<p>If one thinks about &quot;hard&quot; as solid, having stiffness/rigidity, an ability to retain a certain shape when subjected to anisotropic stress, then the answer is NO, it is not hard.</p>
<p>The solid state emerges from having directional interactions between the particles. The solid substances in our daily life are solid either because of specific ordering of their atoms (particles that have size and don't allow penetration of each other) or because of the directional covalent bonds, or a combination thereof.</p>
<p>This is where your wrong assumption starts:</p>
<blockquote>
<p>the immense amount of atoms packed together</p>
</blockquote>
<p>No atoms there, at all. Atoms require much quieter environment in order to form. 3 orders of magnitude lower temperature could be a good start. Lower density advisable as well, or else one gets an electron-degenerate matter.</p>
<p>What exist in the Sol's core is an extremely compressed and hot gas of free electrons and free atomic nuclei.</p>
<p>Reasonable models for solid degenerate matter do exist and some properties of astronomic objects containing degenerate matter (<a href="https://arxiv.org/pdf/1908.00370.pdf" rel="nofollow noreferrer">white dwarfs</a>, <a href="https://en.wikipedia.org/wiki/Nuclear_pasta" rel="nofollow noreferrer">neutron stars</a>) are explained by partial crystalization.</p>
<p>The core of a main sequence star (e.g. Sun) is considered not degenerate. It is a simple plasma where particles only isotropically scatter against each other.</p>
<p>(It is simple in regard to not having some underlying structure like e.g. in metals where we have plasma behavior of the electron gas but we also have a crystal lattice)</p>

CC BY-SA 4.0
2023-01-09T15:56:38.937Z
a neutron star isn't that some extremely packed(compact)(heavy) object, ? if I remember correctly
William Martens
CC BY-SA 4.0
2023-01-08T23:06:09.007Z
2023-01-10T12:21:17.593Z
2023-01-10T12:21:17.593Z
fraxinus
<p>It is not solid. But it is hard. Its Young's modulus is about <span class="math-container">$10^{16.5}\text{ Pa}$</span>. It is thousands or millions of times more than the Young's modulus of any ordinary solid matter. For comparison, the largest Young's modulus is approximately <span class="math-container">$10^{12}\text{ Pa}$</span> for diamond.</p>

CC BY-SA 4.0
2023-01-08T16:25:26.550Z
Interesting,  I couldn't find any information on it's hardness, viscosity or other similar properties.  How is Young's modulus defined for a plasma, It's often said that young's modulus is not defined for a gas or liquid.  How is it defined in the sun.  Can you cite these values ?
James K
CC BY-SA 4.0
2023-01-08T17:45:09.957Z
I agree that the Young's modulus is not used for gas. But if an ideal gas is compressed by a piston in a cylinder isothermally, then it will be felt as a solid body with an corresponding Young's modulus at small changes of piston position. This value is approximately equal to the gas pressure. The solar core presure is [https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html] $2.477 \times 10^{11}\text{ bar} \approx 10^{16.5}\text{ Pa}$
Imyaf
CC BY-SA 4.0
2023-01-08T18:53:23.960Z
But "gas pressure" is still fluid.  I don't think this pressure value corresponds to "hardness" or "resistance to flow", which would be characteristics of a solid.  Does this answer the question "is the sun's core a solid", because it seems to say "yes it is".  I have doubts about that.
James K
CC BY-SA 4.0
2023-01-09T08:54:40.603Z
@Imyaf saying that "it would be stiff it were contained" and "it is hard" are quite different assertions. Specifically, in the contexts of the behaviours the question asks about, this question has relatively little relevance.
Will
CC BY-SA 4.0
2023-01-09T15:54:40.687Z
@Imyaf good answer!  and pointing out the last example for the diamond too - but the **https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html]** link you posted **is dead** - care to update? :)
William Martens
CC BY-SA 4.0
2023-01-09T15:59:42.830Z
@William Martens Link is https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html without ]
Imyaf
CC BY-SA 4.0
2023-01-08T14:52:31.757Z
2023-01-09T09:18:24.097Z
2023-01-09T09:18:24.097Z
Imyaf
<blockquote>
<p>The Sun's core is mind-bogglingly dense, about 150,000 kg/m3 (about
15x denser than lead, 7x denser than uranium, 6x denser than osmium).</p>
</blockquote>
<p>Such a high pressure would compress matter into a <a href="https://en.wikipedia.org/wiki/Wigner_crystal" rel="nofollow noreferrer">wigner crystal</a> (but with nuclei against an electron background instead of the other way around). But the core of the sun is also mind-bogglingly <em>hot</em>. The thermal energy prevents this from happening.</p>
<blockquote>
<p>By hardness, I mean having stiffness/rigidity, an ability to retain a
certain shape when subjected to anisotropic stress.</p>
</blockquote>
<p>No. The core of the sun is a gas and has a low viscosity. Ideal gas viscosity increases with the square root of temperature <em>and doesn't depend on pressure</em>, so a naïve ideal gas formula predicts a viscosity about 225 times more viscous than air. This is somewhere between water and cooking oil.</p>
<blockquote>
<p>Supposing we got the indestructible observer to throw a punch randomly
inside the Sun, what would this observer feel?</p>
</blockquote>
<p>The trouble would be the <em>density</em>. Swimming would be hard, much worse than in mercury, and you could only swim or punch at the center. Each <em>g</em> of gravity gives you up to 140 <em>gs</em> of buoyant forces. And gravity rises to over a hundred <em>gs</em> (compared to &quot;only&quot; 27<em>g</em> at the surface) at the right distance from the center. Your <em>unobtanium</em> body would survive these extreme forces but fluid dynamics alone would dictate your posture.</p>

