Young and dense star clusters such as R136 in the Large Magellanic Cloud (LMC) and NGC 3603 in the Milky Way are known to contain the most massive stars known with masses that well exceed 100 times the Sun’s mass. These massive stars are thought to have formed through multiple stellar collisions in the dense environments found in such star clusters. An extremely high-mass star may also lead to the formation of an intermediate-mass black hole (IMBH) with 100 to 1000 times the Sun’s mass if the stellar collision rate is high enough to overcome the extraordinary mass-loss rate that such a massive star experiences. This process of multiple stellar collisions occurring in the heart of a dense star cluster is known as core-collapse.
To form extremely massive stars, the process of core-collapse in a dense star cluster has to take place early and quickly enough as the main-sequence lifetime of such stars is only a few million years. One way for this to happen is by the merging of a number of sub-clusters into one single star cluster. This is because the process of core-collapse takes place earlier and quicker in a sub-cluster than in a larger cluster. The merging of sub-clusters into one single star cluster creates an environment where the growth of extremely massive stars through multiple stellar collisions can take place much more efficiently. However, the merger of sub-clusters does not always lead to the efficient formation of extremely massive stars.
If the sub-clusters merge after each one has already experienced core-collapse (“late-assembling” case), multiple stellar collisions will result in the formation of a number of very massive stars instead of a single extremely massive star and the growth of very massive stars comes to a halt after the sub-clusters have merged. In this case, the most massive stars are expected to have around 200 to 400 times the Sun’s mass. Furthermore, very massive stars in each sub-cluster tend to form massive binaries. As the sub-clusters merge, many of these massive binaries in each sub-cluster will gravitationally interact with one another, causing some of the massive stars to collide and others to be ejected from the cluster.
The formation of extremely massive stars through multiple stellar collisions occurs most efficiently when the sub-clusters merge into a single cluster before core-collapse occurs (“early-assembling” case). In this case, the stellar collision rate is efficient enough to form a few or a single extremely massive star with around 1000 times the Sun’s mass. Such a massive star can collapse directly to form an IMBH.
An image from Hubble showing the star cluster R136 and its surroundings. (Credit: NASA and ESA)
R136 in the LMC is a dense cluster which contains some of the most massive stars known and it is more consistent with the “late-assembling” case. It consists of 5 very massive stars, each with over 100 times the Sun’s mass. The most massive member is R136a1 which is estimated to have 320 times the Sun’s mass at birth and has lost about 50 solar masses over the past million years or so. R136 has no evidence for any extremely massive star with 1000 times the Sun’s mass.
M. S. Fujii and S. Portegies Zwart (2012), “The Growth of Massive Stars via Stellar Collisions in Ensemble Star Clusters”, arXiv:1210.3732 [astro-ph.GA]