Caption: This X-ray image of the Cassiopeia A (Cas A) supernova remnant is the official first light image Chandra X-ray Observatory. Two shock waves are visible: a fast outer shock and a slower inner shock. The bright object near the center may be the long sought neutron star or black hole that remained after the explosion that produced Cas A.

  NEUTRON STARS 

  At the end ... gravity always wins



Neutron Stars: 

A neutron star is the stellar remnant left behind of one of the most cataclysmic events in the Universe: a Supernova explosion. As the nuclear fuel in the star is exhausted, gravity takes over and drives the star into its final state. The final state is a compact star,  such as a White Dwarf, a Neutron Star, or a Black Hole. Stars support themselves against gravitational collapse by generating an enormous pressure in the interior of the star. The three most important sources of pressure are:
 
"Radiation" Pressure  Scales as T4
"Classical" Pressure  Scales as T and N/V
"Quantum" Pressure  Scales as (N/V)5/3 atT=0

Our Sun is now supported by radiation and classical pressure. Yet, at the final stages of its existence - as a white-dwarf star - it is the quantum mechanical pressure from its electrons that will support the Sun. For very massive stars, namely, those exceeding the Chandrasekhar limit (of  1.4 Solar masses) electron pressure is not effective and the star collapses under its own weight.


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In a fraction of a second the following very dramatic changes ensue:

In 1931 Chandrasekhar predicted that a star exceeding 1.4 solar masses is too heavy to support its weight by the quantum pressure from its degenerate electrons and will not die as a white-dwarf star (he did not say what its ultimate fate was going to be). The neutron - one of the two constituents of the atomic nucleus - was discovered by Sir James Chadwick in 1932. In that same year the Russian physicist Lev Landau predicted that neutron stars might exist, with their weight supported by the quantum pressure from the neutrons. Only two years later Walter Baade (Mount Wilson) and Fritz Zwicky (Caltech) suggested that neutron stars might be the end point in the evolution of a very massive that dies in a majestic explosion - known asa Type II Supernova. They speculated that the collapsed core could be dense enough to fuse together electrons and protons to form an extremely dense star: A Neutron Star.
 

The Nature of a Neutron Star


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As the core of a very massive star undergoes a collapse and its radius shrinks to an incredible size of only 10 km (the radius of the Sun is 700,000 km) its surface temperature will increase to about a few million degrees. According to Wien's law, this very hot object will radiate most of its energy at a wavelength of about 1 nm, or in the X-ray part of the electromagnetic spectrum. Thus, most of the radiation emitted from this very compact object  is undetected by terrestrial Optical and Radio telescopes. Moreover, while the gravitational energy is being converted to kinetic energy (rising the temperature of the star) other quantities are being conserved (i.e., remain unchanged in the collapse). These are:

  • Angular Momentum: Scales as Radius2/Period
  • Magnetic Flux: Scales as Radius2*MagField
  • Mass of the Star: Scales as Radius3*Density
    •  Properties  Sun  Neutron Star
     Radius (in km)  700,000  10
     Temperature (in K)  5,800  1,000,000
     Magnetic Field (Bsun)   1.000  1,000,000,000
     Density (grams/cm3)  1.409  100,000,000,000,000

    A 50-kg lady on a neutron star will weigh approximately 1000 billion kg ! Such an enormous gravitational attraction is needed for the star not to fly apart as it rotates 10 or 100 times every second.

    Hence, neutrons stars have extremely large temperatures, magnetic fields and densities. Yet in spite of being so hot, the neutron star has low luminosity because of its very small size. Neutron Stars are extremely difficult to detect, indeed! However, in 1967 a graduate student by the name of Jocelyn Bell detected a very peculiar radio signal made out of very regular pulses of about one second.

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    At one point she and her Professor speculated that the signal might have come from a distant civilization and considered naming it LGM for the     "Little Green Men". As many more pulsars were found - with periods ranging from few milliseconds to seconds - astronomers were convinced that these were radio signals from astronomical bodies which they named pulsars, 

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    for "pulsating" stars. Now we know that calling a neutron star a pulsar is a misnomer, as the radio signal is due to the rotation of the star, rather than to a pulsation. The rapidly-rotating neutron star emits beams of radiation that sweep around the sky such as lighthouses do. Once the beam of radiation crosses our line of sight, we detect these periodic "pulses".  The best known pulsar to date lies in the Crab Nebula which is the remnant of a star whose explosion was witnessed by Chinese astronomers in 1054 AD.

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