Neutron star interiors... what the hell!?

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SolarpunkFan
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Neutron star interiors... what the hell!?

Postby SolarpunkFan » 2017-03-01 11:14am

A while ago I was browsing the Wikipedia article on neutron stars for fun and I saw this diagram:

Image

My interest was piqued because as a child I had learned a very simplified model of a neutron star as a crust of iron over a liquid of neutrons from a Dorling Kindersley book I used to have, with a diagram similar to this:

Image

I hadn't known of Fermi liquids and gasses until then, so I did the only thing someone like me would do in this situation and hopped on over to Wikipedia's pages on Fermi liquid theory and Fermi gasses. And I discovered that I truly am an idiot because the pages were completely confusing to me. There was something on the Fl page about similarities to metals. I vaguely remember a diagram I saw in my high school chemistry textbook about metallic bonds basically acting like a "sea" of electrons that flow freely throughout a metal (but I'm probably wrong about that), but beyond that I'm completely lost.

And in searching for one of the diagrams for this post things got even worse:

Image

So now not only do I know that I don't know what Fermi liquids and gasses are, but now I know that I don't know what hyperons and deltas are. Though everything else in that last diagram makes sense to me which probably puts me ahead of the majority of my countrypeople (not that that's a high bar to clear, United States and all that).

So at this point I'm looking for very simplified explanations on the following:

  1. Fermi fluids
  2. Fermi gasses
  3. Hyperons
  4. and Deltas

Since I know that everyone here who might be able to answer are busy with jobs and family and probably don't want to talk to a mental midget like me (I'm not bitter about this BTW, I completely understand how you feel) so maybe if there's one or more sources of information on these beasts I would like to know about them.
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Simon_Jester
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Re: Neutron star interiors... what the hell!?

Postby Simon_Jester » 2017-03-02 09:06pm

Just to give you a very 'quick and dirty' starting point, bearing in mind that I did my master's degree work some years ago and there are a few others here who could give better answers, at least if they want:

===========================================

"Fermi fluid" and "Fermi gas" refer to states of matter where the dominant behavior of the matter can only be understood in terms of quantum mechanics (classically defined atoms do not exist and are either crushed by pressure, split up by heat, or both). Such substances are governed by what is called "Fermi-Dirac statistics." Basically, Fermi-Dirac statistics are set up on the proposition that no two things can occupy the exact same 'state' at the same time, where 'state' is construed to include both physical location AND other properties that particles have in quantum mechanics.* Two particles can be in the same place, but only if they have different amounts of energy stored up in their physical state, are spinning with different amounts of angular momentum, et cetera.

Electrons are a good example of a particle that obeys Fermi-Dirac statistics, which is why (for example) the orbital shells in an electron* can "fill up" with electrons. There are only so many possible combinations of angular momentum and 'spin' available to an electron with a specified amount of potential energy in orbit around an atom, so there are only so many 'slots' for the electron to occupy.

Fermi "fluid" and "gas" would have different properties governed by the same basic set of relevant laws. Just as in real life liquids and gases have different properties that are consequences of the same laws being applied to the same substance at different temperatures and pressures. The difference is that neutrons are very much not molecules in a gas and don't behave like molecules in a gas. They behave like fermions (because they are, they obey the "two neutrons can't have the same state at once" rule). But to know what fermions will do under certain conditions, you use Fermi-Dirac statistics.

The dominant force driving the behavior of matter under Fermi-Dirac statistics is that while all particles are free to "wash around" and go wherever they 'like' within the material... The physics requires that the lowest-energy 'states' fill up first, then higher and higher energy states are the only ones available. This governs how the material stores and exchanges energy with other substances, and what processes can occur within the material.

Just like how the behavior of ice, water, and steam is governed by temperature, how much energy is available for individual water molecules to bounce around, and to what extent the water molecules "stick together." In ice, the molecules have little energy individually and are firmly stuck together; in water they can move around each other freely and the material 'flows,' but cannot be squeezed or compressed; in gas the molecules move around with great speed and do not stick together at all.
_________________

*[By contrast there is also 'Bose-Einstein' statistics, which describes the behavior of particles that ARE allowed to occupy the same 'state' at the same time.

===========================================

NOW, that leaves 'hyperons,' 'delta particles,' and so on.

Most of the exotic particles referred to are things that normally would undergo radioactive decay very quickly, so we only observe them in particle accelerators as byproducts of slamming protons together at stupidly high energies.

However, the reason these particles undergo decay is because doing so is 'favored' in terms of energy. An unattached neutron not part of a nucleus, moving through empty space, occupies a 'higher energy' state than it would if the same neutron split up, decaying into a proton (which carries away most of the mass) an electron (to balance out the electric charges), and an antineutrino and a gamma ray (to carry away the remaining energy, make sure momentum balances out, et cetera)

Free, unattached neutrons have an average lifetime of about ten minutes before this spontaneously happens to them.

