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The star-forming nebulae NGC 6334 (UKS 11) and NGC 6357 show no evidence of a blue component in their colour - indeed, the blue-light plate used in these 3-colour pictures had no nebulous image at all - nor is there any obvious sign of the bright blue stars normally found in these objects. They are excellent examples of the phenomenon known as 'interstellar reddening', the selective removal of blue light by minute particles of dust in the line of sight. This accounts for both the ruddy hue and apparent absence of blue stars. The hot stars are present but only some of the red part of their light is seen so they are not conspicuous. These nebulae are quite nearby (5500 light years) but located close to the Galactic plane and are buried in the dust of the Milky Way. Careful measurement of the colour of stars associated with the nebulae indicate that they are dimmed by a factor of about 10 in the green part of the spectrum, much more in the blue, but relatively little at red wavelengths. |