Death and rebirth among the stars The Sun is a middle-aged star. It was formed about 4.5 billion years ago, when the Universe was not much more than half its present age. In the Sun, the Earth and our own bodies, all atoms heavier than than the primeval hydrogen and helium were made in stars of the Milky Way that expired before the Sun came into existence. Grains of different origins, found in meteorites and distinguished by atomic fingerprinting, confirm that many individual stellar ancestors contributed to the Solar System's stock of elements. The ashes of the ancestral stars are too dispersed to be identifiable in the Galaxy today.
Astronomers can nevertheless find their analogues among more recent stars.
ISO gives them special access to the stages between stellar death and rebirth. An old star expiring scatters chemically enriched material into the interstellar medium, which concentrates again at the origin of new stars and planets. As described in previous Information Notes, materials identified by their infrared signatures include carbon monoxide and water in vapour or icy form, tarry carbon-rich compounds, and minerals including olivine, which is a major constituent of the Earth's rocky mantle. When the Sun itself grows old, it will swell and cool, and will eventually puff much its material into space. Its burnt-out core will collapse to make a white dwarf star.