The James-Webb Space Telescope has found traces of ancient stars in the clouds of gas surrounding the black hole at the center of our galaxy.
At the heart of the Milky Way lies Sagittarius A*, a black hole about four million times more massive than the Sun. It attracts matter that passes near it, but swallows far less gas than the highly active black holes observed at the centers of other galaxies. Researchers are trying to understand why.
Pixabay illustration
To investigate, a team from McGill University in Canada studied the clouds of gas circulating in its vicinity. Using James-Webb, they observed their infrared light. This makes it possible to identify different chemical elements, even in this region of the sky obscured by large amounts of dust.
The results show that these clouds are particularly rich in heavy elements. Astronomers call them “metals,” but the word has a broader meaning here than in everyday life: it refers to all elements heavier than hydrogen and helium, including oxygen and carbon.
Where do these elements come from? A significant portion of them is produced inside stars. When some massive stars explode at the end of their lives, they scatter this material into space. Researchers believe that such explosions enriched the gases near Sagittarius A* over time.
The observations also reveal that the composition of these clouds resembles that of several stars orbiting the black hole. The gas and these stars could therefore share a common history. This remains an interpretation: James-Webb did not directly observe explosions that would have produced this material.
Why study the composition of the gas? Because it influences how the gas cools. Gas that loses heat can more easily contract and migrate toward the black hole. Understanding its properties could therefore help explain why Sagittarius A* feeds so slowly.
The study also made it possible to distinguish several clouds and better understand their movements. Some appear to be subject to particularly turbulent conditions, perhaps under the influence of winds from nearby stars or shocks in the gas. Researchers cannot yet precisely reconstruct the path of all this material.
The next step will be to compare these observations with simulations to track the evolution of the gas over long periods. The center of our galaxy thus provides an observable environment for understanding phenomena that also occur around much more distant black holes.