The Hubble Space Telescope, a joint project of NASA and the European Space Agency, has shown a new image of the star cluster Messier 30 (M30). It is located in the constellation Capricornus and is named after the French astronomer Charles Messier, who catalogued nebulous objects easily confused with comets in old telescopes.
M30 is a globular cluster: a dense, almost spherical cloud of tens of thousands, and sometimes millions, of stars. Such clusters usually orbit the outskirts of galaxies in a vast, sparse halo that has been compared to a swarm of moths around a light bulb. At least 150 globular clusters are known in the Milky Way.
The age of these objects is impressive: astronomers estimate it at 11.5 to 13.5 billion years. Some clusters are almost as old as the Universe. It is their antiquity that makes them valuable witnesses of the past: clusters that formed inside our Galaxy work like time capsules, preserving the original chemical composition of the young Milky Way.
However, not all globular clusters in orbit around our Galaxy could have been born in it. Researchers have found that M30 and many other similar objects move in the direction opposite to the rotation of the Milky Way. This may mean that they did not form here but were captured by our Galaxy in the distant past.
Hubble has observed M30 several times. One project used the Advanced Camera for Surveys (ACS): it studied dozens of globular clusters of the Milky Way in red and near-infrared light and distinguished stars with a mass of only 20 percent of the Sun's. Later, another program used the Wide Field Camera 3 with three filters that capture bluer light. These data helped study exotic objects, such as blue stragglers - stars that look younger and hotter than they should be for the age of the cluster - as well as multiple stellar populations within a single cluster.
Very recently, ACS looked again at many of these objects. The goal was to trace how different groups of stars in the clusters moved over the 15 years since the first observations. Such a comparison became possible thanks to Hubble's long operation and its ability to distinguish the smallest details in distant objects.
The new image is assembled from data from all three observing programs and includes a wide range of wavelengths available to the telescope: ultraviolet, visible and near-infrared light. Each of them helps to better understand how these stellar communities formed and evolved.