AAO image reference AAT 112. « Previous || Next »
Top left is NE. Image width is about 15 arc min
Image and text © 1992-2010, Australian Astronomical Observatory, photograph by David Malin.
Messier saw this fine open cluster in 1780, indeed it is hard to miss, being almost 30 arc minutes in diameter. The cluster is about 2500 light years away in the direction of Cancer. NGC 2682 is among the oldest open clusters, with an age of about 4 billion years, almost as old as the Sun. Being so old (for an open cluster) and so nearby it has been extensively studied since all its stars are at essentially the same distance. Because this eliminates two variables, distance and age, studying stellar evolution in such a cluster is greatly simplified. Nonetheless there remain large differences in values for the absolute age of the cluster and its mass, which is over 1000 solar masses.
This photograph deliberately was taken on the AAT under conditions of poor seeing and with a series of short (4 minute) exposures. The idea was the let the seeing spread out the star images so the colour would be more obvious. In most photographs the centres of bright stars are completely over-exposed so they appear white. however there is some colour information in the 'wings' of the stellar image, where the effective exposure gradually tails off. The wings are bigger in poor seeing, spreading the starlight over a larger area. The colours in this picture are hardly visible, so the experiment did not work too well. A rather better result was obtained with the next photograph M93 (NGC 2447).
AAT 91. The old open cluster, Trumpler 5
Constellation of Cancer (external site)
For details of photographic exposure, search technical table by AAT reference number.
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