General discussion

In the previous sections we have discussed stars which are generally considered to be normal stars, which means their spectra fit into the two-dimensional classification scheme according to spectral type and luminosity. True, the weak-lined stars, or population II stars, do not fit into that scheme, but generally their peculiarity can be understood by the change of just one parameter, the ratio of the metal abundance to the hydrogen abundance, though recently it has been found that this may not always be the case. More than one parameter may actually be necessary to describe the abundances of the heavy elements. The population II stars are still generally considered to be ‘normal’ stars because we believe that all their peculiarities can apparently be traced back to different chemical abundances. For the stars we are going to discuss in this chapter, this does not seem to be the case. There are, of course, a large number of different kinds of peculiar stars, but we are not able to discuss all of them in the framework of this introduction to stellar astrophysics. We shall only discuss the most frequent kinds of peculiar stars and those which are of special interest in the framework of understanding stellar structure and evolution.

Peculiar A stars, or magnetic stars

The observations

In the previous section we saw that some stars with very strong magnetic fields are found among the early A stars.