by Ann » Fri Mar 29, 2013 7:24 pm
Thank you for that image of NGC 2547 by José Joaquín Pérez, starsurfer. His image is lovely indeed.
Anyway, I need to object a bit to this part of the ESO caption:
While NGC 2547 contains many hot stars that glow bright blue, a telltale sign of their youth, you can also find one or two yellow or red stars which have already evolved to become red giants.
I agree that the red stars that can be seen in the image are probably all red giants, but it is not clear that any of the red stars in the image are members of NGC 2547. Note that the red stars that are very close to the cluster center are relatively faint. This suggests that they have not evolved from any of the young bright blue stars in the cluster, since this kind of evolution should not make the stars fainter. Besides, the brightest blue stars should use up their core hydrogen first and turn into quite bright red giants, so any member of NGC 2547 that had turned into a red giant should be at least as bright as the brightest blue stars of the cluster.
Other red stars in the image are relatively bright, but they are also rather far away from the obvious center of NGC 2547, which again suggests that they are not members.
Since it isn't clear that any of the red stars in the image are members of the young cluster NGC 2547, we can't really say that the red stars have
already evolved to become red giants. These red stars may all be considerably older than the members of NGC 2547.
Ann
Thank you for that image of NGC 2547 by José Joaquín Pérez, starsurfer. His image is lovely indeed.
Anyway, I need to object a bit to this part of the ESO caption:
[quote]While NGC 2547 contains many hot stars that glow bright blue, a telltale sign of their youth, you can also find one or two yellow or red stars which have already evolved to become red giants.[/quote]
I agree that the red stars that can be seen in the image are probably all red giants, but it is not clear that any of the red stars in the image are members of NGC 2547. Note that the red stars that are very close to the cluster center are relatively faint. This suggests that they have not evolved from any of the young bright blue stars in the cluster, since this kind of evolution should not make the stars fainter. Besides, the brightest blue stars should use up their core hydrogen first and turn into quite bright red giants, so any member of NGC 2547 that had turned into a red giant should be at least as bright as the brightest blue stars of the cluster.
Other red stars in the image are relatively bright, but they are also rather far away from the obvious center of NGC 2547, which again suggests that they are not members.
Since it isn't clear that any of the red stars in the image are members of the young cluster NGC 2547, we can't really say that the red stars have [b][i]already[/i][/b] evolved to become red giants. These red stars may all be considerably older than the members of NGC 2547.
Ann