Galaxies are thought to have formed from clouds of hydrogen and helium gas that lacked heavy elements, such as oxygen and iron. Later, some of the stars born from this gas created heavier elements deep in their cores. If these stars explode, they eject the heavy elements into the galaxy. When new stars form from these ejected materials, they acquire more of these heavy elements. In this way, a galaxy’s supply of heavy elements increases in time: old stars have fewer heavy elements, whereas younger stars — such as the Sun — have more.

But it turns out that oxygen and iron arise from two different types of exploding stars, or supernovae. Oxygen gets ejected into a galaxy from massive stars — those born with more than eight times the Sun’s mass. Such a high-mass star builds its fuel quickly and dies quickly, exploding as a so-called Type II supernova and forming a compact neutron star or a black hole.

Before it explodes, the high-mass star creates iron. But oxygen is lighter than iron and floats up outside the core of the star, whereas the iron remains at the star's centre. When the star explodes, the oxygen is set free, while the iron gets sucked into the newly formed neutron star or black hole. High-mass stars contribute a lot of oxygen to a galaxy, but little iron.