[Edited to remove incorrect geological assumptions]

The sun is self-rotating, i.e. it spins on its axis. When something spins, there's an apparent force away from the centre, and matter gets flung out. The sun's surface is gas; it follows that beneath the gas surface there could only be void with the density less than the surface. So, is the sun hollow, and if it isn't, what's the reason that the centrifugal force doesn't spin the matter outwards?

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    $\begingroup$ Possible world's record for most wrong claims in one paragraph. $\endgroup$ Apr 27, 2018 at 15:09
  • $\begingroup$ Welcome. Please edit if you can/may, thanks @CarlWitthoft $\endgroup$ Apr 27, 2018 at 15:21
  • $\begingroup$ Density of liquid iron is still near 7gm/cc. Pv = nrT tells You that gases get denser as pressure increases. $\endgroup$ Apr 27, 2018 at 21:00
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    $\begingroup$ I've proposed a fairly substantial edit to the question that I hope expresses Mishu's intentions. There's a lot of misunderstanding about centrifugal pseudo-forces, centripetal forces and rotating reference frames, so it might be an opportunity for someone to add a third answer addressing this in layperson's terms. $\endgroup$ Apr 28, 2018 at 12:06
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    $\begingroup$ There is nothing flat earth-like thing in this question! It is an ordinary astronomics question, and thus it deserves a reopen. $\endgroup$
    – peterh
    Apr 28, 2018 at 22:49

2 Answers 2


As the surface of the earth is solid the crust,


underneath the crust the mantle which is liquid,

No the mantle is also solid, although more plastic than the crust. There is a liquid core below the mantle

the liquid density is smaller than the crust.

No the core is made of iron and considerably denser than the mantle rocks above it.

This way the earth can exercise self-rotation.

I don't know what you mean. Fully solid bodies, liquid bodies, gaseous bodies can all rotate.

Therefore, since the sun also self-rotating, the sun's surface is gas;

Actually plasma, but that is a detail

it follows that beneath the gas surface there could only be void which the density smaller than gas, otherwise it wouldn't be possible for the sun to exercise self-rotation.

No. This doesn't follow. It is not the case that there is a void, for if there were, the plasma would flow into it. In fact density increases with depth.

So, is the sun hollow? Or any reason that the sun can only be hollow inside?

No the sun is not hollow.

  • $\begingroup$ but self-rotation will result a force that need absorption therefore the underneath has to be less dense, that's my point, no? $\endgroup$ Apr 27, 2018 at 14:45
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    $\begingroup$ @Mishu米殊 What do you mean by "self rotation" ? $\endgroup$ Apr 27, 2018 at 15:03
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    $\begingroup$ +1 for a step-by-step answer brimming with crystal clarity. $\endgroup$ Apr 27, 2018 at 15:08
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    $\begingroup$ @StephenG I fear we're being trolled here. Let's just let it drop. $\endgroup$ Apr 27, 2018 at 15:09
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    $\begingroup$ @CarlWitthoft I don't think we have a troll. We probably just have someone with little science knowledge and possibly a language difficulty - not a native English speaker. The OP seems to have reasonable reps on other SE sites. $\endgroup$ Apr 27, 2018 at 18:40

With most celestial bodies above a certain size limit, you can safely assume they only become more and more dense the closer you get to the center. This is true for most planets and it's definitely true for the Sun.

But the aggregation state (solid, liquid, gas) does not matter for the spin (I believe that's what you mean by "self-rotation"). If it's spinning, then it's spinning, no matter what it's made of.

What keeps the Sun in one piece is only its gravity, nothing else. The various parts of the Sun are compressed together by the overall gravity of this object. Spin doesn't matter here.

Spin would matter if the Sun was spinning incredibly fast. But that's not the case, it's not even close; it would have to spin by orders of magnitude faster in order to break up because of that. Like, thousands of times faster, I'm not even sure what the number is but it's huge.

Finally, aggregation states are not very important at cosmic scales. Planets like the Earth are technically "solid", but they behave more like fluids, or more like piles of sand, during an impact. Also, the "solid" parts of the Earth actually do flow like liquids over many millions of years. There are no clearly defined rules here.

  • $\begingroup$ Thanks. But I get that if a large celestial object spins, it will need an inner part that's less solid than the surface to balance off the spinning, or to absorb and re-adjust to keep the status quo, otherwise it may grid, i.e., skipping off from the same spot. What do you think? $\endgroup$ Apr 28, 2018 at 16:26
  • $\begingroup$ @Mishu米殊 I honestly don't understand the observation. A solid steel ball can spin just fine, and it's completely solid. $\endgroup$ Apr 30, 2018 at 19:38

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