Chemical Evolution of the Universe Flashcards
why was the early universe the only period which overwhelmingly favored the formation of massive and supermassive stars?
- due to the lack of metals which radiate away energy efficiently, the gas in early universe had a high T of 100K
- today metals, recent molecular clouds are much cooler
- as a consequence gas clumps needed large masses to initiate collapse to form protostars (hotter gas moves quicker and therefore needed large masses to initiate collapse to form protostars)
how did the ccSN of first-generation stars enable the formation of low-mass stars?
first generation stars produced metals for the first time, and when they ccSN, they released these metals and were incorporated into new stars, which could be much lower mass as they metal content radiated heat better and cooled them down, meaning they needed less mass to collapse
why do heavy elements have much lower abundances compared to lighter elements?
lighter elements e.g. C, O are produced in stars of most masses
however the heavier elements are produced during the r-process which requires two massive stars to explode and form a binary system that merges (neutron star-neutron star merger) therefore heavy element production is obviously limited, as the process which forms them is limited
why were mergers possible in the first ~10Ma
stars were more massive due to higher temperatures, and massive stars die quickly compared to low mass stars. therefore they died quick and often and formed neutron stars after core collapse, which merge, as there are more neutron stars.
why did it take a while for lower-mass stars to contribute elements to the universe?
(s-process nuclei, Li, N, C).
were only released to universe when the lower mass stars finally died, as they can fuse elements for a much longer period of time compared to massive stars, which burn through their H supply very quickly.
first was from 8-10solar mass stars ~30Ma after first ccSN of massive stars
what does nucleosynthesis in Type Ia supernova require
- the full evolution of a lower mass star to produce a white dwarf
- additional mass transfer to the white dwarf until this becomes gravitationally unstable
- exploding white dwarfs thus took place ~1Ga after star formation started
why has planet formation become much more likely with time since universe formation?
universe metallicity has increased after multiple star cycles, which are able to form planets
when did star formation peak?
about 10-11Ga, when the universe was 3-4Ga old
which isotopic/elemental ratios did contributions from lower mass stars and SNIa increase? given the fact that up until this point, most element contributions to the universe were from massive star ccSN and neutron star mergers, which provided all elements but not all isotopes
the Fe/Mg ratio increased (s-process forms Fe)
and Pb/Pt increased (s-process forms Pb)
the r-process dominated the early universe, so when s-process contributions through time from AGB stars, increased, the amount of Fe and amount of Pb increased.
why is the metal contributions to the universe going to decrease over time? i.e. why will the universe die (when?)
elliptical galaxies, which have very little interstellar matter (gas/dust) and hence v low rates of star formation, will become much more common with time
eventually no new stars will form, and the only live nucleosynthesis sites will be the lowest mass stars which contribute very little metal to the universe.
however, older low mass stars will continue to shed their envelopes and white dwarfs can still explode for a while
predicted to happen 10^13 (10trillion) years after the big Bang
the chemical composition of the universe will essentially stop to evolve.
