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According to https://www.britannica.com/science/Schwarzschild-radius, the Schwarzschild radius for a black hole with the mass of the Sun is about 2 miles. My question is what is inside of it? Is whatever is inside of it similar to what surrounds it? Can particles escape? Or does the whole sun have to collapse into it for particles to be trapped inside of it?

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    $\begingroup$ Search term: shell theorem. $\endgroup$
    – rob
    Sep 17 '20 at 17:28
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It's not relevant to the physics of the sun. Nothing happens there.

You can construct an exact solution to general relativity describing an idealized star (spherically symmetric, nonrotating, surrounded by vacuum). You do this by combining an interior solution describing the matter with an exterior solution describing the vacuum around it. The vacuum part of the solution is the Schwarzschild geometry.

If you extend the Schwarzschild geometry inward to the center, strange things happen at the Schwarzschild radius. But in the stellar model, that part of the solution isn't used; a different solution to general relativity applies there. The odd behavior of the vacuum solution would only be relevant if the star collapsed to a size comparable to that radius, which never happens to solar-mass stars.

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  • $\begingroup$ So what is inside of the Schwarzschild radius of the sun? Vacuum or particles? $\endgroup$ Sep 17 '20 at 22:07
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    $\begingroup$ @CraigFeinstein Particles. $\endgroup$
    – benrg
    Sep 17 '20 at 23:05
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The schwarzschild radius defines the location of the event horizon that surrounds a black hole. Note that there is no black hole inside the sun because the density of the material in the sun's core is insufficient to create one.

But if there were, the rest of the sun would fall into it quickly and from our point of view all that matter would get squashed onto the event horizon and flattened to zero thickness.

As stated in your question, for that to happen, the mass of the entire sun would need to be compressed down into a sphere 2 km across. This will never occur in the sun's lifetime.

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