CC BY-SA 4.0
2023-01-15T09:58:06.737Z
2023-01-15T10:37:29.390Z
2023-01-15T10:37:29.390Z
Kevin Kostlan
<p>Solid is one of the four states of matter which is <strong>uncompressible</strong> with atoms that are <a href="https://en.wikipedia.org/wiki/Chemical_bond" rel="nofollow noreferrer">bound</a> together. <strong>The sun's core is compressible</strong> and its atoms are completely unbound. Apply a bit of extra force or vacuum and the core will compress or expand.</p>
<p>Solids comprise the following: ionic solids, molecular solids, network covalent solids, metallic solids, and amorphous solids like glass, rubber, gel, plastics can't be compressed either.</p>
<p>There is thermal energy equivalent to about 2700 megawatt hours per kilo, and an atom bomb is about 3500 megawatt hours per kilo of plutonium, so your immutable object would have to apply a higher kinetic force in order to move it.</p>
<p>The atoms of the core are like a billiard table knocking at 100ds of kilometres per second. If you put a space-time based object in hot gas higher than 100,000'C like a human hand, the hand has vaporized in a millionth of a second, only confined by other atomic explosions, the atoms fly off in all directions, hydrogen protons at 610KMS in a vaguely <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.75.4214" rel="nofollow noreferrer">brownian motion</a>.</p>
<p>If you imagine a solid, i.e. salt crystal and compare the sun core matter, the expanding atomic force is far greater than the force of contact of a hammer hitting a diamond, perhaps by millions of times.
The speed of nuclei of atoms changes relative to temperature... Most matter in the sun's core travel at an average of 610km/s and sometimes 1200km/s:
<a href="https://i.sstatic.net/5AnTP.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/5AnTP.jpg" alt="enter image description here" /></a></p>
<p>To qualify the sun's core, you can say it's thermally caustic and amorphous and dissolves anything very fast so that atoms dissipate pretty fast, immutable hands could compress or expand the core physically if they have a force higher than an atomic explosion, a real hand would vaporize outwards into a cloud that expands at 1-2 kilometres per second according to vaguely brownian motion, there's also gamma rays, x-rays, UV, visible light, at higher quantities than the lasers used for slicing steel.</p>

