NASA's Hubble Space Telescope has studied 37 nearby dwarf galaxies and found that much of the oxygen expected to be present is missing. The observation raises questions about the elemental balance in these systems, which are far smaller and simpler than galaxies like the Milky Way and whose chemical evolution should, in principle, be easier to trace.

Dwarf galaxies are privileged laboratories for studying how elements forged in stars are formed and distributed. Oxygen, produced in the cores of massive stars and released into the interstellar medium when they explode as supernovae, is one of the most widely used tracers of that evolution. The fact that a significant fraction of the expected oxygen is not detected suggests that some of the element may have been expelled beyond the galaxy, may reside in a state that is hard to observe, or that the models linking star formation to oxygen production need revisiting.

The observations were carried out with Hubble, whose ability to analyse the ultraviolet and visible light of nearby galaxies allows the chemical composition of their gas to be measured in detail. Studying 37 objects also provides a sample large enough to test whether the discrepancy is an isolated case or a common pattern among dwarf galaxies in the local universe.

If confirmed, the result would affect how we reconstruct the chemical history of small galaxies, which are also the most abundant in the cosmos and which feed star formation in larger systems. It remains to be determined where the missing oxygen is and whether the cause is gas loss through stellar winds, energy injection from supernovae, or a limitation of the observations themselves.