Saturday, March 08, 2014

GHGs to the Stars

Metals, in Astro speak, are any elements other than hydrogen and helium. Since they weren't synthesized in the big bang (except for traces of lithium) the cosmos started out without any of them. The metals (in the astronomical use of the word) out of which we and our planet are made were created inside of stars and later dispersed in the interstellar medium through stellar winds and supernova explosions.
The oldest stars in our current universe, some of them over twelve billion years old, have much smaller metallicities (metal contents) than our own star, the Sun, which wasn't formed until the Universe was already about 9.4 billion years old. None of those stars, however, have the zero metallicities expected from the first born stars, so it looks like none of those survived to the present. Since a star with a mass of 85% of that of the Sun (or less) would be expected to live longer than the present age of the Universe, that suggests that no such stars were made in that first generation.
The absence of metals makes some significant differences in the formation and behaviors of stars. To condense into a star, a cloud of hydrogen needs to dissipate a bunch of heat. Hydrogen and helium are relatively inefficient at that. Metals make a big difference in the opacity of stars - one might think of them as the green house gases of the stars - or, more precisely, the greenhouse plasmas of the stars.
Consequently, it is plausible that most of those first generation stars were extremely massive, perhaps anywhere from 100 to 10000 Solar Masses. Not only that, but their lesser opacity made it easier for radiation to leak out of them, so for a given mass and luminosity, these so-called Population III stars were smaller and hotter than a population I (modern) counterpart, so they would put out a bluer spectrum.
Such stars lived fast and died hard, and were millions to billions of times as bright as the Sun. Depending on their mass, they could end their short (a million years or less) lives in a variety of catastrophes, including gamma ray bursts that seem to be the most distant stellar phenomena known, 8 or more red shifts away, back when the universe was only a few hundred millions of years old.