Tuesday, March 2, 2021

Neutron Stars: The Densest Objects in the Universe

A neutron star is a cosmic object that forms after a supernova explosion. It is a degenerate star from neutron gas, the final stage in the evolution of massive stars. Its mass is greater than 1.4 masses of the Sun, but less than 3 masses of the Sun. Neutron stars have a diameter of 10-30 kilometers. About 5% of all known neutron stars are members of a binary system. We can also observe neutron stars called pulsars.

Observing solitary neutron stars is difficult because they have only a very small surface and this makes them very faint objects. Neutron stars that can be observed are very hot and typically have a surface temperature of around 600000 K.

Neutron stars are composed of material with extreme density, mostly neutrons. This material is formed during the supernova explosion by extreme pressure, which causes electrons to be pushed into atomic nuclei and the subsequent conversion of protons into neutrons.

It is estimated that there are about 30 million neutron stars in the Milky Way (the galaxy we are in).

Pulzar is a rotating neutron star that can be observed as a source of electromagnetic radiation.

The events leading to the formation of a pulsar begin when the core of a massive star is compressed during a supernova, which collapses into a neutron star. The neutron star retains most of its angular momentum, and since it has only a tiny fraction of its progenitor’s radius (and therefore its moment of inertia is sharply reduced), it is formed with very high rotation speedThe neutron star rotates so fast that the centrifugal force shapes the radiation that the star emits into cones at the equator, which regularly, like a beacon, strike a certain part of the universe.

Pulsars were first discovered at radio wavelengths in 1967, and have since been found in X-rays and gamma rays.

To this day, we know three types of pulsars, which differ in the source of energy that supplies energy to their radiation:

Rotation-driven pulsars- in which the energy of the radiation is supplied by the loss of the star’s rotational energy.

X-ray pulsars- in which the source of energy is the gravitational potential energy of the accumulated mass.

And also Magnetars whose main manifestation is an extremely strong magnetic field. The decay of the unstable crust of the magnetar is accompanied by massive high-energy discharges, especially X-rays and gamma rays. When in a supernova, a star collapses to a neutron star, and its magnetic field increases dramatically in strength through conservation of magnetic flux. Halving a linear dimension increases the magnetic field fourfold.

As of March 2016, 23 magnetars are known, with six more candidates awaiting confirmation.

In August 2017, LIGO and Virgo made first detection of gravitational waves produced by colliding neutron stars.

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