Question: How does a super giant become a neutron star?

James Verdon answered on 28 Jun 2012:

HI bhavinamistry,

Red giants are formed when stars run out of hydrogen fuel. Normal stars work through nuclear fusion, fusing hydrogen atoms to form helium, which gives out huge amounts of energy. The force of the fusion reactions pushes the star outwards, and counterbalances the force of gravity which is trying to make the star collapse. When hydrogen runs out in the core of the star, fusion begins to happen in the outer part of the star. This causes the outer parts to expand, so they become larger than ordinary stars, but not as hot. These are called red giants.

Once all the hydrogen has been used, the force of gravity has no counterbalance, so the star collapses in on itself. A number of more exotic fusion reactions can take place as the star collapses – fusing helium to make carbon and oxygen, and fusing carbon and oxygen to make heavier elements, until iron is created. Iron is the most stable in terms of fusion, so once iron is been created, no more fusion can occur.

At this point the force of gravity takes over completely, forcing the star to collapse on itself. If the star is smaller, then the compressive gravitational force is not so large, so the outer material is shed, leaving a small, earth-sized mass of star remnants call a white dwarf. However, for larger stars the gravitational force is so strong that something far weirder happens:

If the star’s mass is large enough, the gravitational force overcomes the forces that keep atoms in shape. The electrons in the atoms get forced to join with the protons, which forms neutrons and neutrinos. The energy involved in this process is huge, blasting out a shockwave and radiation in all directions. This is called a supernova, and they can be the brightest things in the galaxy!

What’s left after the supernova is a neutron star. They are small, usually smaller than earth, but incredibly dense, weighing more than the sun! All the neutrons are completely packed in on each other because there are no electrons to keep each atom apart. Usually, if you imagine an atom is a little ball, then if it were the size of a football on the centre spot of Wembley, the electrons would be whizzing round the edge of the stadium. Electrons usually keep each atomic nucleus apart. However, with the electrons gone, the neutrons are packed in together so tightly, that neutron stars are the densest things in the universe. A sand-grain sized piece of a neutron star would weigh more than a 747 jet.

I hope that explanation made sense. Supernovae and neutron stars are some of the most incredible things in the universe!!