by Ann » Tue Oct 02, 2012 2:51 am
This is an interesting picture, where some foreground stars look really strikingly red. The reason for the unusual HEIC globular cluster palette is that this is a four-filter picture, imaged through blue, green, orange (or clear) and infrared filters. This makes it possible to bring out a wide range of colors in the finished picture.
Why has M69 been imaged through four filters, even though most clusters get visited by only two filters, typically clear (or orange) and infrared ones? I would say that the probable reason is that M69 is unusually colorless as globulars go, and one of the aims here was to find out exactly how colorless it really is.
Most globulars are extremely metal-poor. They therefore typically have a rich population of blue horizontal branch stars, blue stars which represent a late evolutionary stage in very metal-poor stars. While I, of course, find the blue stars very beautiful, the blue stars are important for astronomers because they are associated with white variable stars called RR Lyra variables. The RR Lyra variables are very important as distance indicators in globulars, and they, like the blue horizontal branch stars, can only exist in very metal-poor populations.
Since M69 is very metal-rich as globulars go, we should not expect to find many, or any, blue horizontal branch stars in it. Therefore we should not expect to find many, or any, RR Lyra variables either.
I googled "M69 globular RR Lyrae", but came up empty-handed. I checked M69 with my Sky Catalogue 2000.0 Volume 2, which says that M69 has an intrinsic B-V index of +0.82, which is red for a Milky Way globular. I checked Burnham's Celestial Handbook, which mentions M69 but says nothing about any RR Lyra variables in it.
So I would indeed guess that the fact that M69 was imaged through four filters, among them a blue one, was to find out exactly how non-blue and RR Lyra-free this globular cluster really is.
Ann
This is an interesting picture, where some foreground stars look really strikingly red. The reason for the unusual HEIC globular cluster palette is that this is a four-filter picture, imaged through blue, green, orange (or clear) and infrared filters. This makes it possible to bring out a wide range of colors in the finished picture.
Why has M69 been imaged through four filters, even though most clusters get visited by only two filters, typically clear (or orange) and infrared ones? I would say that the probable reason is that M69 is unusually color[i]less[/i] as globulars go, and one of the aims here was to find out exactly how colorless it really is.
Most globulars are extremely metal-poor. They therefore typically have a rich population of blue horizontal branch stars, blue stars which represent a late evolutionary stage in very metal-poor stars. While I, of course, find the blue stars very beautiful, the blue stars are important for astronomers because they are associated with white variable stars called RR Lyra variables. The RR Lyra variables are very important as distance indicators in globulars, and they, like the blue horizontal branch stars, can only exist in very metal-poor populations.
Since M69 is very metal-rich as globulars go, we should not expect to find many, or any, blue horizontal branch stars in it. Therefore we should not expect to find many, or any, RR Lyra variables either.
I googled "M69 globular RR Lyrae", but came up empty-handed. I checked M69 with my Sky Catalogue 2000.0 Volume 2, which says that M69 has an intrinsic B-V index of +0.82, which is red for a Milky Way globular. I checked Burnham's Celestial Handbook, which mentions M69 but says nothing about any RR Lyra variables in it.
So I would indeed guess that the fact that M69 was imaged through four filters, among them a blue one, was to find out exactly how non-blue and RR Lyra-free this globular cluster really is.
Ann