* Astronomy

The largest known swarm of red supergiant stars has been found near the central bulge of our galaxy. It offers a rare glimpse of massive stars on the verge of exploding.
Red supergiants are among the largest stars in the universe and in fact are second in size only to rare 'hypergiant' stars such Eta Carinae. Spanning several hundred times the diameter of the Sun, each could fit millions of Sun-like stars inside it.
These stellar titans are extremely rare. Only very massive stars, more than 10 times as heavy as the Sun, turn into red supergiants. And the red supergiant phase lasts just 100,000 years before ending in a supernova, a fleeting moment compared to the overall lifespan of the star.
Only about 200 red supergiants have been identified in our galaxy. In 2006, a team led by Don Figer of the Rochester Institute of Technology (RIT) in New York, NY, US, reported finding a massive cluster of thousands of stars that included 14 red supergiants the biggest collection of these rare stars then known.
Now, Ben Davies, also of RIT, and a team that includes Figer have identified an even larger group of 26 red supergiants.
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Title: A massive cluster of Red Supergiants at the base of the Scutum-Crux arm
Authors: Ben Davies (RIT), Don F. Figer (RIT), Rolf-Peter Kudritzki (IfA, Hawaii), John MacKenty (STScI), Francisco Najarro (CSIC, Madrid), Artemio Herrero (IAC, Spain)

We report on the unprecedented Red Supergiant (RSG) population of a massive young cluster, located at the base of the Scutum-Crux Galactic arm. We identify candidate cluster RSGs based on {\it 2MASS} photometry and medium resolution spectroscopy. With follow-up high-resolution spectroscopy, we use CO-bandhead equivalent width and high-precision radial velocity measurements to identify a core grouping of 26 physically-associated RSGs -- the largest such cluster known to-date. Using the stars' velocity dispersion, and their inferred luminosities in conjunction with evolutionary models, we argue that the cluster has an initial mass of ~40,000 solar masses, and is therefore among the most massive in the galaxy. Further, the cluster is only a few hundred parsecs away from the cluster of 14 RSGs recently reported by Figer et al (2006). These two RSG clusters represent 20% of all known RSGs in the Galaxy, and now offer the unique opportunity to study the pre-supernova evolution of massive stars, and the Blue- to Red-Supergiant ratio at uniform metallicity. We use GLIMPSE, MIPSGAL and MAGPIS survey data to identify several objects in the field of the larger cluster which seem to be indicative of recent region-wide starburst activity at the point where the Scutum-Crux arm intercepts the Galactic bulge. Future abundance studies of these clusters will therefore permit the study of the chemical evolution and metallicity gradient of the Galaxy in the region where the disk meets the bulge.

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