Observation of
stellar spectra has revealed that stars older than the Sun have fewer heavy elements compared with the Sun. This immediately suggests that metallicity has evolved through the generations of stars by the process of
stellar nucleosynthesis.
Formation of the first stars Under current cosmological models, all matter created in the
Big Bang was mostly
hydrogen (75%) and
helium (25%), with only a very tiny fraction consisting of other light elements such as
lithium and
beryllium. When the universe had cooled sufficiently, the first stars were born as population III stars, without any contaminating heavier metals. This is postulated to have affected their structure so that their stellar masses became hundreds of times more than that of the Sun. In turn, these massive stars also evolved very quickly, and their
nucleosynthetic processes created the first 26 elements (up to
iron in the
periodic table). Many theoretical stellar models show that most high-mass population III stars rapidly exhausted their fuel and likely exploded in extremely energetic
pair-instability supernovae. Those explosions would have thoroughly dispersed their material, ejecting metals into the interstellar medium (ISM), to be incorporated into the later generations of stars. Their destruction suggests that no galactic high-mass population III stars should be observable. However, some population III stars might be seen in high-
redshift galaxies whose light originated during the earlier history of the universe. Scientists have found evidence of
an extremely small ultra metal-poor star, slightly smaller than the Sun, found in a binary system of the spiral arms in the
Milky Way. The discovery opens up the possibility of observing even older stars. Stars too massive to produce a pair-instability supernova would have likely collapsed into
black holes through a process known as
photodisintegration. Here some matter may have escaped during this process in the form of
relativistic jets, and this also could have distributed the first metals into the universe.
Formation of the observed stars The oldest stars observed thus far, known as population II, have very low metallicities; as subsequent generations of stars were born, they became more metal-enriched, as the
gaseous clouds from which they formed received the metal-rich
dust manufactured by previous generations of stars from population III. As those population II stars died, they returned metal-enriched material to the
interstellar medium via
planetary nebulae and supernovae, enriching further the nebulae, out of which the newer stars formed. These youngest stars, including the
Sun, therefore have the highest metal content, and are known as population I stars. ==Chemical classification by Walter Baade==