Neutron Star

In 1932, the neutron was discovered. As mentioned above, two years later, Walter Baade and Fritz Zwicky at the California Institute of Technology hypothesized stars consisting essentially of neutrons. The idea was that if one compresses a star to a density of 10^14 g cm^-3-the density of the nuclei of atoms we are familiar with then the repulsive force between the neutrons. practically touching one another. would prevent any further compression, and the star will be stable. It is just like packing a football with ball bearings; when the ball is jam-packed. it will be quite incompressible. This hypothesis led physicists to the conclusion that even stars more massive than 1.4M☉  could find ultimate peace. not as white dwarfs but as neutron stars. A density of 10^14 g cm^-3 implies that such stars would be incredibly small, with a radius of mere 10 kilometres!

So it appeared that all stars will find peace after all, either as white dwarfs or neutron stars, but this turned out to be not true. Chandrasekhar's discovery implied that there must be a maximum mass for neutron stars also. The maximum mass of neutron stars was discovered by Oppenheimer and Volkoff in 1938. We shall turn to the consequence of this discovery in the next section.

Neutron stars were eventually discovered in 1968 by a young student by name locelyn Bell at Cambridge University. Today, their population is nearly a thousand. In the intervening years between their prediction in 1934, and their discovery in 1968, many of their properties had been theoretically predicted. All of these have now been verified by actual observations. Their masses are remarkably close to 1.4M☉ , which, you will recall, is the Chandrasekhar limit for white dwarfs. They are endowed with incredibly strong magnetic fields of the order of 5 x 10^12 gauss. They are very rapid rotators, the fastest among them spinning 640 times a second! Their period of rotation can be measured with remarkable precision. To give an example, the rotation period of the neutron star known as PSR 1937+21 has been measured to be P = 0.001557806472448830(3) seconds. Indeed, the accuracy with which the rotation period can be determined is limited only by the precision to which the unit of time itself is presently defined!

You will recall that a magnetized rotating sphere is a dynamo. A dynamo, such as the one you may have in your cycle, generates a few volts-enough to light a bulb. Given the strong magnetic field and rapid rotation. neutron stars function as incredibly powerful dynamos; the voltage drop between the poles and the equator can be as large as 10^16 volts! Not surprisingly. many exotic phenomena can occur near the surface of neutron stars. As a result, rapidly rotating neutron stars emit electromagnetic radiation over a broad spectrum-radio, infrared, visible, ultraviolet, X-rays and gamma rays-due to a variety of processes.

Many neutron stars are in binary systems with a gaseous star like the Sun as a companion. When matter from the gaseous companion is sucked by the neutron star’s strong gravity and falls on its surface, the neutron star will turn into a powerful X-ray source. The reason for this is quite simple. Since the radius of a neutron star is a mere ten kilometres, the surface gravity is incredibly strong. The gravitational potential energy of a mass m at the surface of the star is approximately equal to 10 percent of its rest mass energy mc^2! This means that if we drop a mass m onto a neutron star, the potential energy released is a staggering 0.1 mc^2. This energy comes out mainly as X-rays. This is why neutron stars in binary systems, accreting from their gaseous companion, are powerful X-ray sources. Many hundreds of such Xray-emitting neutron stars in binary systems are now known, a large number of them in external galaxies.

Also see - 
Can Star find peace ?
https://forastrophysicist.blogspot.com/2019/08/can-star-find-peace.html?m=1

What are the Stars?
https://forastrophysicist.blogspot.com/2019/08/what-are-star.html?m=1

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