CC BY-SA 4.0
2023-01-15T17:55:25.093Z
Thanks for the info's. A kilo of the sun material contains ~2.7 Gigawatt-hours of stored thermal energy, A kilo of Plutonium 239 can give 23 GWh, and a bomb is 15% efficient so it's only 3.45 Gigawatt for the bomb versus 2.7 gigawatt for the sun. Vaguely the same range.
bandybabboon
CC BY-SA 4.0
2023-01-15T18:11:36.430Z
No you are confusing energy and power. In fact you seem to be confusing force, pressure, power, velocity and energy in your answer.
ProfRob
CC BY-SA 4.0
2023-01-15T19:29:41.690Z
You are changing the subject from the atomic bomb comparison. Why? That was in fact, correct, wasn't it. The maths is of the thermal energy content are quite accurate, and changing the subject vaguely to avoid aknowledgeing that is silly. The protons move at 610 KMS and the latent the latent energy of the material is 2-3 gigawatts per kilogramme, exactly the same as an atomic bomb. It's a fair newbie reply. Why risk me labelling 500 pasting all over the website?
bandybabboon
CC BY-SA 4.0
2023-01-15T07:20:53.577Z
2023-01-15T19:35:35.520Z
2023-01-15T19:35:35.520Z
bandybabboon
<p>The solar core can be considered soft in a relative sense (compared to other materials at the same density), but hard and incompressible in an absolute sense. The material behaves almost exactly like a perfect <em>gas</em> but would be as viscous as ketchup.</p>
<p>The equation of state is that of a perfect monatomic gas and thus the pressure <span class="math-container">$P \propto \rho^{\alpha}$</span>, with <span class="math-container">$\alpha \sim 4/3$</span> in the solar core, where heat transport is dominated by radiative diffusion. This is a &quot;soft equation of state&quot; - the material is highly compressible - it takes a small <em>fractional</em> increase in pressure to produce a compression. For most solids, <span class="math-container">$\alpha$</span> would be in double figures and they are approximately incompressible.</p>
<p>However, what you are asking about could be represented by the &quot;bulk modulus&quot; (Young's modulus and Shear modulus are not meaningfully defined for a fluid). This is roughly equal to the pressure of a gas and is a measure of how much force <em>in an absolute sense</em> is required to change the volume of something. At the centre of the Sun, this is <span class="math-container">$2\times 10^{16}$</span> Pa. This can be compared with the bulk modulus of diamond which is <span class="math-container">$4\times 10^{11}$</span> Pa. Thus in that sense, the solar core is much harder than a diamond to compress.</p>
<p>In terms of viscosity, the microscopic <a href="https://en.wikipedia.org/wiki/Viscosity" rel="noreferrer">kinematic viscosity</a> in the solar core is of order <span class="math-container">$10^{-4}$</span> m<span class="math-container">$^2$</span>/s (<a href="https://ui.adsabs.harvard.edu/abs/1996ApJ...466.1078R/abstract" rel="noreferrer">Ruediger &amp; Kitchatinov 1996</a>) and hence a dynamic viscosity of 15 Pa s. For comparison, the kinematic viscosity of water at 293K is <span class="math-container">$10^{-6}$</span> m<span class="math-container">$^2$</span>/s and the dynamic viscosity is  <span class="math-container">$10^{-3}$</span> Pa s. Thus the solar interior fluid is 100-10000 times more viscous than water, depending on how viscosity is defined. Fluids of comparable viscosity would be <a href="https://en.wikipedia.org/wiki/List_of_viscosities#Substances_of_variable_composition" rel="noreferrer">honey or ketchup</a>. The viscosity of solids (like rocks) meanwhile, is of order <span class="math-container">$\sim 10^{20}$</span> Pa s.</p>
<p>To understand why the Sun behaves like a perfect gas, one must compare the interaction energies between the particles with their kinetic energies. At a density of 150000 kg/m<span class="math-container">$^3$</span> the mean separation of protons and electrons is <span class="math-container">$\sim 2\times 10^{-11}$</span> m, with a mutual Coulomb energy of about 100 eV. The kinetic energy of the particles is <span class="math-container">$3k_BT/2$</span>, and with temperature <span class="math-container">$T \sim 1.5\times 10^7$</span> K in the solar core, this is about 1000 eV. Thus the kinetic energies are much greater than the Coulombic binding energies and so the particles behave like a gas. To &quot;freeze&quot; into a solid you would need the binding energy to be about 100 times the kinetic energy, which would require <em>much</em> higher densities at that temperature.</p>