But if we take the same neutron and stick it into an atomic nucleus, the decay process is no longer favorable. The neutron will not decay on its own, as long as the nucleus itself is stable and in balance.

So we can imagine that other subatomic particles which would decay even more quickly in their 'free' state might become stable when 'bound' under unusual conditions of high density, high pressure, high energy, et cetera.

This is where we get the prediction that things like hyperons and delta particles would exist in the core of a neutron star. Because the pressures and energies down there SHOULD be so great that these short lived, ultra-unstable particles would be able to take on a more lasting existence. Or be spawned into existence so frequently that they are constantly found there. Not sure which from the information available.

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SolarpunkFan
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Re: Neutron star interiors... what the hell!?

Postby SolarpunkFan » 2017-03-02 09:22pm

Thank you! :)

Though I feel like in retrospect that maybe I shouldn't have posted this topic at all. I mean, I just feel like I'm the dunce here that's bothering everyone else by asking for information like this. :?
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"You drongos will have to do better than that if you want to beat the devil!" - Hugh Dawkins, also known as "The Tasmanian Devil"

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Re: Neutron star interiors... what the hell!?

Postby SpottedKitty » 2017-03-02 11:55pm

SolarpunkFan wrote:I mean, I just feel like I'm the dunce here that's bothering everyone else by asking for information like this. :?

Don't feel too bad about it, I thought I did have at least a layman's grasp of stellar anatomy and recent discoveries of the particle zoo, but a fair amount of the material upthread was new to me as well. Fascinating. I can see I've a lot of reading to catch up with (in my copious free time...).
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Simon_Jester
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Re: Neutron star interiors... what the hell!?

Postby Simon_Jester » 2017-03-07 09:12am

SolarpunkFan wrote:Thank you! :)

Though I feel like in retrospect that maybe I shouldn't have posted this topic at all. I mean, I just feel like I'm the dunce here that's bothering everyone else by asking for information like this. :?
You're letting the insecurities get to you.

The fact that I can answer this question, and that some other people on this forum could have potentially answered it in more depth, does not mean it's a stupid question.

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Lord Revan
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Re: Neutron star interiors... what the hell!?

Postby Lord Revan » 2017-03-07 10:12am

Simon_Jester wrote:
SolarpunkFan wrote:Thank you! :)

Though I feel like in retrospect that maybe I shouldn't have posted this topic at all. I mean, I just feel like I'm the dunce here that's bothering everyone else by asking for information like this. :?
You're letting the insecurities get to you.

The fact that I can answer this question, and that some other people on this forum could have potentially answered it in more depth, does not mean it's a stupid question.

Simon is correct, as long as you're being honest about your desire to find the information there's no such thing as an "unworthy" question. I'm sure if you asked why the sky is blue there would be people willing to explain it in detail (FYI:it's due to how light refracts in air).

The problem with Archinist and people like him isn't that they asked stupid questions but rather that they clearly were asking those to either to troll us or looking for an ego boost By claiming they've found some big secret and we should praise them for it aka the questions were inherently dishonest.

That said there's some questions where asking it here shouldn't be your first option, but stellar physics related questions aren't those.
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Oh wait, that's marijuana..."Einhander Sn0m4n

Simon_Jester
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Re: Neutron star interiors... what the hell!?

Postby Simon_Jester » 2017-03-07 11:19am

Agreed.

I mean, some of the stuff I summarized above is stuff that it took me years to learn. Copious amounts of calculus were involved. Reaching that level of understanding is not simple or easy. I had the benefit of experts whose understanding far surpasses my own. Even then, I only really scratched the surface of all that exists to be known on the subject.

The uttermost limits of my own physics knowledge would permit me to prove that neutron stars should exist, that they are a thing that can exist in the universe. I could not even begin to predict what happens inside a neutron star in formal, mathematical terms; I barely know enough to even understand what is said by those who can.

And my own understanding of such a strange thing, primitive and limited though it is, is such that I would not be able to explain it in a comprehensible way, even now, except for one thing. Namely, the fact that writing explanations of things is one of my favorite hobbies, and I've spent over a decade practicing writing explanations.

Unless you are some kind of mental god, you do not obtain an understanding of such things without others to explain it you you. Certainly not starting from scratch with a high school education, or even a college education in a non-physics field.

So SolarpunkFan, unless you are a mental god, you should not be ashamed to ask questions about what goes on inside a neutron star.

And if you are such a mental god, then that is fucking awesome. And you should never be ashamed of anything, ever, except deliberate harm inflicted on other people.


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