by Ann » Sun Nov 18, 2012 7:08 am
I'm always very interested in high-mass stars, so of course I find this APOD hugely interesting. It is a joy to see, once more, Hubble ability to sharply resolve fascinating objects. NGC 6357 is far away and also considerably dimmed by intervening dust. Astrophotographer David Malin once demonstrated that NGC 6457 lacks the magenta hue that we associate with emission nebulae like the Lagoon Nebula. Instead, the blue-green light of hydrogen beta emission has been scattered away by dust, making NGC 6357 glow deep-red.
As always, though, I'm confused by the color of this picture. How can the star in the "cave" at bottom be so very much bluer than the stars of the cluster, even though the star in the "cave" can be assumed to be even more reddened than the others. The bright stars of the clusters "above the cave" are not blue at all, but white, and the brightest ones are the most non-blue. Well, this is actually possible. In
NGC 869, one of the two Double Clusters, the brightest stars have evolved off the main sequence and are less blue than many of the fainter stars. Yet the difference is slight. In the Hubble image, the difference in color between the non-blue brightest stars and the intensely blue bluest stars is huge.
So what about the filters that were used to produce this image? Well, an image taken through a green filter at 550 nm was mapped as blue. Another image taken through a similar but sligthly bluer green filter at 547 nm was mapped as green. Ha and SII filters at 656 and 658 nm were mapped as red, as well as an infrared filter at 850 nm. I don't understand how these filters could produce the differences in star colors that we see in this picture, unless the brightest stars are actually red giants.
Maybe the star in the "cave" is the bluest of the lot. But I'm not convinced.
Ann
I'm always very interested in high-mass stars, so of course I find this APOD hugely interesting. It is a joy to see, once more, Hubble ability to sharply resolve fascinating objects. NGC 6357 is far away and also considerably dimmed by intervening dust. Astrophotographer David Malin once demonstrated that NGC 6457 lacks the magenta hue that we associate with emission nebulae like the Lagoon Nebula. Instead, the blue-green light of hydrogen beta emission has been scattered away by dust, making NGC 6357 glow deep-red.
As always, though, I'm confused by the color of this picture. How can the star in the "cave" at bottom be so very much bluer than the stars of the cluster, even though the star in the "cave" can be assumed to be even more reddened than the others. The bright stars of the clusters "above the cave" are not blue at all, but white, and the brightest ones are the most non-blue. Well, this is actually possible. In [url=http://www.capella-observatory.com/images/OpenClusers/NGC869.jpg]NGC 869[/url], one of the two Double Clusters, the brightest stars have evolved off the main sequence and are less blue than many of the fainter stars. Yet the difference is slight. In the Hubble image, the difference in color between the non-blue brightest stars and the intensely blue bluest stars is huge.
So what about the filters that were used to produce this image? Well, an image taken through a green filter at 550 nm was mapped as blue. Another image taken through a similar but sligthly bluer green filter at 547 nm was mapped as green. Ha and SII filters at 656 and 658 nm were mapped as red, as well as an infrared filter at 850 nm. I don't understand how these filters could produce the differences in star colors that we see in this picture, unless the brightest stars are actually red giants.
Maybe the star in the "cave" is the bluest of the lot. But I'm not convinced.
Ann