CC BY-SA 4.0
2023-01-08T18:50:46.760Z
Is this the right way to answer the question.  The"bulk modulus" would distinguish between a gas and liquid, but at solar core temperatures and pressures, I doubt words like "gas" and "liquid" have any meaning.  The distinguishing factor of a solid is its resistance to flow.  Reading between the lines I think you are saying that there is very little resistance to flow, and the core of the sun is "fluid".  Normal fluids have viscosities.  I'm not sure if viscosity makes sense for the kind of supercritical plasma, but if it does, what would be the viscosity of the core?
James K
CC BY-SA 4.0
2023-01-09T07:31:02.953Z
@JamesK the interior of the Sun is a perfect *gas* at the 1% perturbation level.
ProfRob
CC BY-SA 4.0
2023-01-09T13:46:10.110Z
@ScienceAJ the conditions inside are not much affected from the fusion process. These 150 metric tons of matter in 1 cubic meter produce some 300 watts of heat.
fraxinus
CC BY-SA 4.0
2023-01-09T15:54:06.217Z
I also think the sun burns fuel to heavier elements (e.g iron) (wouldn't that make it more "hard") ? (I can be wrong - something just lightly tells me this)
William Martens
CC BY-SA 4.0
2023-01-09T15:59:32.793Z
@WilliamMartens except that it doesn't produce iron or anything heavier the helium.  However, that would make no difference to any of the arguments above in any case.
ProfRob
CC BY-SA 4.0
2023-01-09T21:14:10.343Z
@WilliamMartens to clarify, every nucleus -- hydrogen, helium, carbon, iron, or uranium -- that is in the core of the sun is a bare nucleus with no electron shells, because it is a plasma. Therefore it behaves as a perfect gas.  (Yes, there are likely a few nuclei of every stable arrangement in the core, because the sun is a third generation star.  But only hydrogen gets fused.)
Ross Presser
CC BY-SA 4.0
2023-01-09T23:06:25.770Z
I highly doubt I can introduce enough water vapor to push away from the idea gas law. I don't think it would stay water vapor.
Joshua
CC BY-SA 4.0
2023-01-10T09:00:47.867Z
As a comparison, the dynamic viscosity of honey is around 10 Pa s, so on the same ballpark as the interior of the Sun.
nonDucor
CC BY-SA 4.0
2023-01-10T10:09:20.760Z
@nonDucor interesting - I'll add that.
ProfRob
CC BY-SA 4.0
2023-01-11T09:32:07.453Z
What about the kinetic energy of the bullet that would traverse the stuff... Would it need need to be close to or exceed the speed of light? The has to counter the pressure of an atomic explosion right, because thats the resistice force there.
bandybabboon
CC BY-SA 4.0
2023-01-11T11:04:13.957Z
@ArktourosUltorMaximus7600  any reference ? just curious. +1  for commenting! :)
William Martens
CC BY-SA 4.0
2023-01-31T08:00:05.827Z
So, that means that the Solar core is viscous as honey, but much harder to compress than diamond if you put it in a imaginary piston
Arcturus
CC BY-SA 4.0
2023-01-08T17:18:35.793Z
2023-01-10T15:23:28.447Z
2023-01-10T15:23:28.447Z
ProfRob
<p>This may seem like a weird question, but something got me thinking about it just recently.</p>
<p>The Sun's core is composed of mainly hydrogen and helium, and is present in the form of a extremely hot supercrushed plasma. The Sun's core is mind-bogglingly dense, about 150,000 kg/m<sup>3</sup> (about 15x denser than lead, 7x denser than uranium, 6x denser than osmium). The density can get extremely high at the center of stars. This leads me to think that the solar core, due to the immense amount of atoms packed together would behave like a extremely hard solid, as per my understanding, most dense metals (excluding gold) are extremely hard, like tungsten.</p>
<p>I decided to dig into it a bit on the Internet, but whatever information I got were merely about the pressure at the center of the Sun, and not about the hardness of the solar core. By hardness, I mean having stiffness/rigidity, an ability to retain a certain shape when subjected to anisotropic stress.</p>
<p>To clarify things a bit: Supposing we submerged an &quot;indestructible&quot; observer really deep into the Sun, just inside the solar core. Supposing we got the indestructible observer to throw a punch randomly inside the Sun, what would this observer feel? More specifically, would the observer perceive the solar core material as being extremely hard, like a chunk of steel, or would it act like an extremely thick fluid?</p>
<p>TL;DR</p>
<p><strong>Would the solar core be extremely stiff and hard? Or would it simply behave like a dense and viscous gas?</strong></p>

