What is a Neutron Star?

Neutron stars are very dense and spin very fast and are typically only 10-15 km in radius. Because neutron stars form from burnt-out stars, they do not glow. The collapse of the star causes the matter to be converted into mostly neutrons, and that explains the name neutron star.

Some neutron stars emit radio waves that pulse on and off. These stars are called pulsars. Pulsars don't really turn radio waves on and off--it just appears that way to people on Earth because the star is spinning. What happens is that the radio waves only escape from the North and South magnetic poles of the neutron star. If the spin axis is tilted with respect to the magnetic poles, the escaping radio waves sweep around like the light beam from a lighthouse. Far away on Earth, radio astronomers pick up the radio waves only when the beam sweeps across the Earth.

Here is a Q - A type thing... from Rick's Page in Neutron Stars!

Almeida asks: "I understand that a star collapses (due to gravity) until it becomes a Black Hole, a Neutron Star, or a white dwarf. Yet does it not undergo a supernovae and THEN become a black hole or neutron star?"

You are right: black holes and neutron stars (including pulsars) undergo supernovae explosions as they form. Young stars are hydrogen, and the nuclear reaction converts hydrogen to helium with energy left over. The left over energy is the star's radiation--heat and light. When most of the hydrogen has been converted to helium, a new nuclear reaction begins that converts the helium to carbon, with the left over energy released as radiation. This process continues converting the carbon to oxygen to silicon to iron. Nuclear fusion stops at iron. If you could slice a very old star in half, you may see serveral layers of elements.

The star now has layers of different elements. The outer layers of hydrogen, helium, carbon, and silicon are still burning around the iron core, building it up. Eventually, the massive iron core succumbs to gravity and it collapses to form a neutron core. The outer layers of the star fall in and bounce off the neutron core which creates a shock wave that blows the outer layer outward. This is the supernovae explosion.

When a white dwarf forms, however, a lot of gas is blown off (but not in a supernova explosion) to form a "planetary nebula," which looks like a giant smoke ring. The still-glowing core that is left over is a white dwarf. When it cools and no longer glows, it is a black dwarf.

How Neutron Stars Form

Black holes and neutron stars form when stars die. While a star is burning, the heat in the star pushes out and balances the force of gravity. When the star's fuel is spent, and it stops burning, there is no heat left to go against the force of gravity. Whatever material is left over collapses in on itself. How much mass the star had when it died determines what it becomes. Stars about the same size as the Sun become white dwarfs, which glow from left over heat. Stars that have about 3 times the mass of the Sun compact into neutron stars. And a star with mass greater than 3 times the Sun's gets crushed into a single point, which we call a black hole.

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