The infinite universe never ceases to amaze. The latest discovery was made using the European Space observatory’s Very Large Telescope in Chile, and it’s a special one: A cluster of stars around the giant galaxy Centaurus A. These clusters, however aren’t an average grouping of stars. They’re known as “dark” globular star clusters, and they give off less let but contain far more mass than a normal star cluster. They may also contain two of the most mysterious phenomena in space: Dark matter and black holes.
“Globular clusters and their constituent stars are keys to understanding the formation and evolution of galaxies. For decades, astronomers thought that the stars that made up a given globular cluster all shared the same ages and chemical compositions—but we now know that they are stranger and more complicated creatures,” said Matt Taylor, a PhD student at the Pontificia Universidad Catolica de Chile, Santiago, Chile.
As the closest giant galaxy neighbor to our own galaxy, the Milky Way, Centaurus A is of particular interest to astronomers. However, though the Milky Way only has 150 or so of these dark globular clusters at its outskirts, Centaurus A is believed to have as many as 2,000. So far, Taylor and his team have produced some of the most detailed studies ever of about 125 of the clusters (and counting.
In order to determine the size of the clusters, scientists typically measure the brightness: A brighter cluster has more stars in it, and more stars mean more mass. However, the dark globule clusters surrounding Centaurus A didn’t make sense – compared to their brightness, these clusters appeared to be much more massive. The discrepancy was a source of confusion for Taylor and his research colleagues.
The best possibility, they say, are black holes, which are themselves very massive and practically invisible to human instruments. However, the researchers note that the clusters are so massive that it’s impossible for black holes to be the sole culprit. Dark matter, about which scientists understand very little, is also a possibility, but historically speaking astronomers have found little if any of the mysterious substance in similar star clusters.
“Our discovery of star clusters with unexpectedly high masses for the amount of stars they contain hints that there might be multiple families of globular clusters, with differing formation histories. Apparently some star clusters look like, walk like, and smell like run-of-the-mill globulars, but there may quite literally be more to them than meets the eye,” said Co-author Thomas Puzia.
As of now, the reason for the unexpected mass remains unknown. Hopefully, a larger survey of similarly strange star clusters will reveal more information.