surface gravity is a few ×10¹⁴ cm/s² = a few ×10¹² m/s². And you are right, the range of 2×10¹¹ to 2×10¹² g given a few lines above is incorrect as well. According to the family of the equations of state presently considered in the literature, it is possible for a neutron star to have the surface gravity from 0 (for the minimum-mass neutron star) to about 7×10¹⁴ cm/s² (see, e.g., Bejger, M.; Haensel, P. (2004). Surface gravity of neutron stars and strange stars. Astronomy and Astrophysics 420, 987-991). However, most typical neutron stars with masses of 1 to 2 solar masses should have surface gravity somewhat between 1×10¹⁴ cm/s² and 5×10¹⁴ cm/s². I have therefore corrected that point in the "Formation" section. Potekhin (talk) 18:24, 29 January 2009 (UTC)

"The nuclei become smaller and smaller until

the core is reached, by definition the point where they disappear altogether." (a quote from current article) I wonder about the accuracy or at least the clarity. The sentence seems to be saying that a "neutron star" must "by definition" have at least some location where matter exists only as neutrons(and thus must at least have it in the core), but I doubt astronomers think that way. Astronomers probably identified some objects that they suspected had that property, and called them neutron stars, but they're not defined by that, but probably by observational characteristics, whether or not astronomers now know if some or all neutron stars have matter of this form.Astronomers don't define their universe, they (try to) describe it.--Richard Peterson75.45.97.146 (talk) 18:10, 7 May 2009 (UTC)Rich (talk) 21:12, 7 May 2009 (UTC)

Neutron stars are defined as bodies supported by neutron-degeneracy pressure (though the term as used by astronomers will also cover things like quark stars and other stars with exotic hadrons). Whether the nuclei grow smaller as you move deeper within the star is debatable, as that depends on the equations of state for nuclear matter, which are poorly understood. What people will agree on is that near the surface, you'll have ordinary nuclei and a degenerate gas of electrons. Deeper in, it becomes energetically favourable to convert electrons and protons into neutrons, so you end up with nuclei past the neutron drip line that are stabilized by external fluid pressure. A bit deeper, and it's energetically favourable for some of these neutrons to un-bind from nuclei and be shared as a degenerate gas of neutrons (like the Fermi gas of electrons in a metal). This may happen piecemiel, with smaller nuclei within the gas (either as a solid or moving as a fluid), or it may happen all at once, with a sharp transition between matter composed of nuclei and a neutron-degenerate fluid (someone's probably figured out which of these happens, but you'll have to dig up relevant papers to find out which occurs). Either way, past a certain depth, you have neutron-degenerate matter without distinct nuclei. I'm told this ends up acting like a superfluid (behavior is dominated by quantum effects rather than classical). Even farther in, the neutron-degenerate matter may or may not convert to other forms of matter (quark soup or strange matter). Whether or not it does this depends on the equations of state of all of these forms of matter (it'll happen if and only if it's energetically favourable to do so at the pressures and temperatures found within the star). --Christopher Thomas (talk) 20:18, 7 May 2009 (UTC)

you obviously know a lot more than me about it, and what you said seems to partially support my point, so let's fix the sentence in question.75.45.97.146 (talk) 21:03, 7 May 2009 (UTC)Rich (talk) 21:12, 7 May 2009 (UTC)

I just changed it to something I think is a better approximation to current understanding, but you probably should write it the way you think best.Rich (talk) 21:12, 7 May 2009 (UTC)

Stability

Neutron stars are very hot and are supported against further collapse because of the Pauli exclusion principle. - This statement is not entirely correct. Of course the exclusion principle is important here, but it's too weak to support the star. The major contribution to force that counters gravity are repulsive nuclear forces which come into the game because of huge density. I think it should be corrected. Any comments? Siberie (talk) 14:57, 25 May 2009 (UTC)

The "repulsive nuclear force" you refer to is degeneracy pressure. As density increases, confinement requires energy to increase as well. Net result is a repulsive force. See degenerate matter for the derivation. The Strong force is purely attractive, and the Weak force is mostly mediating the transformation between neutrons and electron-proton pairs.--Christopher Thomas (talk) 21:42, 27 May 2009 (UTC)

Yup, You're obviously right. I had to be really tired to write such a BS. Thanks for correcting it. Siberie (talk) 13:31, 30 May 2009 (UTC)

Enormous Magnetic Fields

This subject is not even discussed and needs to be a prominent part of the article. Magnetic fields of 10×10⁸ Tesla are common or 100 million times greater than a rare earth magnet This is one of the MAIN properties of a neutron star. Magnetic poles are usually not aligned with the axis of rotation which gives a pulsar.Trojancowboy (talk) 03:02, 28 May 2009 (UTC)