The early universe had a problem. The first generations of stars, composed almost entirely of hydrogen and helium, produced heavier elements through nuclear fusion. When those stars exploded as supernovae, they seeded the surrounding gas with metals, carbon, oxygen, silicon, that would eventually coalesce into planets and new stars. But between the explosion and the next generation of star formation, something crucial had to happen. The metals had to clump together into dust grains.
Dust is not optional in galaxy evolution. It cools gas clouds, enables molecule formation, and provides the raw material for planetary systems. Without it, star formation is inefficient and planets cannot form. How the first cosmic dust factories worked, however, has been difficult to study because the galaxies that produced it are too far away and too faint for detailed observation.
JWST found a workaround by looking at a galaxy much closer to home.
A proxy for the early universe
Sextans A is a dwarf galaxy 4.6 million light-years from Earth, located at the edge of the Local Group. Its chemical composition is unusual for a nearby galaxy: it contains only 1 to 7 percent of the heavy elements found in the Sun. This makes it an excellent analog for the metal-poor galaxies that populated the early universe, where heavy elements were scarce.
A team led by Claudio Gavetti at Italy’s National Institute for Astrophysics (INAF) used JWST’s NIRCam and MIRI instruments to map Sextans A’s full stellar population. They were looking for stars in the asymptotic red giant branch (AGB) phase, a late stage of stellar evolution in which stars larger than the Sun exhaust the helium in their cores and begin to puff outward, growing up to a thousand times brighter. During this phase, AGB stars are known to produce and expel significant amounts of dust.
The survey found that roughly 90 percent of the AGB stars in Sextans A had no detectable dust envelopes. But about 20 stars were embedded in thick shells of dust, formed 2 to 3 billion years ago from stars with initial masses of about 1.5 times the Sun’s mass.
Identifying the real dust producers
The finding narrows down which stars in the early universe were responsible for enriching the cosmos. The conventional view held that AGB stars were the primary dust factories, but the JWST data shows that only a specific subset, intermediate-mass stars in a narrow mass range, produces the thick dust shells that can seed subsequent star formation.
Flavia Dell’Agli, also of INAF, said the data allowed the team to compare observations directly with theoretical models of stellar evolution. “The value of these data lies not only in the images, but in the ability to compare them with theoretical models and verify how correctly they describe the evolution of stars,” she said.
The study, published in the Astrophysical Journal, demonstrates a method for studying the early universe without needing to look at distant galaxies directly. By finding the right local analog, JWST can resolve individual stars and their dust output, a level of detail that remains impossible for galaxies billions of light-years away.