A tedious lesson

Amed with these suggestive prolegomena, we may now turn to the key problem of chemical (isotopic) evolution in galaxies, starting with our own. 

The second group, with masses between one and eight times the solar mass, experience an agitated evolution, tacking back and forth across the HR diagram, so that it is difficult to keep track of them. At the end of their evolution, they move into a planetary nebula phase, eventually leaving a white dwarf for posterity. 

However, the work awards go, without possibility of dissension, to stars in the third group. With masses between 8 and 100 times the solar mass, these are the stars that will one day explode. Their self-sacrifice earns them special attention and this is indeed what we have been attempting throughout, with all due respect. 

By good fortune, the effects of metallicity on stellar lifetimes are rather limited, being only of the order of a few percent. In a first analysis, it is enough to apply the mass-lifetime relationship calculated for stars with solar metallicity. 

To sum up, high-mass stars are the main suppliers of complex isotopes. All elements from carbon to calcium are synthesised inside them by the relatively gentle and slow process of hydrostatic combustion, whilst iron and its kin. 

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