Abstract
In this chapter we will give an overview on the different types of host stars of planetary systems. The Hertzsprung Russel diagram showing the physical properties of stars and their relation is reviewed. Most stars can be found on the main sequence in this daigram. We concentrate on main sequence stars of spectral classes F, G, K, and M. Their main sequence lifetimes are larger than 1 Gyr. This time range was needed on Earth for life to develop and seems adequate for life to evolve on possible planetary systems around other stars1.
The main message given in this chapter is that possible host stars of earthlike planetary systems might be the majority of all stars since the must be low mass stars like our Sun. However, we do not know yet the percentage of habitable earthlike planets around them.
