Because starlight takes time to reach us, astronomers can observe the history of the cosmos by catching the light of distant galaxies. That's why observatories like the James Webb Space Telescope (JWST) are so useful. With it, we can study in detail how galaxies formed and evolved. We are now at a stage where observations allow us to confirm long-standing galaxy models, as a recent study shows.
This particular model concerns how galaxies become chemically enriched. The early Universe was mostly just hydrogen and helium, so the first stars were massive and planetless. They died quickly and ejected heavier elements from which more complex stars and planets could form. Each generation added more elements to the mix.
But since there are many stars in a galaxy, from blue supergiants to red dwarfs, which stars play the biggest role in chemical enrichment?
One model claims it's the most massive stars. This makes sense because giant stars explode as supernovae when they die. They eject their enriched outer layers deep into space, allowing the material to mix into large molecular clouds from which new stars can form. But about 20 years ago, another model argued that smaller, Sun-like stars play a larger role.
However, stars like the Sun do not die in powerful explosions. In billions of years, the Sun will become a red giant star. In its desperate attempt to keep burning, the core of a Sun-like star heats up significantly to melt helium, and its diffuse outer layers swell. On the Hertzsprung-Russell diagram they are known as asymptotic giant branch (AGB) stars.
Although each AGB star may eject less material into interstellar space, they are much more common than giant stars. The model therefore argues that AGB stars play a larger role in galaxy enrichment.
Both models have their strengths, but proving the AGB model over the giant star model will prove difficult. It is easy to observe supernovae in galaxies billions of light years away. Not so with AGB stars, however. Thanks to JWST, we can now check the AGB model.
With the help of JWST, the spectra of 3 young galaxies were examined in this study. Because the James Webb NIRSpec camera can capture high-resolution infrared spectra, the team can see not only the presence of certain elements, but also their relative abundance. They found a strong presence of carbon and oxygen bands, which is common in AGB remnants, but also the presence of rarer elements such as vanadium and zirconium. Overall, this points to a type of AGB star known as a thermally pulsating AGB or TP-AGB.
Many red giant stars enter a pulsating phase late in their lifetime. The hot core swells up in the outer layers, things cool down a bit and gravity compresses the star a bit, which heats up the core and the whole process starts all over again. This study shows that TP-AGBs are particularly effective at enriching galaxies, thus confirming the 20-year model. | BGNES
