When a galaxy welcomes new stars, they are usually formed in star clusters inside vast gas clouds. While some of these stars inside such clusters are small, cool and dim, others possess 10 times the mass of our Sun and a hundred thousand times higher brightness —but also a shorter lifespan as a result. These differences in initial mass have a significant influence on a galaxy’s luminosity. “The total mass of a dwarf galaxy is relatively low, so it won’t produce any extremely massive stars that’d be brighter than our Sun,” explains Professor Pavel Kroupa from the Helmholtz Institute for Radiation and Nuclear Physics at the University of Bonn, who is also a member of its Transdisciplinary Research Area (TRA) “Matter”. “By contrast, very massive elliptical galaxies, which formed almost 10 billion stars in just 10 million years during the early stage of the Universe, generate millions of these ultra-bright stars.”
The actual mass of each individual star within a cluster is a matter of chance. This assumption was tested by Kroupa and his then doctoral student Carsten Weidner in 2006. They discovered that the mass of the most massive star is determined by the mass of the star cluster.
Kroupa used the pair’s discovery to develop a concept called optimal sampling, which enables the distribution of stars in a young population to be calculated. “When stars are formed from a gas cloud, their masses aren’t decided at random but follow a precise order that leaves no room for statistical fluctuations,” he says. “This can only happen if the star-formation process is extremely self-regulating.”
Up until recently, however, physicists were at a loss to explain this self-regulation. Dr. Eda Gjergo from Nanjing University in China, first author of the study, has now used Shannon entropy (also known as information entropy) to come up with an approach according to which a star cluster develops in line with an especially efficient principle. As she explains: “Out of all the possible mass distribution scenarios, what actually plays out is the one that’s the most natural for large scales and the least dependent on microscopic details.”
Kroupa enthuses: “The work is opening up a new way of formulating theories about star populations. We now only need to have a number, namely the mass of the star population, in order to know what kinds of stars and how many of them will form from a gas cloud. This is enabling us to make highly efficient calculations about the evolution of galaxies, because we no longer have to perform thousands of calculations for a single one, saving much of supercomputing time and thus energy as well.” As a result, a whole host of calculations on the evolution of galaxies will need to be redone: “Contrary to what the previous theory suggested, small dwarf galaxies do not form any massive stars, and this has a fundamental impact on the theory of the matter cycle in the Universe,” Kroupa explains. Co-author Professor Zhiyu Zhang from Nanjing University adds “that these findings will lead to observation projects in order to study the non-random formation of stars in greater detail.”