CC BY-SA 4.0
2023-01-08T14:08:09.923Z
I’m voting to close this question because you're asking about things which are impossible.
StephenG - Help Ukraine
CC BY-SA 4.0
2023-01-08T14:23:37.343Z
What does this mean: "What would it feel like?" Do you literally mean what would it feel like personally to you? But that seems to be ill-defined given your assumption that the hand is an unrealistic ideal (ie indestructable). Are you actually asking what would happen to the cube of stellar core material if it were perturbed by some force?
Daddy Kropotkin
CC BY-SA 4.0
2023-01-08T17:07:10.000Z
@DaddyKropotkin What I am trying to say is that whether the solar core behaves like a solid i.e. extremely hard and tough, or does it simply behave like a viscous liquid?
Arcturus
CC BY-SA 4.0
2023-01-09T08:14:23.627Z
Note that 150 tons/m^3 isn't that extreme. Platinum weighs 21 tons / m^3. Sure, it's much more, but a factor of eight isn't _that_ much more
JollyJoker
CC BY-SA 4.0
2023-01-09T10:59:19.747Z
There seems to be a lot of confusion in the answers about hardness versus incompressibility. Many of the answers say the core of the Sun is very hard to compress, but I think that's pretty orthogonal to what's being asked about here. If the core of the Sun is a fluid then I think this question is really about its viscosity.$$\,$$For example, the Earth's mantle is a fluid, but its viscosity is so high that if you would be able to interact with it directly on human time scales it would behave like a rock. I think the question is about whether the Sun's core is similar.
N. Virgo
CC BY-SA 4.0
2023-01-09T15:52:51.677Z
@StephenG-HelpUkraine I don't think closing this will solve anything, that is;and the answers is very good to, and especially  since it has many answers; and this was only asked yesterday
William Martens
CC BY-SA 4.0
2023-01-09T17:37:27.670Z
"per my understanding, most dense metals (excluding gold) are extremely hard, like tungsten": Some of the hardest pure metals are also the densest, but several dense metals (>10 g/cm^3) are ductile (platinum, rhenium, uranium), outright soft (gold, thorium, lead, silver), or even liquid at room temperature (mercury). And alloys can have very different properties
Christopher James Huff
CC BY-SA 4.0
2023-01-09T18:16:09.700Z
@WilliamMartens It is long established policy on SE that a question having lots of view, upvotes or answer does not in and off itself make it a good question *for the purposes of the site*.  People may be stimulated by it, but it's clear that the interpretation of the question is unclear to many people.  Note that closing the question will not make the question or answers disappear.  The edited question does seem more focused, but the OP's references to "how would it feel" don't make much sense (IMO) - this is way outside of any meaningful human scenario.
StephenG - Help Ukraine
CC BY-SA 4.0
2023-01-09T19:01:00.153Z
@StephenG-HelpUkraine I think the last sentence how it would feel was more a figure of speech(or what to call it), not exactly practically possible either. I think if was more indirectly, like, "I can't obviously touch it but if I could,in theory (...)"  :)  it was that I meant
William Martens
CC BY-SA 4.0
2023-01-10T09:14:04.957Z
I am voting to NOT close this question. In fact, this is a fabulous question. It leads you to imagine and define what exactly "hard" would mean, whether we can even define it in such extreme conditions, if not are there there any analogous properties, etc. Perhaps more of a Physics question than an Anstronomy question, nonetheless, it is a great question.
KalEl
CC BY-SA 4.0
2023-01-10T11:27:32.200Z
@KalEl **agreed**. totally.
William Martens
CC BY-SA 4.0
2023-01-11T10:58:52.183Z
as someone else pointed out somewhere, we don't like to loose great answers ^_^
William Martens
CC BY-SA 4.0
2023-01-11T12:54:00.187Z
@N.Virgo The Earth's mantle is not a fluid. It is a viscoelastoplastic material.
David Hammen
<the-sun><core><plasma-physics>
Is the Solar core hard?
Arcturus
# Is the Solar core hard?

(Asked by: username_0 on 2023-01-08)

This may seem like a weird question, but something got me thinking about it just recently.

The Sun's core is composed of mainly hydrogen and helium, and is present in the form of a extremely hot supercrushed plasma. The Sun's core is mind-bogglingly dense, about 150,000 kg/m3 (about 15x denser than lead, 7x denser than uranium, 6x denser than osmium). The density can get extremely high at the center of stars. This leads me to think that the solar core, due to the immense amount of atoms packed together would behave like a extremely hard solid, as per my understanding, most dense metals (excluding gold) are extremely hard, like tungsten.

I decided to dig into it a bit on the Internet, but whatever information I got were merely about the pressure at the center of the Sun, and not about the hardness of the solar core. By hardness, I mean having stiffness/rigidity, an ability to retain a certain shape when subjected to anisotropic stress.

To clarify things a bit: Supposing we submerged an "indestructible" observer really deep into the Sun, just inside the solar core. Supposing we got the indestructible observer to throw a punch randomly inside the Sun, what would this observer feel? More specifically, would the observer perceive the solar core material as being extremely hard, like a chunk of steel, or would it act like an extremely thick fluid?

