Degenerate Objects

Introduction

We tend to sort the objects in the sky into those that generate their own light?the stars?and those that reflect sunlight?the planets, but another way to sort these objects is by their structure, by the source of pressure that prevents each of these objects from collapsing under its self-gravity.  In this picture, most stars are supported by the pressure of gas heated by thermonuclear fusion, and the smaller planets are supported by the pressure exerted by atoms.  But these two mechanisms do not encompass all of the long-lived objects in our galaxy.  There exist three other classes of object: the electron-degenerate object, the neutron star, and the black hole.  The black hole, which is discussed under the General Relativity path, is purely a gravitational object; whatever material structure it may have is hidden from view.  The remaining two classes of object are subclasses of degenerate object.  They are manifestations of the same physical principal?particles like electrons, protons, and neutrons exert a pressure even when cold.  This pressure is called degeneracy pressure, and it is a consequence of quantum mechanics.

The class of electron-degenerate objects is broad, comprising the giant gaseous planets, the brown dwarfs, and the white dwarfs (in these pages, white dwarfs are generally called degenerate dwarfs).  We are all familiar with the giant gaseous planets Saturn and Jupiter.  Saturn is as light as an electron-degenerate object can get, so it defines the low-mass end of the electron-degenerate-object mass range.  The degenerate dwarfs are rather common objects, although they are all too faint to see without a telescope.  The most brilliant star in the sky, Sirius, is a nearby double star composed of an A-type star and a degenerate dwarf.  A degenerate dwarf is a remnant star, the final stage of some star's life once thermonuclear fusion has ceased.  White dwarfs are the heaviest of the electron-degenerate objects.  The brown dwarfs are objects that lie in the intermediate mass range between the giant gaseous planets and the degenerate dwarfs.  These objects were unseen until recently, although they were predicted to exist.  These objects are infrared emitters, and are too light to burn hydrogen in thermonuclear fusion.

The neutron stars constitute the other class of degenerate object found in the universe.  We see numerous neutron stars in our Galaxy, usually as strong radio, x-ray, and gamma-ray sources.  Young neutron stars are usually strong, pulsed radio sources, and they are called radio pulsars.  A familiar example of a radio pulsar is the Crab pulsar, which provides power to the Crab nebula.  Many older neutron stars are seen in binary systems, pulling gas from companion stars onto themselves.  These systems are x-ray binaries, and they are the brightest x-ray sources in the sky.

The reason two types of degenerate object exists is that two types of particle can provide degeneracy pressure.  As implied by the name, electron-degenerate objects are supported against their own gravity by the degeneracy pressure exerted by electrons.  The neutron star, on the other hand, is supported against gravity by the degeneracy pressure exerted by neutrons and protons.  The consequence of this difference is that the neutron star is much smaller than a degenerate dwarf, smaller by the ratio of the electron mass to the proton mass (1/2,000), so while the degenerate dwarf is comparable to Earth in size, the neutron star has a radius of order 15 kilometers, which is not much larger than the last stable orbit of a black hole of comparable mass.  Because of the small sizes, degenerate dwarfs and neutron stars are often called compact stars.

The upper mass for the degenerate dwarfs is similar to the upper mass of the neutron stars.  This upper limit exists because degenerate bodies more massive than this limit are unstable to gravitational collapse.  This instability is responsible for the supernovae; it causes massive stars to collapse to neutron stars, generating type 1a supernovae, and it causes degenerate dwarfs to collapse and explode in type 2 supernovae.

The compact size of both the degenerate dwarf and the neutron star is a source of energy exploited by close binary systems.  When the distance between a degenerate object and a companion star is small enough, the atmosphere of the companion star falls onto the degenerate object in a process called accretion, releasing gravitational potential energy as intense radiation.  These binary systems are called cataclysmic variables when they contain white dwarfs, reflecting the strong fluctuations in their brightness.  Cataclysmic variables release most of their energy as ultraviolet light and x-ray radiation.  As stated above, the accreting binary systems containing neutron stars are known as x-ray binaries, and they emit most of their energy at x-ray and gamma-ray energies.