A Chemical Census of the Milky Way’s Nuclear Star Cluster

The Milky Way’s Nuclear Star Cluster (NSC) is a densely packed group of stars at the very center of our galaxy. Understanding how this cluster formed and evolved is crucial to piecing together the history of the Milky Way itself. However, studying the NSC has always been difficult because its stars are hidden behind thick clouds of gas and dust. In this study, Govind Nandakumar and colleagues used high-resolution infrared spectroscopy to analyze the chemical composition of nine stars in the NSC. By examining 19 different elements, they uncovered new insights into how the NSC compares to other parts of the Milky Way, such as the inner bulge and the surrounding disk.

The Analysis of the Stellar Spectra

To study these stars, the researchers used the IGRINS spectrometer at the Gemini South telescope, which allows scientists to detect light in the infrared spectrum—wavelengths that can pass through the thick dust of the Galactic Center. The team measured the abundances of 19 elements, including common ones like iron (Fe), magnesium (Mg), and calcium (Ca), as well as rarer elements like fluorine (F) and yttrium (Yb). The researchers then compared the chemical signatures of NSC stars to those found in the inner-bulge and solar-neighborhood stars.

Results and Discussion

One of the most important findings of the study is that most elements in the NSC follow the same chemical patterns seen in the inner bulge. This suggests that the NSC and the inner bulge likely share a common evolutionary history, meaning they may have formed from similar gas clouds or undergone similar enrichment processes over time.

However, the study also identified one key difference: NSC stars contain significantly more sodium (Na) than expected. This is an unusual result because sodium is not typically enhanced in this way in other parts of the Milky Way. Similar sodium enhancements have been observed in certain globular clusters, which are tightly bound groups of old stars. This could mean that the NSC formed from multiple smaller clusters merging together or that a unique enrichment process occurred in this region.

The researchers also found that elements produced by slow nuclear reactions in stars, such as barium (Ba) and cerium (Ce), follow expected trends. This supports the idea that the NSC has undergone a steady process of chemical evolution similar to the rest of the inner Milky Way.

Conclusions

This study provides one of the most detailed chemical analyses of the NSC to date, significantly expanding our knowledge of its composition. The fact that most elements follow expected patterns suggests that the NSC shares a close connection with the Milky Way’s inner regions. However, the high sodium levels remain a mystery, hinting at an unusual process in this region’s formation history. Future studies with larger samples of NSC stars will help refine these findings and shed more light on the unique role the NSC plays in our galaxy’s evolution.

Source: Nandakumar