Argonavis
24-12-2007, 08:12 AM
Is the Jewel Box full of jewels?
This is the question posed by David Riddell, a northern hemisphere observer visiting the south.
David writes: ".....One of the most celebrated open clusters in the heavens is NGC 4755 -- the Kappa Crucis cluster commonly referred to as the "Jewel Box."
How did this cluster receive its reputation as being spectacularly colorful?
Obviously, John Herschel's description of NGC 4755 as a "rich piece of jewelry" (in his Outlines of Astronomy) and "a superb piece of fancy jewelery" (Cape Observations) advanced the idea that has been repeated over and over in observer's manuals. Nowhere can I find that Herschel actually used the phrase "jewel box", so this nickname's origin remains obscure.
Later observers disagreed about the intensities of colors. W. C. Russell (1872) thought Herschel's remarks to be fully justified, citing a number of red, yellow, blue, green and blue-green stars visible with a 7 1/4" Merz refractor and an 8 1/2" reflector.
It must be confessed that, with moderate telescopic apertures, it fails to realise the effect of colour implied by Sir John Herschel's comparison..."
Using 25inch and 12 inch telescopes in South Africa, David noted only 3 stars with a particular colour, the numerous other members remained essentially colorless. He notes that the close proximity of relatively bright stars introduces some interesting physiological effects giving the illusion of a variety of colors. He thought the colors of Messier 41 to rival those of NGC 4755.
I tend to agree. For modest aperature telescopes, the jewel box does not to my eye show a wealth of colour. Maybe I am a little insensitive to star colours, but I have always preferred the open clusters ngc2516, ic2602 (southern pleadies) and ngc3532. They are just as colourful and richer.
I must admit that with substantial aperatures, like 25inch, some of the stars in ngc4755 do show a green cast, and it is then and only then that you can appreciate the name jewel box. But with the wealth of rich open clusters in the southern milky way the name "jewel box" could only belong to a former era of long refractors with narrow fields of view. We are spoilt. Our modern era of large reflectors with wide field eyepieces show off the sky like nothing before.
This is the question posed by David Riddell, a northern hemisphere observer visiting the south.
David writes: ".....One of the most celebrated open clusters in the heavens is NGC 4755 -- the Kappa Crucis cluster commonly referred to as the "Jewel Box."
How did this cluster receive its reputation as being spectacularly colorful?
Obviously, John Herschel's description of NGC 4755 as a "rich piece of jewelry" (in his Outlines of Astronomy) and "a superb piece of fancy jewelery" (Cape Observations) advanced the idea that has been repeated over and over in observer's manuals. Nowhere can I find that Herschel actually used the phrase "jewel box", so this nickname's origin remains obscure.
Later observers disagreed about the intensities of colors. W. C. Russell (1872) thought Herschel's remarks to be fully justified, citing a number of red, yellow, blue, green and blue-green stars visible with a 7 1/4" Merz refractor and an 8 1/2" reflector.
It must be confessed that, with moderate telescopic apertures, it fails to realise the effect of colour implied by Sir John Herschel's comparison..."
Using 25inch and 12 inch telescopes in South Africa, David noted only 3 stars with a particular colour, the numerous other members remained essentially colorless. He notes that the close proximity of relatively bright stars introduces some interesting physiological effects giving the illusion of a variety of colors. He thought the colors of Messier 41 to rival those of NGC 4755.
I tend to agree. For modest aperature telescopes, the jewel box does not to my eye show a wealth of colour. Maybe I am a little insensitive to star colours, but I have always preferred the open clusters ngc2516, ic2602 (southern pleadies) and ngc3532. They are just as colourful and richer.
I must admit that with substantial aperatures, like 25inch, some of the stars in ngc4755 do show a green cast, and it is then and only then that you can appreciate the name jewel box. But with the wealth of rich open clusters in the southern milky way the name "jewel box" could only belong to a former era of long refractors with narrow fields of view. We are spoilt. Our modern era of large reflectors with wide field eyepieces show off the sky like nothing before.