Small galaxies have lost most of the oxygen their stars made, Hubble-based study finds

Small galaxies near our own have held on to only a sliver of the oxygen their stars have made over billions of years, astronomers reported Monday in The Astrophysical Journal.  The study of 37 nearby dwarf galaxies found they kept between 4% and 25% of it, about 11% on average.

Dwarf galaxies hold anywhere from a few thousand to hundreds of millions of stars, compared with hundreds of billions in the Milky Way.  All 37 sit within about 20 million light-years, close enough for the Hubble Space Telescope to pick out individual stars.  A light-year is about 5.9 trillion miles, the distance light covers in a year.

Why oxygen, and where did it go?

Oxygen is made inside stars and is the most common element after hydrogen and helium.  It is easy to measure, so astronomers use it to track all the heavier elements stars produce.  Knowing how many stars a galaxy has formed tells researchers roughly how much oxygen it should contain.  Most of it was missing.

The likely culprit is supernovas, exploding massive stars whose blasts drive winds of gas out of a galaxy.  "We know that the oxygen had to be inside of the galaxies at some point in the past, so the oxygen must have been ejected," said co-investigator Grace Telford of the University of Utah.  Those winds appear to strip small galaxies far more effectively than computer simulations predict.  Some oxygen has been detected in the thin gas halo around dwarf galaxies, but the team says it is still unclear whether most of it is there or has escaped into intergalactic space entirely.  Galaxies in crowded neighborhoods, or near a big galaxy, tended to keep more.

The galaxy in the picture

The image shows NGC 4449, one of the study's galaxies, about 12.5 million light-years away in the constellation Canes Venatici, the Hunting Dogs.  Its light left when our ancestors were still millions of years from walking upright.  Hubble's Advanced Camera for Surveys photographed it in blue, visible, infrared and hydrogen-alpha light, the red glow of hydrogen gas.  Blue-white clumps are clusters of hot, massive stars; pink-red patches are clouds where new stars are forming now; dark streaks are dust.

How they did it

The project, called GLOW for Galaxies Losing Oxygen via Winds, used archived observations rather than new ones:  Hubble images to reconstruct each galaxy's star-forming history, plus data from NASA's retired Spitzer infrared telescope and two radio arrays, the Very Large Array in New Mexico and the Australia Telescope Compact Array, to weigh their gas.  Hubble, a joint NASA and European Space Agency telescope, has orbited Earth since 1990.

This is one peer-reviewed study, the second paper from the GLOW team, and has not been independently repeated.  A 2014 study found Milky Way-size galaxies also lost much of their oxygen.  Principal investigator Kristen Wingfield McQuinn of the Space Telescope Science Institute and Rutgers University plans to use NASA's new Roman Space Telescope to find even smaller galaxies to test.

Source:  McQuinn and others, "GLOW. II. A Census of Oxygen in Low-mass Galaxies," The Astrophysical Journal 1011, published Nov. 2, 2026 (doi:10.3847/1538-4357/ae93a6, CC BY 4.0); NASA Science, "NASA's Hubble Uncovers Case of Missing Oxygen in Nearby Dwarf Galaxies" (Nov. 2, 2026); ESA/Hubble release heic0711, "Stellar fireworks are ablaze in galaxy NGC 4449" (2007).  Drafted by AI for San Jose using NASA source material.  NASA did not write or review this story.