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Issue 6.01

The Astrophysics Spectator

January 19, 2009

Happy New Year and welcome back.  In my first issue of the year, I write about how the radii of degenerate dwarfs (white dwarfs) and neutron stars are set by the fundamental constants of physics.

The degenerate objects of the universe are direct expressions of some very basic physics.  When a star becomes degenerate, the complications of generating and transporting heat out of the object fade away, and the properties of the object become fixed by the physics of cold matter, just as the properties of a cold steel bar—density for instance—are set by the physics of the atoms within the bar.  In particular, the mass range spanned by degenerate objects and the radii of degenerate dwarfs and neutron stars are directly set by the properties of the electron and the proton.

The radii of neutron stars are set by the scales of quantum mechanics and gravitation, the speed of light, and the mass of the proton.  A neutron star's radius is only a little larger than the event horizon of a black hole of comparable mass.  This would be the case regardless of the value of any of the constants that set the size of the neutron star.  A degenerate dwarf at its maximum possible mass, on the other hand, has a radius that is about a factor of 2,000 larger than a neutron star's radius, making the degenerate dwarf comparable to Earth in size.  This larger size is tied to the electron's mass, which is smaller than the proton's mass by a factor of 2,000.  The sizes of the remnant stars in our universe are therefore directly set by the masses of the electron and the proton.

Next Update:  The next issue of the web site should appear on or shortly after January 28.

Jim Brainerd

Degenerate Objects

The Radii of Degenerate Objects.  The radii of degenerate dwarfs and of neutron stars are fundamentally linked to the fundamental constants of physics.  The neutron star is about the size of a black hole of comparable mass.  The degenerate dwarf, on the other hand, has a radius that is of order 2,000 times larger.  This difference in radius is a direct consequence of the proton being more massive than the electron by this factor.  The mass of the proton sets the absolute scale for these objects.  The radius of the neutron star is of order 15 km, and the radius of the degenerate dwarf is comparable to Earth's.  (continue)

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