Definition of Neutron Star

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A neutron star is a type of dead star, or stellar remnant, which forms during the death of a star much larger than our own, but smaller than the massive blue giants which (it is believed) collapse into black holes. The resulting neutron stars are small, extraordinarily heavy objects, weighing several times the mass of our own Sun yet compressed into a dense, hot sphere as little as just a few miles wide.

- Neutron Star Formation -

Stars are massive, glowing balls of gas which sustain themselves through a continuous process of nuclear fusion in their cores, where hydrogen atoms are forced together to form helium. For as many as several billion years, in the case of stars like our Sun, this process can continue effectively unchecked, allowing us the stable light source we rely on for all life on Earth. All stars, however, will eventually run out of hydrogen, and die. What happens next depends on just how large the star is.

Without the heat and energy of hydrogen fusion to sustain them, stars that are roughly two to three times the size of our own Sun begin to collapse inward under the force of their own gravity, forming neutron stars. Smaller stars, like our own Sun, puff up into red gas, blow off their surface to form beautiful planetary nebulae, and then shrink into a gradually cooling ember known as a white dwarf. In much larger stars, there is too much mass for the gravitational collapse to stop at the neutron star level; these massive stars will continue collapsing, forming an extraordinarily dense field of matter and energy known as a black hole.

Large stars, both those that form neutron stars and those that form black holes, explode spectacularly when they run out of fuel; from Earth, these explosions are sometimes so bright that they are visible to the naked eye, and are known to astronomers as supernovae. At the centre of this supernova lies what remains of the core of the star, spinning extremely quickly and attempting, through gravity, to pull back the exploded matter.

This works - but with a price. Under the extraordinarily strong gravity of the large stellar remnant, matter is sucked in at extraordinarily high speeds. Normally, stars form gradually over time, so that their gravity does not reach great strength until they are more or less fully formed; at the end of their life, the process works in reverse, pulling in matter so quickly that, as it collides with the surface, the atoms begin to break apart.

The immense gravity, ignoring this, continues to pack particles more and more densely. Normally, an atom consists of a very small nucleus surrounded by a much larger sphere, which is entirely empty except for electrons. In a neutron star, gravity essentially compresses all of the nuclei together, eliminating the empty spaces inside each atom. The result is a small but unbelievably heavy, unbelievably solid, and extremely radioactive mass known as a neutron star.

- Composition -

All large celestial objects have gravity, and therefore have an associated "escape velocity": the speed which must be travelled in order to escape the gravity well. On Earth, rockets are used to accelerate spacecraft until they achieve escape velocity. In the vicinity of a neutron star, this is simply impossible: because of their incredibly densely packed mass, one would need to travel at nearly the speed of light to escape a neutron star. (The reason black holes form from much larger stores is that their escape velocity is even higher, so high that light itself cannot escape and is pulled into the centre of the black hole.)

At the moment they form, neutron stars are rotating extremely quickly, usually one rotation in less than a second. Some omit a single, continuous beam of radiation from near one pole as they do so; the result, from a vantage point like Earth, is a lighthouse-like beam which seems to blink on and off, called a pulsar. Based on scientists' observations of these pulsars, we currently believe that neutron stars will gradually slow down, and perhaps, in billions of years, come to a complete standstill.

- Examples of Neutron Stars -

Neutron stars were first theoretically proposed in the 1930s, on the basis of advances in nuclear physics. In 1965, two astronomers finally located the first neutron star in the beautiful Crab Nebula. It was subsequently shown that the neutron star they had discovered was the remnant of the giant star which exploded in 1054, in a supernova which was recorded by ancient and medieval astronomers across the planet for nearly two years, forming what is now still recognizable in telescopes as the Crab Nebula.

Since then, several thousand neutron stars have been identified in the Milky Way alone, most of them pulsars. One of the closest is Calvera, or 1RXS J141256.0+792204, which is located in the Little Dipper constellation and is 450 light-years from the Earth; another of similar distance goes by the similarly technical name PSR J0108-1431, and is found in the constellation of Cetus (the Whale).

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