Beyond Maturity

Stellar Evolution Beyond the Main Sequence

2. Heavyweight stars

What happens to a star heavier than five solar masses? A lot depends on how much heavier. Stars about ten solar masses probably behave like our five solar mass star until they reach the end of core-helium burning. Then, the growing carbon core will heat up to temperatures around one billion degrees, pairs of carbon atoms begin to combine, and a new series of nuclear reactions begins.

Carbon and oxygen burning leads on to neon and silicon burning. More and more quickly, the light elements are transformed until the core is made of iron. By now the inside of the star will be a series of shells inside one another, with hydrogen on the outside, helium next, carbon and oxygen, silicon and finally iron in the center.

Supernovae

The Crab nebula was formed after a massive explosion ripped a star apart in 1054. For a few days, the exploding star could be seen during the day. Today, the glowing outer layers of the star are still rushing away from a tiny star in the centre. This remnant is a neutron star, the Crab pulsar, first discovered vbecaus it emits regular pulses of radio waves.

So far, reactions between two or more atomic nuclei have taken place when the temperature is high enough. The star gets more heat out of the reaction than it puts into it, and this heat helps to support the star. Iron cannot do this, since it costs energy to make heavier atoms.

Temperatures in the core now exceed 3 billion degrees. Up to now, the pressure due to photons - particles of light - and electrons, which are very energetic, has been holding the star up. Now, these energetic particles are easily converted into neutrinos - a different type of particle with no mass. Neutrinos do not help to hold the star up. In fact they leave the star immediately, taking energy with them, working like the coolant in a very efficient refrigerator. This refrigeration causes the core of the star to collapse suddenly.

In the next one tenth of a second, a fantastic series of events takes place. The hydrogen and helium-rich outer layers of the star fall inwards. The iron in the core gets even hotter and breaks up into helium and then into protons and neutrons. At 40 billion degrees, the neutrinos are recaptured and the temperature rockets upwards. At 100 billion degrees and a density ten thousand million times that of water, the core becomes incompressible. Light elements slamming into this incredibly hard hot core react with the protons. The nuclear energy released has nowhere to go except outwards. An incredible explosion rips through the outer layers of the star, releasing colossal amounts of energy in a supernova explosion. That explosion will release as much energy as the Sun radiates through its main-sequence lifetime, or as much energy as 10 billion Suns radiate in one year.

Within that explosion, also known as a core-collapse or Type II supernova, such extremes of heat and density are experienced that hundreds of different nuclear reactions are possible. A supernova explosion is the only place in the Universe where many elements can be manufactured. Every atom of gold on earth was created billions of years ago in supernova explosions.

Neutron stars, pulsars and black holes.

The supernova will shine brilliantly for a few days or weeks as the hot gas blown out by the explosion cools down. The outer layers of the star will be visible years later as a supernova remnant like the Crab nebula.

Left behind will be a bizarre object weighing two to three solar masses, with a radius of just a few kilometers. Inside, atoms have been crushed so completely that the electrons have combined with protons to form neutrons - hence the name neutron star.

Nuclear forces between the neutrons prevent the star collapsing any further. Densities are similar to those inside atomic nuclei, but this nucleus is a billion trillion trillion trillion trillion times more massive than a neutron.

It is rotating up to a thousand times a second and it has a huge magnetic field. Above the surface, trapped electrons spiral around in the spinning magnetic field, emitting pulses of light and radio waves - hence their other name - pulsars.

Neutron stars are exotic places. Gravity is so strong that the escape velocity is about one third the velocity of light. It bends light over the horizon so that an observer on the surface would think he was standing at the bottom of a hollow rather than outside a sphere. Unfortunately, no human observer could ever survive on the surface of a neutron star!

Other exotic stars

Unlike our five solar mass star, very massive stars will not make it to become red giants. Helium burning can start while the star is a blue supergiant still close to the main sequence or evolving towards the giant branch. Very bright hypergiants like Eta Carinae are more than 50 times as massive than the Sun, and are so luminous that their outer layers are barely held on to the star by gravity. Radiation is rapidly blowing away the outer layers. Wolf-Rayet stars have blown off so much material that helium-rich layers beneath have been exposed.

 The fate of these massive stars is sealed. They will explode as supernovae. If, after the supernova, the mass of the surviving neutron star is greater than about three solar masses - the explosion will blow all the remaining outer layers into space - then even nuclear forces can't hold the star up. It will continue to collapse until it becomes a black hole.

• Table of Contents
• 1. After Hydrogen Burning
• 2. Heavyweight Stars
• 3. Lightweight Stars
• The end