by Ann » Sun Aug 15, 2010 8:20 am
That's a fascinating image of NGC 663. I don't think I have seen this cluster photographed many times before, if at all, at least not in color. The colors here are very subtle and interesting. Clearly the cluster is strongly reddened. The brightest stars, the ones bright enough to have "spikes", are all B-type supergiants, according to my software. Their B-V index is generally between 0.6 and 0.8, which suggests a fairly high degree of reddening.The colors of the stars of intermediate brightness is fascinating. The brightest of the stars that are not bright enough to have spikes appear to be slightly redder than the brightest stars. But fascinatingly, there is a large population of still fainter stars which are noticably bluer.
Disregarding the color of the brightest stars, the colors of the stars of this cluster seems fairly typical of a metal-poor old globular cluster. In such a cluster the brightest stars are clearly orangish, since they are so-called Asymptotic Giant Branch stars, which have swollen to a gigantic size just before shedding their atmospheres and becoming planetary nebulae and white dwarfs. Then there are somewhat fainter yellow-orange stars which have begun their climb up the Asymptotic Giant Branch ladder, and they are still in the process of swelling and reddening. But there is also a relatively large population of still fainter Horizontal Branch stars, which are blue in a metal-poor cluster. If the brightest stars in NGC 663 were orange instead of off-white, this cluster would have the color distribution of a globular cluster. In an open cluster, by contrast, the brightest stars are usually blue. And if the brightest stars have started evolving off the main sequence, it is usually the second-brightest stars which are the bluest.
NGC 663 looks really rich in this image, and that also makes it slightly similar to a globular cluster. Fascinating!
Ann
That's a fascinating image of NGC 663. I don't think I have seen this cluster photographed many times before, if at all, at least not in color. The colors here are very subtle and interesting. Clearly the cluster is strongly reddened. The brightest stars, the ones bright enough to have "spikes", are all B-type supergiants, according to my software. Their B-V index is generally between 0.6 and 0.8, which suggests a fairly high degree of reddening.The colors of the stars of intermediate brightness is fascinating. The brightest of the stars that are not bright enough to have spikes appear to be slightly redder than the brightest stars. But fascinatingly, there is a large population of still fainter stars which are noticably bluer.
Disregarding the color of the brightest stars, the colors of the stars of this cluster seems fairly typical of a metal-poor old globular cluster. In such a cluster the brightest stars are clearly orangish, since they are so-called Asymptotic Giant Branch stars, which have swollen to a gigantic size just before shedding their atmospheres and becoming planetary nebulae and white dwarfs. Then there are somewhat fainter yellow-orange stars which have begun their climb up the Asymptotic Giant Branch ladder, and they are still in the process of swelling and reddening. But there is also a relatively large population of still fainter Horizontal Branch stars, which are blue in a metal-poor cluster. If the brightest stars in NGC 663 were orange instead of off-white, this cluster would have the color distribution of a globular cluster. In an open cluster, by contrast, the brightest stars are usually blue. And if the brightest stars have started evolving off the main sequence, it is usually the second-brightest stars which are the bluest.
NGC 663 looks really rich in this image, and that also makes it slightly similar to a globular cluster. Fascinating!
Ann