Have you ever wondered that the death of something could possibly give life to something new which could prove as a gift from nature to us?
Indeed neutron stars are cosmological gifts to test our knowledge at the extreme conditions that cannot be created in the laboratory. Neutron stars are objects created after the death of a star in supermassive explosion. When an ordinary star of mass of 8 - 25 solar masses runs out of its fuel means no longer can produce enough heat to sustain itself against gravity and collapses under its own gravitational pull. At this point first the core of the dying star collapses and releases a ripple of energy to the outer layers and blows them away into the Universe, as a supernova explosion. What is left after the explosion a tiny, sometime called microscopic, super-dense and hot star is left that is a neutron star. The star is named a neutron star because it is made of neutrons mostly. If the mass of progenitor star is less than 8 solar masses or greater than 25 solar masses then explosion results a white dwarf or black hole respectively.
In the formation electrons and protons are squeezed together and they form neutrons and anti-electron neutrinos which leaves the star as a flash of light. Neutron stars are not ordinary stars because of the extreme conditions like highest order of energy density, temperature, spin, magnetic field and gravity.
As we know neutron stars come into existence when an ordinary star cannot sustain against gravity then what makes neutron star sustained against gravity?
The answer to this question is the reason we call neutron stars, 'Cosmological gifts'. Neutron stars are super dense just beaten by black holes in terms of energy density. One teaspoon of neutron star material weighs 4 billion tons. They produce a gravity so high that if you ever fall in the neutron star you might be decomposed in the constituent particles before even hitting the surface. At this energy density actually we do not know what are laws of physics make then sustain. One most agreed answer is, the neutrons react with each other and make new particles and develop a degeneracy pressure, which make then sustain against the gravity, most physicist agree on this. Different scientist propose different approaches and try to calculate the neutron star properties and test them against the neutron star observational data. This is how we understand, test the new approaches and we get to know how physics behave at the extreme energy density.
Under these conditions developing the models of structure of neutron stars is great research challenge and particularly to get information about the core of the neutron star is a matter of great speculation. After the discovery of the neutron particles by James Chadwick the existence of neutron stars was proposed by Baade and Zwicky. They both suggested that they could be mainly made of neutrons with very high energy density. A few years later in order to understand the structure of neutron stars Tolman and Oppenheimer-Volkoff derived the equation of state of the non rotating neutron star assuming neutron stars are made of ideal gas of neutrons at very high energy density.
Although the existence of neutron stars had been theoretically suggested yet the possibility was ignored for almost 3 decades due to their tiny size and they are thought as impossible to detect. In 1967 first radio Pulsar PSR B1919+21 was discovered by Bell and Hewish having rotation period of 1.4sec. Gold showed that pulsars are in fact rotating neutron stars.
Up to now over 200 rotating neutron stars have been discovered in the Universe. X-ray astronomy has played important role in observing the properties like radius, mass, magnetic field, temperature and chemical composition, of neutron stars.
How does these cosmological gifts look like?
Mass - Since the discovery of neutron stars, it has been a puzzle to answer, how massive a neutron star can be? Unlike black holes, neutron stars cannot have arbitrary mass because there is no physical force excluding neutron degeneracy pressure, to counter the gravitational collapse. Most of the neutron stars found in the Universe have masses around 1.4 solar masses while recent discovery of pulsar PSR J0348+0432 surprised the astronomical community with the mass of 2.01 solar masses. Upper limit of mass, a neutron star can have, is not known. But PSR J0348+0432 sets up maximum mass constraint of at least 2 solar masses. A successful physics model that explain physics at extreme density must predict maximum mass of 2 solar masses at least.
Spin - Even the fastest grinder on Earth does not rotate that fast. Neutron stars rotate with a typical frequency of around 700Hz, that is 700 rotations/sec. This incredible high rate of rotations can be understood by the law of conservation of angular momentum, because the initial star has low spin and large radius as its radius reduces to 10-15 kms its spin goes up. Fastest spinning neutron star till today is PSR J1748-2446ad with rotation period of 1.33 miliseconds.
Magnetic Field - Why neutron stars have magnetic field? They are made of neutrons which is neutral so how can a neutron star produce a magnetic field? but strangely neutron stars were discovered due to their high magnetic field. Rotating neutron stars are also known as pulsars. Also, the magnetic field axis is not the same as the axis of rotation. It is tilted slightly and the reason for this is unknown. They are detected when radiation is pointed towards Earth. They serve as natures light house. They have magnetic field of around 10^12 Gauss at their surface. This magnetic field value is so high that if a neutron star near Earth at the distance of Moon then none of the electric appliance on Earth will work. Such a high magnetic field is quite strange property because neutron star matter is supposed to be neutral. The possibility of such magnetic fields has been addressed by many authors yet the mystery is still not solved.
Glitch - In 1967, Pacini suggested that rapidly rotating neutron stars with strong dipole magnetic fields can transfer their rotational energy into electromagnetic radiation and they can emit particles to a very high energy. Neutron stars by losing their rotation energy steadily decrease their frequency. But Neutron stars show incredible accuracy in their rotational period, and on the contrary there is small increment in the rotational frequency of a rotating neutron star observed after a certain time that is called a glitch. The reason for this is not known.
Radius - Neutron stars are microscopic objects because of their tiny size with radius ranging 10-15 kms. Direct determination of their size is not possible due to their tiny size and large distance, which is why it took more than 30 years to find one after the prediction. If you want to imagine them then double the mass of the Sun and squeeze it into the size approximately 1/3rd of London city.
It is a gift form nature to us. It is nature's laboratory to let us discover beyond what we can produce in our labs. Thorough understanding of neutron star may lead us to have a glimpse of what black holes could have hidden inside them and we may speculate what would have happened in the early Universe.
