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Old 07-01-2010, 09:51 AM
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ngcles
The Observologist

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Join Date: Dec 2006
Location: Billimari, NSW Central West
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Eeeeexcellent write up

Hi Paddy,

Thanks for taking the time to post you observations -- well done an excellent report.

Re NGC 419, this is an O.C not a G.C -- not your fault at all because most maps get this wrong as well as most cattledogs. The SMC only has one genuine true G.C -- NGC 121. All the others marked on many (most) maps and planetarium software as G.C's are in error. All these other clusters marked as G.C's are simply extremely large, middle-aged, exceptionally rich open clusters. NGC 121 is the only one certainly older than 10gyr in the SMC. Many were designated as G.C's in the old catalogues based on appearance only and this has carried over to a lot of the modern catalogues despite the object's spectra proving it is an O.C.

Both the LMC and SMC have many of these "super-clusters" -- ie clusters of >10,000 stars that look like G.C's but their spectra shows them to be relatively metal rich, middle aged (2-6 gyr) and not to have a "horizontal branch". They came about as a result of star-burst episodes in the relatively recent past. The LMC has about 6-7 genuine G.Cs but I can't remember off-hand which are which at the moment. The LMC is at present building a globular-sized cluster in the centre of the Tarantula Nebula (NGC 2070), but it is decidedly "open" in population.

To be a G.C the object must be very old (>10gyr) and exceptionally metal-poor (ie they are all Population II stars). It's not just "how they look in the ep". They also need RR Lyrae stars which indicates a proper horizontal-branch in their H.R diagram not merely a "red-clump".

But great report -- very much enjoyed mate .


Best,

Les D

Last edited by ngcles; 07-01-2010 at 10:15 PM.
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