Stars with a mass of between 8 and 20 solar masses have a more complex evolution. Initially, they evolve in the same way as low mass stars, turning into red giants and undergoing a core helium burning phase.

In medium mass stars, however, the burning of helium into carbon is no longer the end phase of stellar evolution. When the core helium supply is exhausted, the additional mass allows stellar collapse to take place and the outer layers to reignite. A cross section through the star at this point would show an outer shell of hydrogen burning, an inner shell of helium burning and the core, where there is now sufficient energy for the carbon to fuse with helium into oxygen.

Once the carbon supply is exhausted so oxygen fuses into neon, the helium shell becomes a carbon burning shell, the hydrogen shell a helium shell and a new outer layer of hydrogen burning forms. Neon can then fuse into magnesium, into silicon, and so on to chromium into iron. Each of these stages produces less energy than the previous stage and lasts for less time. During these final stages the star expands to thousands of times the diameter of the Sun, becoming a red supergiant like Betelgeuse.

The star finally hits a problem. To fuse iron into heavier elements requires an input of energy. Separating into the lighter elements again requires an input of energy. So as iron burning stars the core cools down – it draws in heat from its surroundings to power the fusion. Suddenly the outward radiative pressure, which has supported the star for many millions of years, ceases and the star undergoes a free fall gravitational collapse.

The core, which represents a large percentage of the stellar mass, now exceeds the 1.44 Chandrasekhar mass limit for a white dwarf. The protons and electrons in the core are compressed into a ball of neutrons, the size of a large city and the density of an atomic nucleus, held up by neutron degeneracy pressure. Such an object is a neutron star.

As a result of the core collapse a shock wave forms and blasts out through the star releasing an enormous amount of energy in a few seconds. All the outer layers of the star become superheated plasmas, with temperatures high enough to fuse iron and heavier elements, like gold and uranium. These outer layers brighten rapidly and are ejected into the interstellar medium at speeds approaching the speed of light. Such events are witnessed as Type II Supernovae.

Pulsars

A pulsar is a neutron star that produces pulsed emissions across the entire electromagnetic spectrum. Neutron stars spin rapidly and have large magnetic fields. Radiation channelled down the field lines is beamed out across the Universe like the light from a lighthouse. If the Earth happens to be in the line of sight then the emission is seen. Many pulsars spin rapidly with pulse durations of the order of fractions of a second.

	Solar Mass Stars
	High Mass Stars