The Milky Way is one of the most studied galaxies in the universe, but until now it was not clear how unique it is. Recent research by astronomers has shown that the Milky Way is indeed unusual in its chemical composition. These results may help to understand how galaxies form in the universe and how they evolve over time.
Chemical composition of the Milky Way
Hydrogen and helium are the most abundant elements in the universe. They appeared shortly after the Big Bang, which occurred about 13.8 billion years ago. However, all the other elements, which astronomers call “metals,” are created in stars and stellar processes.
The metallicity of galaxies tells us a lot about how they formed and evolved over time. A study published in Nature Astronomy found that the chemical composition of the Milky Way is not unique, but it is definitely unusual.
The metallicity of the Milky Way increases from the center of the galaxy to about 23,000 light-years away, then stabilizes at a solar-like metallicity. The sun is about 26,000 light-years from the center of the galaxy. However, the metallicity then drops again, dropping to about one-third of the solar one at a distance of 50,000 light-years at the edge of the Milky Way.
Unusualness of the Milky Way
Compared to other galaxies, the Milky Way is unusual in its chemical composition. Only one percent of surveyed galaxies and only 11 percent of modeled galaxies are very similar to ours. The discrepancy between the two numbers is due to uncertainties in the survey and the limitations of realistic modeling of the universe.
One possible reason for the Milky Way’s singularity could be due to the activity of a supermassive black hole at its core. This could cause more metal-rich gas to be pushed outward, stopping the formation of younger, metal-rich stars. In addition, a collision of the Milky Way with a small galaxy that lacks metals could bring in more intact material, creating a larger population of low-metal stars.
The significance of the study
This is the first time astronomers can meaningfully compare the detailed chemical composition of the Milky Way with measurements of many other galaxies. These results are important for the next generation of comprehensive studies of galaxy formation. Data from future large-scale observing programs targeting the Milky Way or distant galaxies will inform these studies. The study also shows how to intelligently combine the two types of data sets.