dougettinger wrote:How do astronomers tell the age of these open cluster stars in the center of the Rossette cluster ? These stars are listed as being 4 million years old which is extremely specific for cosmic time.
Doug Ettinger, Pittsburgh, PA 02/14/11
Indeed, four million years old is very specific. I don't know enough to explain how astronomers arrived at exactly that figure. I know, however, that it can't be off by too many million years.
Astronomers use color-magnitude diagrams to date clusters:
Young clusters are rich in bright blue stars. The brighter and bluer a star is, the younger it must be, because the the brighter and bluer a star is, the more massive it is, and the faster it evolves and dies.
You can see in the color-magnitude diagram that NGC 2362 contains very bright blue stars. Here is a picture of the cluster, centered on O9 supergiant Tau Canis Majoris:
Mighty Tau Canis Majoris is surrounded by a cluster of blue but not quite so massive B-type stars. You can see some reddish stars in the outskirts of the cluster, too: they are either background stars or relatively low-mass and therefore redder members of the cluster.
So how old is the Tau Canis Majoris Cluster? According to Professor Emeritus Jim Kaler, who has written several books about stars, Tau Canis Majoris itself is estimated to be five million years old - in other words, it would be one million years older than the Tau Canis Majoris cluster. See
http://stars.astro.illinois.edu/sow/taucma.html
So why should the NGC 2244 be one million years younger? Well, for starters, all the bright members of this cluster are hot blue stars. Oh, but surely the brightest member of this cluster is an evolved red giant? No, because the yellow star right next to the blue ones is a foreground star and not a member of NGC 2244. As for the true members of the cluster, at least three are O-type stars. The fact that there are three O-type stars and not a single red supergiant in this cluster is in itself proof that the cluster is very young. Let's compare NGC 2244 with the famous Double Cluster in Perseus:
As you can see, there are some prominent red giants that appear to be members of the Double Cluster. Interestingly, however, Wikipedia says about the Double Cluster that NGC 884, the cluster that seems to contain all the red giants, is only 3.2 million years old and therefore younger than the Rosette Nebula Cluster. This doesn't seem right to me, particularly in view of the fact that Wikipedia also says that the neither of the two clusters of the Double Cluster contain any O-type stars.The fact that these clusters are so rich strongly suggests to me that they must originally have contained some O-type stars, which have now evolved and become cooler. The red giants could be such former O-type stars, although we might perhaps expect them to be more centrally placed in the clusters if they were the original O-type members. Anyway, I question the suggestion that NGC 884 would be only 3.2 million years old. NGC 869, the "other member of the Double cluster", clearly contains some centrally placed bright and slightly yellowish members, which may indeed be former O-type stars which have now evolved into, say, A- or F-type supergiants. NGC 869 is estimated to be 5.6 million years old, according to Wikipedia.
Please note that neither the Tau Canis Majoris cluster nor the Double Cluster are surrounded by any nebulosity. These clusters must have been born out of massive gas clouds, but these gas clouds are now completely gone. NGC 2244, on the other hand, is surrounded by the magnificent Rosette Nebula, the nebula it was born from. NGC 2244 is so young that it has not had time to "blow away" all its natal gasses. We may note, however, that the hot bright stars have "burned a hole" in the central part of the Rosette.
In short I would say that four million years seems like a reasonable age estimate for the Rosette Nebula cluster.
Ann
The Rosette Nebula
