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.