- Westerlund 1: the most massive young cluster in the Milky Way -


We (Simon Clark and myself, working in collaboration with other people, such as Paul Crowther and Simon Goodwin) have recently discovered that the obscured young open cluster Westerlund 1 is by far the most massive young cluster in the Milky Way. It is probably more massive than any other young cluster in the Local Group of galaxies. Moreover, it is extremely compact, and so looks like the starburst clusters (or super star clusters) generally observed in starburst galaxies. As such, it is the best laboratory for the study of the formation and evolution of massive stars, and also the place where we can test theories of star formation in starburst environments.

This cluster had been initially discovered by Bengt Westerlund in 1961, but it is so obscured by a cloud of interstellar dust that only the most modern instrumentation, such as that on the VLT has allowed its study.

A rather technical summary of our findings can be found in a poster presented at the Starbursts conference in Cambridge in September 2004.

ESO red

Over the last few years, we have collaborated with a number of people in order to study different aspects of this cluster. We try, in this way, to use this natural laboratory to answer different questions of astrophysical relevance.

Papers on Westerlund 1 by our group

False-colour image of the cluster from J,H & K  photometry, obtained with the New Technology Telescope, at the La Silla observatory. See Brandner et al. (2008, A&A 478, 137) for details. Comparing this image with the optical one above is an interesting exercise. In the infrared image, the four red supergiants are extremely bright and have orangish spikes. Another object with an orange glow is W9, a peculiar object surrounded by a cocoon of dust. Two Wolf-Rayet stars with dusty envelopes are distinctly red, though not as bright as the supergiants. The Yellow Hypergiants, which are the brightest stars in the optical image are less prominent here, though still very bright, and appear whitish here. The colour selection here is closer to the true intrinsic colours of the stars.
 This effect is due to extinction by interstellar dust. Dust absorbs blue light much more strongly than red  light. Because of this, the cluster is much brighter in the infrared. As there is a lot of dust between us and Westerlund 1, all the blue light is absorbed, and all stars appear red in the optical image. In the infrared, where extinction is lower, the colours we observe (technically, the spectral energy distribution) are closer to what the stars actually emit.


Distribution of massive stars in Westerlund 1, based on the data in Negueruela et al. (2010).

The moderately evolved OB supergiants are shown as rectangles. The more evolved stars are shown as circles: blue circles are blue hypergiants and LBVs, red circles are A/F hypergiants and M supergiants, while magenta circles are WR stars.
massive stars

Narrow-filter IR image of the cluster showing the enormous difference in brightness between the red and yellow hypergiants and the OB supergiants, which makes imaging with IR devices a very painful task: the red objects saturate very badly the detector and measurements of magnitudes are affected.

IR colour-magnitude diagram based on 2004 SOFI observations. Considering field star contamination and not taking into account the incompleteness due to a substantial number of OB stars being obliterated by the hypergiants in our images, there are no less than 400 stars with M>10 solar masses in Westerlund 1.


Written and maintained by Ignacio N.. Last update on 17th March 2009.