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Colour-magnitude Arrays in the Clusters Tombaugh 5, NGC 7789, NGC 1528 and NGC 2682

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  • V. C. Reddish

Magnitudes and colours of stars in four clusters have been obtained from simultaneous photographs on blue- and red-sensitive plates, using the twin refractor of the University of London Observatory at Mill Hill. The magnitudes have been reduced to the (B, V) system of Johnson and Morgan. The colour–magnitude arrays show the cluster Tombaugh 5 to be of similar type to Praesepe. NGC 7789, previously classified as a galactic cluster, is found to be a globular cluster. Trumpler's classification of NGC 1528, a normal galactic cluster, is confirmed. NGC 2682 is known to be peculiar. The author considers it to be a transition type, between the normal Population I and Population II types. It is tentatively classified as of Population I on the basis of the existence of a narrow Hertzsprung Gap, a basis which the author suggests should be used as the main criterion to differentiate between clusters of the two types of population. A continuous sequence of types of colour–magnitude arrays of clusters is arranged, in which the main variation along the sequence (apart from the brightness of the stars) is the width of the Hertzsprung Gap. This variation is probably a result of differences in the proportions of the stars' masses contained within their convective cores.

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Monthly Notices of the Royal Astronomical Society

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  • Interstellar reddening
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