Westerlund 1 is one of the two or three most difficult clusters to observe. Yet it is also one of the two or three star clusters so important that our knowledge would be incomplete without it. If all this sounds a bit disconnected, we can thank dust for obscuring more than just the view. Wd 1 is a unique natural laboratory for the study of extreme stellar physics. No other star cluster is quite like it. Its massive stars are a compendium of overweight exotics. Yet it is this unabashed weirdness that helps astronomers pin down how the most massive stars in a galaxy live and die. Wd 1’s total number of stars from 0.08 solar masses (msol) to 120 msol is between 20 000 to 45 000 msol. Add the smaller stars and the total swoops quickly past 100,000 stars – this is true globular cluster territory. All this makes Wd 1 about 10 times the stellar mass of the Orion Nebula cluster, and 2 to 4 times the mass of the very young super star cluster NGC 3603 (below). Wd 1 is an oddity in many regards. At roughly 5.5 million years old it has experienced only 1 supernova. In most clusters this massive, supernovae usually get underway at about 3 to 3.5 million years, and Wd 1 should have experienced around 40 to 60 of them by now. Why are there so many slowpokes? One reason could be the cluster’s high proportion of tightly packed binaries and multiple stars. Several other plausibilities exist, but so far none has proven to be the sole actor in this starry stage set. Many of Wd 1’s hundreds of very massive stars shine with a brilliance of almost one million suns. Several are twohundred to a thousand times larger than the Sun. Wd 1’s truly whopper star is a CMDs and stellar age red giant whose photosphere is the size of profiles used in this article from Brandner et al 2008. Jupiter’s orbit.
It is hard to imagine life in a star system like Wd 1. If the Sun were located at the heart of this glittery monster, the sky would bedazzle with hundreds of stars as bright as the full Moon. We would not be able to read a newspaper at night because the sky would be hotter than the ignition temperature of paper, and we would be sizzling cinders. The mass-segregation phase of Wd 1’s evolutionary cycle is well underway, meaning that the heaviest stars are converging into the middle while the least massive are easing outward toward the very edges of the cluster’s halo. The cluster has thus begun the stellar mass loss phase of its evolution, which can go on for billions of years. The end result will be a quiescent, nearly sempiternal globular star cluster whose stars are all 0.8 msol and less. If the above photo suggests that the cluster is elongated, that’s because it is. It has one Wd 1’s bimodal 2-stage star formation sequence. elongation with an eccentricity of The smooth curve is a Gaussian plot of the 0.2 for stars with masses between averages of all the ‘x’ stars at an assumed age of 5 million years. 10 and 32 msol and another of 0.15 for stars with masses in the range 3 to 10 msol. Barring external shocks like supernova shells passing through, the stars in isolated, ageing clusters naturally evolve toward more perfect circles as the clusters they lie within get older — the stellar version of settling into staid suburban life. The fact that Wd 1 has two distinct settlement patterns going on points to two distinct phases of progenitor cloud collapse out of a single huge cloud. Such a bimodal collapse structure is most logically explained by the cloud-collision genesis of massive star cluster formation. Here is an example of turbulence shocks when two clouds collide head-on: Side view. Face-on view.