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Berkeley 20 & Berkeley 29 – Old Open Clusters and the Evolution of the Milky Way

April 30, 2012 Leave a comment

Above: Berkeley 20 Open Cluster in Orion. Imaged  with iTelescope T3  7/10/11  4:06:04 am New Mexico time (UTC -7) RA: 05h 32m 36.0s DEC: 00° 11′ 11″ (J2000) TAK TOA 150, SBIG ST2000 SMC 2x 600 sec Bin 1

Yes I know this isn’t one of the most exciting photographs visually, but that little knot of stars just to the right of centre is one of the top 20 in terms of the oldest star clusters in our galaxy (6 billion years old) and one of the most distant (15.8 kiloparsecs or 51,000 light years). Along with Berkeley 29, which currently holds the record for the most distant open cluster in our galaxy at 22.6 kpc or 73,000 light years, these veteran star communities are providing exciting new evidence for the formation of the outer disk of the Milky Way and perhaps even suggest that our galaxy may be a cannibal !

Current research using photometry and spectroscopy of the stellar makeup of these and a number of other distant clusters (Berkeley 31, NGC 2141, NGC 2808, NGC 5286) suggests that something odd is going on. While they typically have low iron metal content this content should decrease the further you get from the galactic centre, but recent papers have suggested that beyond 40,000 light years the iron content bottoms out and actually remains constant out to the edge of the galaxy at about 30% of the Sun’s iron content. On top of that the clusters have stars with a high Oxygen/Magnesium to Iron ratio which is a signature derived from short-lived massive stars that have gone supernova, so what is going on ? An intriguing possibility is that these distant open clusters may be aliens in our midst, derived by our galaxy either swallowing up the stars from another galaxy in a merger event, or stars accreting to the outer disk of the Milky Way in a collision event with a passing gas cloud that triggered new star formation.

Evidence is also gathering that these older clusters lie in a dispersed arc-like structure in the outer disk known as the Galactic Anti-center Stellar Structure (GASS), sometimes called the Monoceros Ring. It is thought that this structure  could be the remnant of a merger with a dwarf galaxy about 5 billion years ago, similar to the Canis Major dwarf spherical galaxy that was discovered to be merging with our galaxy back in 2003.

Above: Berkeley 29 Open Cluster in Gemini – the most distant open cluster in the Milky Way. Imaged  with iTelescope T3  7/10/11  5:01 am New Mexico time (UTC -7) RA: 06h 53m 04.0s DEC: 16° 55′ 39″ (J2000).  TAK TOA 150, SBIG ST2000 SMC 2x 600 sec Bin 1

With the advent of the 4.1 metre VISTA infrared telescope coming online at the European Southern Observatory last year we may soon discover older and more distant open clusters that will fill out this picture of early galactic evolution. Up to September 2011 VISTA had discovered 96 new clusters previously hidden by thick dust clouds and it is estimated that there may be as many as 30,000 open clusters still waiting to be found in our galaxy – exciting times ahead !!

Categories: Stars & Star Clusters