In The Stellar Zoo

In The Stellar Zoo

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Neutron stars are born as the result of the fatal supernova explosion of a massive star, combined with gravitational collapse, that compresses the core to the density of an atomic nucleus. How the neutron-rich, extremely dense matter behaves is a scientific mystery. This is because it is impossible to create the necessary conditions in any lab on Earth. Although physicists have proposed various models (equations of state), it remains unknown which (if any) of these models actually describes neutron star matter.

Once the neutron star is born from the wreckage of its progenitor star, that has gone supernova, it can no longer actively churn out heat. As a result, these stellar oddballs cool as time goes by. However, they still have the potential to evolve further by way of collision or accretion. Most of the basic models suggest that neutron stars are made up almost entirely of neutrons. Neutrons, along with protons, compose the nuclei of atoms. Neutrons have no net electrical charge, and have a slightly larger mass than protons. The electrons and protons in normal atomic matter combine to create neutrons at the conditions of a neutron star.

The neutron stars that can be observed are searing-hot and typically have a surface temperature of 600,000 K. They are so extremely dense that a matchbox containing its material would weigh-in at about 2 billion tons. The magnetic fields of these dead stars are about 100 million to 1 quadrillion times more powerful than Earth’s magnetic field. The gravitational field at the bizarre surface of a neutron star is approximately 200 billion times that of our own planet’s gravitational field.

As the core of the doomed massive star collapses, its rotation rate increases. This is a result of the conservation of angular momentum, and for this reason the newborn neutron star–called a pulsar–can rotate up to as much as several hundred times per second. Some pulsars emit regular beams of electromagnetic radiation, as they rapidly rotate, and this is what makes them detectable. The beams of electromagnetic radiation emitted by the pulsar are so regular that they are frequently likened to lighthouse beacons on Earth.

The discovery of pulsars by Dr. Jocelyn Bell Burnell and Dr. Antony Hewish in 1967 was the first observational indication that neutron stars exist. The radiation from pulsars is believed to be primarily emitted from areas near their magnetic poles. If the magnetic poles do not coincide with the rotational axis of the neutron star, the emission beam will sweep the sky. When observed from a distance, if the observer is situated somewhere in the path of the beam, it will appear as regular pulses of radiation emitted from a fixed point in space–hence the “lighthouse effect.” PSR J17482446ad is currently the most rapidly spinning pulsar known, and it rotates at the breathtaking rate of 716 times every second, or 43,000 revolutions per minute, giving a linear speed at the surface of almost a quarter of the speed of light.

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