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Examples of Open Clusters. The following Open Clusters are all young (less than a couple of hundred million years old) star clusters. The luminosity is dominated by very hot Main Sequence stars (B or A).
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Examples of Open Clusters • The following Open Clusters are all young (less than a couple of hundred million years old) star clusters. • The luminosity is dominated by very hot Main Sequence stars (B or A). • There are many more cool Main Sequence stars that are not visible because their luminosity is so low. • On occasion, a post-Main Sequence red giant star may be visible
Stellar Content of Open Clusters The following three slides illustrate the typical stellar content of Open Clusters at three different ages. Cluster Age 1 Myrs Cluster Age 10 Myrs Cluster Age 100 Myrs
O B A F G K M
O B A F G K M
O B A F K M G
What stars do we “see” in Open Clusters? • The brightness of a star follows an inverse square law with distance (like the strength of Gravity). • A Main Sequence A5 star has an absolute Magnitude of +2.0. Thus at 10 parsecs (33 lyrs) it would appear as a star with apparent magnitude of +2. • The same star at 100 pcs (330 lyrs) would be 100 times fainter and appear as a magnitude 7 star, invisible to the naked eye. At 1,000 pcs, the same star would appear as a magnitude 12 star. • Most Open Clusters are between 100 pc and a few thousand parsecs away. Thus, we can only “see” the most luminous members of the cluster at these large distances. There are many more stars that simply do not appear, even on photographs, due to their very low luminosity.
