Readers of the interesting paper by D. J. Mullan1 should be aware that a previous explanation of stellar sizes was given over 30 years ago by Victor Weisskopf. Weisskopf’s explanation was included in a talk on “Atoms, mountains and stars” given initially at the University of Michigan in 1972 and subsequently elsewhere. An extended version of this talk was later published in 1975.2 As a member of the audience for the talk at the University of Michigan, I can testify that the elaborations in the published version partially obscure the simplicity and elegance of the original talk. In the following I reconstruct my notes from that talk relevant to stars.

Weisskopf’s explanations of sizes had their basis in the “kinetic energy of confinement,” which results from the uncertainty principle. Given a particle confined within a volume of radius r, the uncertainty in its radial coordinate can be no...

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