This dense cluster is not composed of individual stars, but of entire galaxies. The Coma Cluster lies within the constellation Coma Berenices, which is the only one of the 88 IAU-recognized constellations named after a historical figure. Specifically, it honors Queen Berenice II's hair, with "coma" meaning "hair of the head" in Latin.
Berenice's story is steeped in myth: she offered her hair to the gods when her husband safely returned from war. Although her hair was placed in a temple, it soon disappeared. The court astronomer, Conon of Samos, claimed to have found Berenice's lost hair in the night sky, suggesting that the goddess Aphrodites had transformed it into a constellation. This myth dates back to around 245 BCE, giving the queen's locks celestial recognition for over two millennia.
The detailed image of the Coma Cluster was constructed using data from the Dark Energy Camera (DECam), built by the U.S. Department of Energy and mounted on the Victor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory, a program of NSF NOIRLab. DECam's 570-megapixel camera was developed to conduct the Dark Energy Survey (DES), which ran over 758 nights between 2013 and 2019. DES aimed to deepen our understanding of dark energy, the mysterious force accelerating the expansion of the Universe.
The Coma Cluster is also deeply linked to dark matter, a concept that emerged nearly a century ago when Zwicky observed several galaxies within the cluster. He estimated the cluster's mass based on its observable components but noticed a strange discrepancy: the galaxies behaved as though the cluster contained 400 times more mass than his calculations suggested.
Zwicky reached this conclusion by analyzing the galaxies' velocities. In gravitational terms, a more massive object exerts a stronger pull, meaning less massive objects nearby should be drawn toward it. However, if an object moves quickly enough, it can escape this gravitational pull. Zwicky inferred that the galaxies were moving too fast to be held by the observable mass alone, suggesting the presence of an immense amount of unobservable "dark" matter holding the cluster together - a notion that initially seemed improbable to many astronomers.
It wasn't until the 1980s that the astronomical community largely accepted the existence of dark matter, with further studies revealing similar mass discrepancies at the galactic level. In 1970, U.S. astronomers Kent Ford and Vera C. Rubin found evidence of invisible matter in the Andromeda Galaxy. Later, in 1979, Sandra Faber and John Gallagher's extensive study of the mass-to-light ratio in over 50 galaxies further strengthened the case for dark matter, concluding that "the case for invisible mass in the Universe is very strong and getting stronger."
Today, dark matter and dark energy are widely recognized, with understanding their mysterious nature being a key focus of modern astrophysics. The upcoming 10-year Legacy Survey of Space and Time, to be conducted by the NSF-DOE Vera C. Rubin Observatory, promises to shed more light on these elusive cosmic forces, continuing the legacy of the astronomer for whom the observatory is named.
