Free Astronomy Magazine July-August 2015

27 SPACE CHRONICLES 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.” These objects remain a mystery. The team is also engaged in a wider sur- vey of other globular clusters in oth- er galaxies and there are some intriguing hints that such dark clus- ters may also be found elsewhere. Matt Taylor sums up the situation: “We have stumbled on a new and mysterious class of star cluster! This shows that we still have much to learn about all aspects of globu- lar cluster formation. It’s an impor- tant result and we now need to find further examples of dark clusters around other galaxies.” story. What about dark matter? Globular clusters are normally con- sidered to be almost devoid of this mysterious substance, but perhaps, for some unknown reason, some clusters have retained significant dark matter clumps in their cores. This would explain the observations but does not fit into conventional theory. Co-author Thomas Puzia adds: “Our discovery of star clusters to deduce the mass of the clusters and compare this result with how brightly each of the clusters shines. For most of the clusters in the new survey, the brighter ones had more mass in the way that was expected — if a cluster contains more stars it has greater total brightness and more total mass. But for some of the globulars something strange showed up: they were many times more massive than they looked. And even more strangely, the more massive these unusual clusters were, the greater the fraction of their ma- terial was dark. Something in these clusters was dark, hidden and mas- sive. But what? There were several possibilities. Per- haps the “dark clusters” contain black holes, or other dark stellar remnants in their cores? This may be a factor that explains some of the hidden mass, but the team con- cludes that it cannot be the whole n O bservations with ESO’s Very Large Telescope in Chile have discovered a new class of “dark” globular star clusters around this galaxy. These are marked in red. Normal globulars are marked in blue and globulars showing similar properties to dwarf galaxies are in green. The dark globulars appear very simi- lar to other globulars around this galaxy but contain much more mass. [ESO/Digitized Sky Survey]