TL;DR

**Would the solar core be extremely stiff and hard? Or would it simply behave like a dense and viscous gas?**

**Tags:** the-sun, core, plasma-physics
> **Comments:**
> **username_1**: I’m voting to close this question because you're asking about things which are impossible.  
> **username_2**: What does this mean: "What would it feel like?" Do you literally mean what would it feel like personally to you? But that seems to be ill-defined given your assumption that the hand is an unrealistic ideal (ie indestructable). Are you actually asking what would happen to the cube of stellar core material if it were perturbed by some force?  
> **username_0**: @DaddyKropotkin What I am trying to say is that whether the solar core behaves like a solid i.e. extremely hard and tough, or does it simply behave like a viscous liquid?  
> **username_3**: Note that 150 tons/m^3 isn't that extreme. Platinum weighs 21 tons / m^3. Sure, it's much more, but a factor of eight isn't _that_ much more  
> **username_4**: There seems to be a lot of confusion in the answers about hardness versus incompressibility. Many of the answers say the core of the Sun is very hard to compress, but I think that's pretty orthogonal to what's being asked about here. If the core of the Sun is a fluid then I think this question is really about its viscosity.$$\,$$For example, the Earth's mantle is a fluid, but its viscosity is so high that if you would be able to interact with it directly on human time scales it would behave like a rock. I think the question is about whether the Sun's core is similar.  
> **username_5**: @StephenG-HelpUkraine I don't think closing this will solve anything, that is;and the answers is very good to, and especially  since it has many answers; and this was only asked yesterday  
> **username_6**: "per my understanding, most dense metals (excluding gold) are extremely hard, like tungsten": Some of the hardest pure metals are also the densest, but several dense metals (>10 g/cm^3) are ductile (platinum, rhenium, uranium), outright soft (gold, thorium, lead, silver), or even liquid at room temperature (mercury). And alloys can have very different properties  
> **username_1**: @WilliamMartens It is long established policy on SE that a question having lots of view, upvotes or answer does not in and off itself make it a good question *for the purposes of the site*.  People may be stimulated by it, but it's clear that the interpretation of the question is unclear to many people.  Note that closing the question will not make the question or answers disappear.  The edited question does seem more focused, but the OP's references to "how would it feel" don't make much sense (IMO) - this is way outside of any meaningful human scenario.  
> **username_5**: @StephenG-HelpUkraine I think the last sentence how it would feel was more a figure of speech(or what to call it), not exactly practically possible either. I think if was more indirectly, like, "I can't obviously touch it but if I could,in theory (...)"  :)  it was that I meant  
> **username_7**: I am voting to NOT close this question. In fact, this is a fabulous question. It leads you to imagine and define what exactly "hard" would mean, whether we can even define it in such extreme conditions, if not are there there any analogous properties, etc. Perhaps more of a Physics question than an Anstronomy question, nonetheless, it is a great question.  
> **username_5**: @KalEl **agreed**. totally.  
> **username_5**: as someone else pointed out somewhere, we don't like to loose great answers ^_^  
> **username_8**: @N.Virgo The Earth's mantle is not a fluid. It is a viscoelastoplastic material.  

---

## 5 answers

### Answer by username_15 on 2023-01-08. Score: 53 | Accepted answer

The solar core can be considered soft in a relative sense (compared to other materials at the same density), but hard and incompressible in an absolute sense. The material behaves almost exactly like a perfect *gas* but would be as viscous as ketchup.

The equation of state is that of a perfect monatomic gas and thus the pressure $P \propto \rho^{\alpha}$, with $\alpha \sim 4/3$ in the solar core, where heat transport is dominated by radiative diffusion. This is a "soft equation of state" - the material is highly compressible - it takes a small *fractional* increase in pressure to produce a compression. For most solids, $\alpha$ would be in double figures and they are approximately incompressible.

However, what you are asking about could be represented by the "bulk modulus" (Young's modulus and Shear modulus are not meaningfully defined for a fluid). This is roughly equal to the pressure of a gas and is a measure of how much force *in an absolute sense* is required to change the volume of something. At the centre of the Sun, this is $2\times 10^{16}$ Pa. This can be compared with the bulk modulus of diamond which is $4\times 10^{11}$ Pa. Thus in that sense, the solar core is much harder than a diamond to compress.

In terms of viscosity, the microscopic [kinematic viscosity](https://en.wikipedia.org/wiki/Viscosity) in the solar core is of order $10^{-4}$ m$^2$/s ([Ruediger & Kitchatinov 1996](https://ui.adsabs.harvard.edu/abs/1996ApJ...466.1078R/abstract)) and hence a dynamic viscosity of 15 Pa s. For comparison, the kinematic viscosity of water at 293K is $10^{-6}$ m$^2$/s and the dynamic viscosity is $10^{-3}$ Pa s. Thus the solar interior fluid is 100-10000 times more viscous than water, depending on how viscosity is defined. Fluids of comparable viscosity would be [honey or ketchup](https://en.wikipedia.org/wiki/List_of_viscosities#Substances_of_variable_composition). The viscosity of solids (like rocks) meanwhile, is of order $\sim 10^{20}$ Pa s.

To understand why the Sun behaves like a perfect gas, one must compare the interaction energies between the particles with their kinetic energies. At a density of 150000 kg/m$^3$ the mean separation of protons and electrons is $\sim 2\times 10^{-11}$ m, with a mutual Coulomb energy of about 100 eV. The kinetic energy of the particles is $3k\_BT/2$, and with temperature $T \sim 1.5\times 10^7$ K in the solar core, this is about 1000 eV. Thus the kinetic energies are much greater than the Coulombic binding energies and so the particles behave like a gas. To "freeze" into a solid you would need the binding energy to be about 100 times the kinetic energy, which would require *much* higher densities at that temperature.

