This chapter will look at how stars evolve during the later stages of their lives, and describe the remnants left when they die: white dwarfs, neutron stars and black holes.

For a collapsing mass of gas to become a star, nuclear fusion has to initiate in its core. This requires a temperature of ~10 million K and this can only be reached when the contracting mass is greater than about 1029 kg, about 1/20 the mass of the Sun, or 20 times that of Jupiter.

In low-mass stars, less than ~0.5 solar masses, the conversion of hydrogen to helium by nuclear fusion is the same as in our Sun. However, whereas in stars of greater mass nuclear fusion only converts ~10% of the mass of the star (that residing in its core), in the lowest mass stars it is thought that convection currents mix the star’s interior and so will allow much of the star’s mass to undergo nuclear fusion, so increasing the time during which they can carry out the fusion of hydrogen to helium – a period that is significantly longer than the present age of the Universe. We thus have no direct observational evidence of what happens when fusion ceases in such stars and can only use computer modelling to investigate what might happen.