The amount of
oxygen in a galaxy is determined primarily by three factors: how much
oxygen comes from large stars that end their lives
violently in supernova explosions — a ubiquitous phenomenon in the early Universe, when the rate of stellar births was dramatically higher than the rate in the Universe today; how much of that
oxygen gets ejected from the galaxy by so - called «super winds,» which propel
oxygen and other interstellar
gases out of galaxies at hundreds of thousands of miles per hour; and how much pristine
gas enters the galaxy from the intergalactic medium, which doesn't contain much
oxygen.