We saw in the previous chapters that cosmic fauna is incredibly diverse. But you don't need to be on first name terms with objects as exotic as black holes or pulsars to see this. Even the ‘normal’ star family has too many children to easily keep track of. Some way had to be found to classify this stellar family according to some sensible scheme. Every star is today identified by its colour (or spectral type) and by its luminosity (or absolute magnitude). The different spectral types have each been given a name, in fact a letter of the alphabet, and the sequence is now: OBAFGKM. The O stars are those whose surfaces are much hotter than any of the others. Some of them are well beyond 30000°C. Our Sun, at 5700°C, is in the G class. The M class consists of the coldest stars, with mean surface temperatures of 2600 °C. Whatever their peculiarities and differences, all stars have, however, something in common: the thermonuclear fusion reactions of hydrogen that take place in their cores and which make them members of the main sequence, the club of normal stars.

There are so many stars undergoing nuclear combustion that it seems almost as ordinary as walking the dog. But those on Earth who try to control fusion, which is more powerful and less polluting than the fission used in nuclear power plants, know that it's a very difficult process to tame.