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.
Keywords: Exoplanets: host stars; Main Sequence; Main Sequence lifetime; Hertzsprung
Russell diagram; Life: develop time
