Stars and Galaxies
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Stars and Galaxies
This chapter gives an overview of the cosmic environment of the Earth, the stars and the galaxy. It is important to know their evolution because this contributes to the cosmic circle of matter: stars are born out of interstellar clouds and at the end of their evolution, they enrich the interstellar medium with elements heavier than He. These elements have been produced by nuclear fusion in the stellar interior and are important for the origin of planetary systems and the origin of life itself. The knowledge of stellar evolution is essential for the timescale over which life evolves. If higher lifeforms, as we know from Earth, need several billion of years to evolve, then we must search for a star that does not change in luminosity over such a long time span in order to guarantee a continuous habitable zone.
3.1 Stars: Evolution and Formation 3.1.1 Physical Parameters of Stars In the case that the host star provides the energy source for life, the habitability on planets around that star strongly depends on stellar physical parameters (see also Sect. 9.2.3) like (i) stellar mass, (ii) stellar temperature, (iii) age of the star, and (iv) stellar activity and others. There are relations between these parameters. The activity of a star depends on its rotation and magnetic fields. The lifetime of a star depends strongly on its mass. A summary of stellar parameters and their importance for HZ around the stars is given in Table 3.1.
3.1.2 Spectral and Luminosity Classes The radiation of a star can be analyzed using a spectrograph. According to their spectra, stars can be classified in the following sequence: O-B-A-F-G-K-M. This A. Hanslmeier, Habitability and Cosmic Catastrophes, Advances in Astrobiology and Biogeophysics, DOI 10.1007/978-3-540-76945-3 3, c Springer-Verlag Berlin Heidelberg 2009
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Stars and Galaxies
Table 3.1 Stellar parameters and their influence on habitable zones Parameter Age Mass Temperature Composition Magnetic field Rotation
Influence Important for total lifetime of star Determines the lifetime; stars with high masses evolve too fast Coupled to the mass Nearly identical for all stars Important; determines stellar activity Important; determines stellar activity
is a sequence of temperature (see Table 3.3): O stars are the hottest, M stars the coolest; the number of absorption lines increase from O to M. Some characteristics are given in Table 3.2. The fact that there appear metal lines in cool stars does not imply that they consist of metals. It is only because of the lower temperature that such spectral lines can be formed; the chemical composition is roughly the same for all types of stars (mainly H and He). The luminosity of a star depends on (a) temperature ∼ T 4 and (b) surface which is ∼ R2 . Since, e.g., a K star may be a dwarf main sequence star or a giant, luminosity classes have been introduced. Class I contains the most luminous supergiants and class II the less luminous supergiants. Class III are the normal giants, class IV the sub giants, and class
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