> **Comments:**
> **username_11**: Is this the right way to answer the question.  The"bulk modulus" would distinguish between a gas and liquid, but at solar core temperatures and pressures, I doubt words like "gas" and "liquid" have any meaning.  The distinguishing factor of a solid is its resistance to flow.  Reading between the lines I think you are saying that there is very little resistance to flow, and the core of the sun is "fluid".  Normal fluids have viscosities.  I'm not sure if viscosity makes sense for the kind of supercritical plasma, but if it does, what would be the viscosity of the core?  
> **username_15**: @JamesK the interior of the Sun is a perfect *gas* at the 1% perturbation level.  
> **username_9**: @ScienceAJ the conditions inside are not much affected from the fusion process. These 150 metric tons of matter in 1 cubic meter produce some 300 watts of heat.  
> **username_5**: I also think the sun burns fuel to heavier elements (e.g iron) (wouldn't that make it more "hard") ? (I can be wrong - something just lightly tells me this)  
> **username_15**: @WilliamMartens except that it doesn't produce iron or anything heavier the helium.  However, that would make no difference to any of the arguments above in any case.  
> **username_16**: @WilliamMartens to clarify, every nucleus -- hydrogen, helium, carbon, iron, or uranium -- that is in the core of the sun is a bare nucleus with no electron shells, because it is a plasma. Therefore it behaves as a perfect gas.  (Yes, there are likely a few nuclei of every stable arrangement in the core, because the sun is a third generation star.  But only hydrogen gets fused.)  
> **username_17**: I highly doubt I can introduce enough water vapor to push away from the idea gas law. I don't think it would stay water vapor.  
> **username_18**: As a comparison, the dynamic viscosity of honey is around 10 Pa s, so on the same ballpark as the interior of the Sun.  
> **username_15**: @nonDucor interesting - I'll add that.  
> **username_14**: What about the kinetic energy of the bullet that would traverse the stuff... Would it need need to be close to or exceed the speed of light? The has to counter the pressure of an atomic explosion right, because thats the resistice force there.  
> **username_5**: @ArktourosUltorMaximus7600  any reference ? just curious. +1  for commenting! :)  
> **username_0**: So, that means that the Solar core is viscous as honey, but much harder to compress than diamond if you put it in a imaginary piston  

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### Answer by username_9 on 2023-01-08. Score: 17

If one thinks about "hard" as solid, having stiffness/rigidity, an ability to retain a certain shape when subjected to anisotropic stress, then the answer is NO, it is not hard.

The solid state emerges from having directional interactions between the particles. The solid substances in our daily life are solid either because of specific ordering of their atoms (particles that have size and don't allow penetration of each other) or because of the directional covalent bonds, or a combination thereof.

This is where your wrong assumption starts:

> the immense amount of atoms packed together

No atoms there, at all. Atoms require much quieter environment in order to form. 3 orders of magnitude lower temperature could be a good start. Lower density advisable as well, or else one gets an electron-degenerate matter.

What exist in the Sol's core is an extremely compressed and hot gas of free electrons and free atomic nuclei.

Reasonable models for solid degenerate matter do exist and some properties of astronomic objects containing degenerate matter ([white dwarfs](https://arxiv.org/pdf/1908.00370.pdf), [neutron stars](https://en.wikipedia.org/wiki/Nuclear_pasta)) are explained by partial crystalization.

The core of a main sequence star (e.g. Sun) is considered not degenerate. It is a simple plasma where particles only isotropically scatter against each other.

(It is simple in regard to not having some underlying structure like e.g. in metals where we have plasma behavior of the electron gas but we also have a crystal lattice)

> **Comments:**
> **username_5**: a neutron star isn't that some extremely packed(compact)(heavy) object, ? if I remember correctly  

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### Answer by username_10 on 2023-01-08. Score: 10

It is not solid. But it is hard. Its Young's modulus is about $10^{16.5}\text{ Pa}$. It is thousands or millions of times more than the Young's modulus of any ordinary solid matter. For comparison, the largest Young's modulus is approximately $10^{12}\text{ Pa}$ for diamond.

> **Comments:**
> **username_11**: Interesting,  I couldn't find any information on it's hardness, viscosity or other similar properties.  How is Young's modulus defined for a plasma, It's often said that young's modulus is not defined for a gas or liquid.  How is it defined in the sun.  Can you cite these values ?  
> **username_10**: I agree that the Young's modulus is not used for gas. But if an ideal gas is compressed by a piston in a cylinder isothermally, then it will be felt as a solid body with an corresponding Young's modulus at small changes of piston position. This value is approximately equal to the gas pressure. The solar core presure is [https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html] $2.477 \times 10^{11}\text{ bar} \approx 10^{16.5}\text{ Pa}$  
> **username_11**: But "gas pressure" is still fluid.  I don't think this pressure value corresponds to "hardness" or "resistance to flow", which would be characteristics of a solid.  Does this answer the question "is the sun's core a solid", because it seems to say "yes it is".  I have doubts about that.  
> **username_12**: @Imyaf saying that "it would be stiff it were contained" and "it is hard" are quite different assertions. Specifically, in the contexts of the behaviours the question asks about, this question has relatively little relevance.  
> **username_5**: @Imyaf good answer!  and pointing out the last example for the diamond too - but the **https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html]** link you posted **is dead** - care to update? :)  
> **username_10**: @William Martens Link is https://nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html without ]  

