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Neutron Stars

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Neutron Stars

This image of the crab nebula depicts the X-ray emission from the central pulsar of the region.

Image Credit: NASA

Supernovae are some of the most dynamic events in the Universe. They create such intense explosions that the light that they emit can outshine entire galaxies. But the compact objects that are left behind are also incredibly powerful as well.

Creation of Neutron Stars

Stars, like our Sun, spend most of their lives on what is known as the main sequence. The main sequence begins when the star initially forms -- igniting nuclear fusion in its core -- and ends once the star has exhausted the hydrogen in its core and begins fusing heavier elements.

Once a star leaves the main sequence it will follow a particular path that depends on its mass. Stars that are more than 8 solar masses -- one solar mass is equivalent to the mass of our Sun -- will leave the main sequence and go through several phases as it continues to fuse elements up to iron.

Once the fusion ceases in the core, the core will contract due to the immense gravity and the outer part of the star "falls" onto the core and rebounds to create a massive explosion called a type II supernova. Depending on the mass of the core, it will either become a neutron star or black hole.

If the mass of the core is between 1.4 and 3.0 solar masses the core will become a neutron star. The iron core contracts and undergoes a process known as neutronization, where the protons in the core collide with very high energy electrons and create neutrons. As this happens the core stiffens and sends shock waves through the material that is falling onto the core. The outer material of the star is then driven out into the surrounding medium creating the supernova.

Properties of Neutron Stars

Neutron stars are some of the most difficult objects to study and understand in the Universe. They emit light across a broad spectrum of wavebands -- the various wavelengths of light -- and seem to vary quite a bit from star to star. However, the very fact that each neutron star appears to exhibit different properties, can help us understand what drives them.

Perhaps the greatest difficulty in studying neutron stars is that they are incredibly dense -- so dense in fact that a 14 ounce can of neutron star material would have as much mass as our Moon. We have no way of modeling that kind of density here on Earth, therefore it is difficult to try and understand the physics of what is going on. This is why studying the light from these stars is so important, it can give us clues as to what is going on inside the star.

Because of the high density inside the cores of neutron stars, we don't even know what the cores are made of. Some scientists claim that the cores are dominated by a pool of free quarks -- the fundamental building blocks of matter -- while others contend that the cores are filled with some other type of exotic particle like pions.

Neutron stars also have intense magnetic fields. And it is these fields that are partially responsible for creating the X-rays and gamma-rays that are seen from these objects. As electrons accelerate around and along the magnetic field lines they emit radiation (light) in wavelengths from optical (light we can see with our eyes) to very high energy gamma-rays.

Pulsars

It is believed that all neutron stars rotate, and do so quite rapidly. As a result, some observations of neutron stars yield a "pulsed" emission signature. So neutron stars are often referred to as PULSating stARS (or PULSARS), but differ from other stars that have variable emission. The pulsation from neutron stars is due to their rotation, where as other stars that pulsate (such as cephid stars) pulsate as the star expands and contracts.

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