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Issue 3.20

The Astrophysics Spectator

November 22, 2006

Exotic and complex, the neutron star is my personal favorite among the objects in the sky. Study a neutron star, and you study the cornerstones of modern physics: general relativity and relativistic quantum mechanics.

Only two types of star, the degenerate dwarf and the neutron star, are permanently stable. A degenerate dwarf is stable because the electrons in its core exerts a pressure even when cold. A neutron star is stable for a similar reason: protons and neutrons in its core exert a pressure even when cold. This pressure exists because no more than two electrons, two protons, and two neutrons can occupy the same energy level. This bit of modern physics, the Pauli exclusion principle, sets the final size attained by a star. While the degenerate dwarf is small by comparison to the Sun, it is comparable to the size of Earth. Neutron stars, on the other hand, are incredibly small, packing several solar masses into a sphere of 10 to 20 kilometers radius. This extremely compact size compared to the larger degenerate dwarf is a macroscopic manifestation of a single property of the microscopic universe: the ratio of the electron mass to the proton mass.

Neutron stars give us many reasons to study them. They are seen individually as radio pulsars, generating radio waves as they spin from their powerful magnetic fields. They are the x-ray power plants of compact binary stars, visible across the Galaxy, and as bright as any other object in our x-ray sky. They release a large fraction of their rest mass energy when they are born in the core collapse of a giant star, making them the engine of the supernova explosion. The effects of general relativity are nearly as strong around a neutron star as they are around a black hole. The neutron star is our visible universe at its most extreme.

Next Issue: This issue of The Astrophysics Spectator is the last of the year. The next issue will appear on January 10 I wish all my readers a bountiful Thanksgiving, a merry Christmas, and a happy New Year. I look forward to your return next year, when I hope to add some new simulators of astrophysics to the web site.

Jim Brainerd


Neutron Stars. Neutron stars are born in the supernovae of massive stars. They are small, even on the human scale, having a radius of around 15 km, but they are extremely dense. This compactness makes their physical conditions the most extreme in the visible universe. We see them as radio pulsars in isolation and as brilliant x-ray power plants in compact binary star systems. (continue)

Neutron Star Size. With a radius that is a thousand times smaller than the radius of a degenerate dwarf, neutron stars are the most compact stars that we observe. This difference in size is fundamentally linked to the mass of the proton relative to the electron. Because neutron stars are so compact, they present us with unique conditions in the universe, conditions that foster the effects of general relativity and the physics of the atomic nucleus. (continue)

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