In his award-winning PhD thesis, Kruijssen studied the "Formation and evolution of star clusters and their host galaxies" using star clusters as fossils that tell the story of the formation histories of their host galaxies. The thesis consists of two parts: the formation of stars and stellar clusters and their evolution, and the modelling of isolated and colliding galaxies including their star clusters.

Previously, astronomers thought that all stars are born in clusters with about 90% of them dissolving immediately so that presently only a small fraction of all stars reside in clusters. Detailed computer simulations, however, showed that the processes leading to such dissolution do not work as expected. This means that only a small fraction of all stars is actually born in gravitationally bound star clusters. In the long run, the stellar clusters dissolve due to the tidal forces of their host galaxy as well as the gravitational perturbations caused by passing gas clouds. Kruijssen derived a new physical model, which is computationally more efficient than previous large computer simulations but can still predict accurately which stars escape from a dissolving star cluster. In this way, the properties of a cluster (such as its colour and brightness) can be easily predicted for any phase of its evolution.

Large computer simulations of isolated and colliding galaxies could then capture the influence of the galaxies on the formation and dissolution of star clusters in its entirety by accurately calculating the tidal effects on each cluster. The results showed that the pace of cluster dissolution varies substantially in space and time - sometimes even giving the false impression that the galactic environment plays no role at all. Surprisingly, collisions between galaxies led to a decrease of the number of star clusters in the simulations, whereas observations of colliding galaxies show that many clusters are being born in such environments. However, the destruction of clusters turns out to dominate because of the quickly changing gravitational forces, which cause the newly formed clusters to be rapidly destroyed.

The thesis thus showed that the formation and evolution of star clusters are not as simple as previously thought. In addition, it provides a physical framework to interpret the properties of star cluster populations and in particular the old globular clusters that were formed in young galaxies shortly after the Big Bang.

The Christiaan Huygens Prize is awarded each year by the Dutch Royal Academy of Sciences to a researcher who has made an innovative contribution to science with his or her PhD thesis. In addition to the certificate, the awardee receives a monetary sum of €10,000 and a bronze statue of Huygens. Each year, the prize is awarded in one of the five disciplines in which Christiaan Huygens (1629-1695) was active: actuarial sciences and econometrics, theoretical and applied physics, astronomy and space sciences, information and communication technology, and economical sciences.

For more information:

Christiaan Huygens Prize

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