Pulsar a Neutron star with incredibly high magnetic field

Neutron star
Pulsars are Neutron stars with extremely
high magnetic field. (Credit:Dana Berry/NASA)

Pulsars are one of the mysterious objects of the universe. Pulsars are known for their extremely high magnetic fields and regularly pulsating radio signals which appear as artificial signals. What is pulsar? Pulsars are special type of Neutron stars which spins few times in a second and have incredibly high magnetic field. Before we talk about Pulsars lets first discuss about Neutron star after all pulsar is nothing but a neutron star.

What is Neutron star?

Neutron star is the remnant of star that has undergone supernova explosion. The stars which has core of mass between 1.4 times to 3 times end up into the Neutron star.

The star spends most of its life as a main sequence star and when the hydrogen of the core gets exhausted the process of death of star begins. After the hydrogen the core burns the helium into carbon but for stars having mass less than 1.4 times the solar mass, the carbon reaction never starts. The outward pressure of nuclear reaction stops and the star starts collapsing. The core collapses and becomes so dense that the atoms are as tightly packed as in a crystal. At this point the electron degeneracy pressure acts against the gravity and stops the collapse. According to Pauli excursion principle, no two electron can have same state, so the electron start filling energy state from low energy to high and cannot be force to attain lower filled state. This act as outward pressure called electron degeneracy pressure. The core is prevented to collapse further by electron degeneracy pressure and the outer layer are pushed out by the continue shock wave. The high density core is called white dwarf.

The electron degeneracy pressure is acting against the gravity so if the gravity increases the electron degeneracy will break at some point. Actually the electron degeneracy can support the white dwarf up to the gravity of 1.4 times the solar mass. If the star has more mass then core will collapse further.

For stars having mass more than 10 times the solar mass, the nuclear reactions in the core does not stop at helium but there occur reaction of carbon also. The chain reaction goes till iron but iron is most stable element so reaction of iron does not produce energy instead it requires energy. When the nuclear reaction in core stops, the core starts collapsing. This time the mass of core is more than 1.4 times the solar mass so the electron degeneracy pressure cannot stop the collapse. The core keeps on collapsing and the electrons gets squeezed into their nuclei. The electron and proton combines to form neutron. Now the collapse is stopped by the neutron degeneracy pressure similar to electron degeneracy pressure. The shock wave created after the stop of collapse blows the outer layers of star with big explosion called supernova.

This time, the core is much denser than the white dwarf and is called neutron star. The density of neutron star is very high (3.7 x 1017to 5.9 x 1017 kg/m3) and in the core of neutron star, the density is even larger than the nuclear density of atoms. As an estimate one cube of sugar will be having mass of entire humanity.

The radius of neutron star is about 10 km which is not more than a city. The neutron star of radius about 12-13 km can have mass of two Sun. Such a large mass within small volume creates tremendous gravity on the surface of Neutron star which 100 billion times more than the gravity on the surface of Earth. The temperature of neutron star is very high of the order of 6 x 105 K.

The neutron star spins very rapidly with the time period of the order of seconds and some can spin even 716 times a second. How can neutron star spin at such a high speed? This is possible because of their small size and incredible density. The neutron star forms after the collapse of huge mass of star and the star already has high angular momentum. When star collapses, the angular momentum remains conserved and thus the angular speed increase. If you want to know more about increasing speed of object on collapse you may read “Why do planets spin?

What is Pulsar?

Pulsars are highly magnetised and rotating neutron star that emit beam of electromagnetic radiation. Pulsars emit radiations along its magnetic poles and when Earth is situated in the direction of poles then the pulses of radiation are observed. The time between consecutive pulses is constant because of high and constant rate of rotation.  The periods are extremely regular that only atomic clocks can show lengthening in the period.

Crab Nebula
Crab Nebula also contain pulsar which is remnant of supernova. (Credit: NASA)

Pulsar Discovery

The first pulsar was discovered by the Jocelyn Bell Burnell and Antony Hewish in November 28, 1967. They observed the pulses separated by 1.33 seconds that originated form the same location in the sky. This pulse period was so small that no stars could account for such high rate of rotation. They thought it is originated from some extra-terrestrial life forms and they name it LGM (Little Green Man). When second pulsar was discovered, it was clear that it is not originated from the extra-terrestrial source. Later it was found that these signals must be originated from the neutron stars and these pulsating stars were named as pulsars.

The magnetic axis of neutron star is usually not aligned with the rotation axis and thus we observe beam of radiation once in every rotation. The rotating magnetic field generates electric field which accelerates the protons and electrons on the surface. These accelerated charged particles emit electromagnetic radiation. Since this electromagnetic radiation is powered by rotational energy, the energy of rotation decreases with time. This decreases the speed of rotation of pulsar.

There are three distinct classes of pulsars:

  • Rotation powered pulsars: These pulsars are powered by the loss of rotational energy.
  • Accretion powered pulsars: These pulsars are powered by the gravitation potential energy of the accreted matter. This happens in the binary stars when matter transfers from the one star to the other star which is neutron star. The falling matter is directed along the magnetic field line at the poles and this increases the temperature of matter. This hot matter emits x-ray along the direction of magnetic axis.
  • Magnetar: The pulsars are powered by the decay of extremely strong magnetic field. Magnetars have magnetic fields of 108-1011 tesla which is hundreds of millions of times stronger than any man-made magnet.

The pulse period is very regular and remains constant for very long time that pulsars are used for navigation of space craft. The pulsars can be used as light house and act as an auxiliary device to GPS. Pulsars maps have been included in the two space craft Pioneer and Voyager. These maps include the position of 14 pulsars relative to Sun, so any extra-terrestrial life form can find the location of sun in space and time using the specific pulses of different pulsars.

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Reference:

http://burro.astr.cwru.edu/stu/advanced/stars_neutron.html
http://www.astro.umd.edu/~miller/nstar.html
https://www.e-education.psu.edu/astro801/content/l6_p7.html

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