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### Answer by username_13 on 2023-01-15. Score: 3

> The Sun's core is mind-bogglingly dense, about 150,000 kg/m3 (about
> 15x denser than lead, 7x denser than uranium, 6x denser than osmium).

Such a high pressure would compress matter into a [wigner crystal](https://en.wikipedia.org/wiki/Wigner_crystal) (but with nuclei against an electron background instead of the other way around). But the core of the sun is also mind-bogglingly *hot*. The thermal energy prevents this from happening.

> By hardness, I mean having stiffness/rigidity, an ability to retain a
> certain shape when subjected to anisotropic stress.

No. The core of the sun is a gas and has a low viscosity. Ideal gas viscosity increases with the square root of temperature *and doesn't depend on pressure*, so a naïve ideal gas formula predicts a viscosity about 225 times more viscous than air. This is somewhere between water and cooking oil.

> Supposing we got the indestructible observer to throw a punch randomly
> inside the Sun, what would this observer feel?

The trouble would be the *density*. Swimming would be hard, much worse than in mercury, and you could only swim or punch at the center. Each *g* of gravity gives you up to 140 *gs* of buoyant forces. And gravity rises to over a hundred *gs* (compared to "only" 27*g* at the surface) at the right distance from the center. Your *unobtanium* body would survive these extreme forces but fluid dynamics alone would dictate your posture.


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### Answer by username_14 on 2023-01-15. Score: 2

Solid is one of the four states of matter which is **uncompressible** with atoms that are [bound](https://en.wikipedia.org/wiki/Chemical_bond) together. **The sun's core is compressible** and its atoms are completely unbound. Apply a bit of extra force or vacuum and the core will compress or expand.

Solids comprise the following: ionic solids, molecular solids, network covalent solids, metallic solids, and amorphous solids like glass, rubber, gel, plastics can't be compressed either.

There is thermal energy equivalent to about 2700 megawatt hours per kilo, and an atom bomb is about 3500 megawatt hours per kilo of plutonium, so your immutable object would have to apply a higher kinetic force in order to move it.

The atoms of the core are like a billiard table knocking at 100ds of kilometres per second. If you put a space-time based object in hot gas higher than 100,000'C like a human hand, the hand has vaporized in a millionth of a second, only confined by other atomic explosions, the atoms fly off in all directions, hydrogen protons at 610KMS in a vaguely [brownian motion](https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.75.4214).

If you imagine a solid, i.e. salt crystal and compare the sun core matter, the expanding atomic force is far greater than the force of contact of a hammer hitting a diamond, perhaps by millions of times.
The speed of nuclei of atoms changes relative to temperature... Most matter in the sun's core travel at an average of 610km/s and sometimes 1200km/s:
[![enter image description here](https://i.sstatic.net/5AnTP.jpg)](https://i.sstatic.net/5AnTP.jpg)

To qualify the sun's core, you can say it's thermally caustic and amorphous and dissolves anything very fast so that atoms dissipate pretty fast, immutable hands could compress or expand the core physically if they have a force higher than an atomic explosion, a real hand would vaporize outwards into a cloud that expands at 1-2 kilometres per second according to vaguely brownian motion, there's also gamma rays, x-rays, UV, visible light, at higher quantities than the lasers used for slicing steel.

> **Comments:**
> **username_14**: Thanks for the info's. A kilo of the sun material contains ~2.7 Gigawatt-hours of stored thermal energy, A kilo of Plutonium 239 can give 23 GWh, and a bomb is 15% efficient so it's only 3.45 Gigawatt for the bomb versus 2.7 gigawatt for the sun. Vaguely the same range.  
> **username_15**: No you are confusing energy and power. In fact you seem to be confusing force, pressure, power, velocity and energy in your answer.  
> **username_14**: You are changing the subject from the atomic bomb comparison. Why? That was in fact, correct, wasn't it. The maths is of the thermal energy content are quite accurate, and changing the subject vaguely to avoid aknowledgeing that is silly. The protons move at 610 KMS and the latent the latent energy of the material is 2-3 gigawatts per kilogramme, exactly the same as an atomic bomb. It's a fair newbie reply. Why risk me labelling 500 pasting all over the website?  

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