Neutron Star: The collapsed core.

Neutron Stars

Neutron Stars are city-sized stellar objects that are formed when the core of a relatively smaller star collapses. The surface temperature of a Neutron star is approximately 1,000,000०C which is a lot as compared to the measly six thousand degree on the sun. Being super hot, they also have one of the strongest magnetic field in the universe. The gravitational field of a neutron star is approximately 1 billion times stronger than the earth. They are currently the smallest and densest known class of stellar objects apart from black holes, quark stars, and strange stars. These collapsed cores are similar to white dwarf stars and there are plenty in this universe. Almost 2,000 Neutron Stars are currently known to us that are in the milky way. 

They are so dense that a match-box-sized mass of neutron star weighs approximately 3 billion tonnes. In other words, neutron star mass in the volume of a sugar cube would be the weight of Mount Everest. There are many different types of neutron stars like pulsars, magnetars and binaries. We are able to identify neutron stars in the universe by detecting the radiations radiated by neutron stars. 

Now, neutron star is one of the two outcomes of a Supernova Explosion. When a star dies resulting into a supernova explosion, the cores of smaller stars (smaller than the sun) usually form neutron stars.

Let's see what is inside a neutron star. A star's core is made up of Iron. When a supernova explosion happens, due to the extreme gravitational force, the protons and electrons are compressed into neutrons. All these neutrons get compressed at such a pressure that there are different stages of compressed neutrons. This is called Nuclear Pasta. Neutron stars also have a hard crust, mantle ,and a liquid core. Nuclear pasta is potentially the hardest material in the universe, as predicted by scientists.

• Pulsars: They are highly magnetized, rotating compact stars that emit beams of electromagnetic radiation out of their magnetic poles. This radiation can be observed only when the beam is directed towards the Earth. They are very dense and have a short, regular rotational period. For a neutron star to be identified as a pulsar, it has to have the right combination of magnetic field strength and spin frequency. 
• Magnetars: A Magnetar is an exotic type of neutron star, its defining feature that it has an ultra-powerful magnetic field. The field is about 1,000 times stronger than a normal neutron star and about a trillion times stronger than the Earth’s. Apart from these ultra-powerful magnetic fields, magnetars also release vast amounts of energy in the form of flares, X-rays, and gamma-ray bursts. They are therefore associated with extreme events in the universe, making them perhaps the most bizarre objects in the cosmos next to black holes.
• Neutron Star Binaries: Neutron stars in binary systems (in pairs of two) can undergo acceleration which typically makes the system bright in X-rays telescopes. These binary systems will continue to evolve, and eventually the companions can become compact objects such as white dwarfs or neutron stars themselves. The merger of binary neutron stars may be the source of short-duration gamma-ray bursts and are likely strong sources of gravitational waves. Gravitational waves have also been indirectly observed in a system where two neutron stars orbit each other.