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I just read the Wikipedia article on the Sun. The section on Solar Core states

It has a density of up to 150 g/cm3[47][48] (about 150 times the density of water)

Do the particles (atomic, and sub-atomic) present in the solar core move at the same speed as they would if they were in a vacuum?

EDIT: Just read the article on mean free path; i've read it before, and forgotten. Anyway, mean free path speed of the sub-atomic particles sounds like what i have in mind.

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The mean free path in the core is very short. Are you asking if the particles behave as free particles in between collisions? – John Rennie Apr 9 '13 at 9:20
The mean free path of which particle? Each particle has its own mean free path. – user12345 Apr 9 '13 at 10:12
Is that normal or just a way to be "cool" to use "Sol" for the Sun? Astronomers use "sol" for a solar day, not the star, by the way. – Luboš Motl Apr 9 '13 at 10:26
Dear @user16307, I think that your claim is morally wrong. The mean free path of pretty much all particles in the Sun's core is short, typically around 0.1 mm. – Luboš Motl Apr 9 '13 at 10:28
I'm with John Rennie in that I think the key thing to clarify for the OP would be the difference in speed of propagation between collisions and the slow diffusion after averaging out many collisions. And sure, neutrinos has a large mean free path, but nothing else does (unless you're believing in WIMPs or chameleons or something). – Michael Brown Apr 9 '13 at 12:58

The simple answer is "no".

Velocity depends on the forces constraining/bounding the trajectories of any type of particles. Particles in a bound ensemble, as the ones in the Sun are constrained by the proximity to each other, described by the mean free path that John mentioned in his comment.

For quarks and gluons who are bound with nuclei and hadrons, the concept of velocity is irrelevant.They are off masss shell, their behavior, as far as they are bound in hadrons, is the same for all locations, whether in the Sun or in vacuum. It is only at the extreme energies of the beginning of the Universe where they could be asymptotically free that a velocity could be ascribed to them which of course could not exist in vacuum.

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