- The third cluster of red supergiants -

After we discovered that the obscured young open cluster Westerlund 1 is much more massive than any other young cluster in the Milky Way (with a mass around 100,000 solar masses), many astronomers quickly concluded that our Galaxy contained many clusters much more massive than those normally seen in the Solar neighbourhood. These young, compact clusters, with masses higher than 10,000 solar masses are sometimes called massive clusters, or also starburst clusters.

 Soon afterwards, Don Figer and his collaborators found a cluster that contained a large number of red supergiants, and - with a strong appeal to imagination - called it the Red Supergiant Cluster. This cluster is hidden behind so much insterstellar obscuration that it can only be seen in the infrared. Actually only the red supergiants can be seen at all in infrared photometric catalogues and deep observations will be needed to reveal the rest of the population. In any case, based on the rarity of red supergiants, Ben Davies and co-workers have concluded that the cluster must have around 30,000 solar masses in unseen stars. Shortly afterwards, Ben found that the cluster Stephenson 2, which is also hidden behind huge amounts of obscuration, contains even more red supergiants and is even more massive than the original Red Supergiant Cluster, now re-christened as RSGC1, with Stephenson 2 becoming RSGC2 (as a matter of fact, Ben beat us to this discovery by a narrow margin; Simon had already found the cluster in GLIMPSE/Spitzer images and we had a proposal approved to observ it).

Now we proudly announce the discovery of a third cluster full of red supergiants, which boringly will have to be called RSGC3. This cluster is very close in the sky to the other two and we all think that the three of them are part of a huge region of star fomation that has been excited at the point where the Galactic Long Bar hits the inner spiral arm (the Scutum-Crux Arm).

Our discovery is reported in

J. S. Clark,  I. Negueruela, B. Davies, V.M. Larionov, B.W. Ritichie, D. F. Figer, M. Messineo, P. A. Crowther, A.Arkharov. "A third red supergiant rich cluster in the Scutum-Crux Arm ", 2009. Astronomy & Astrophysics, in press (also at arXiv:0903.1754)

Here is an image of the cluster core. It is a 3-colour composite I made combining UKIDDS JHK images and was later enhanced by Simon's sister-in-law, who is a graphical designer. The ugly blue water-drops are due to the saturation of the infrared detector by the red supergiants, which are so bright that they leave behind long-lived blemishes.


All these discoveries are prompting a real race between professional astronomers to find the most massive clusters in the Galaxy. Indeed we beat a rival group to this discovery by a mere two weeks. They have now also published a paper on the cluster. Fortunately, their conclusions are very similar to ours. We all think that this cluster may have around 30,000 solar masses. Another heavyweight!

We will be working hard to calculate the properties of these clusters and find new massive clusters in the near future.


Written and maintained by Ignacio N.. Last update on 18th March 2009.