NGC 7492
Mellotte 242
Globular Cluster
Aquarius
R.A. 23h08m27.0s
Dec. -15°36'36" (2000)
Magnitude: 11.20
Size: 4.2'
Although the OOTW’s of recent weeks have covered a number of globular clusters for this week
I would like to introduce NGC 7492 in Aquarius. One of three globular clusters in the constellation,
the others being M2 and M72, NGC 7492 was first discovered in 1786 by the indefatigable
William Herschel with his 18.7” Speculum telescope. He described it as extremely faint. It was
also photographed in 1902 by Issac Roberts who required a 90 minute exposure
with his 20" telescope to pick it up and he described it as a cluster of faint stars. NGC 7492 is
an old globular cluster with an age of perhaps 12 billion years or so lying about 84000 light
years from the earth and perhaps the same distance from the galactic centre. As such it
forms part of the group of outlying halo clusters.
ngc7492loc2.jpg
The cluster is very sparse and shows two short tidal tails from stars stripped out of it during
ngc7492.jpg
its passes through the Milky Way’s disk. In this way it is similar to the better-known cluster
Pal 5 and suggests that there is a limited lifetime left for this cluster before it is totally
disrupted. There also appears to be some form of mass segregation of the stars within
the cluster with the more massive ones towards the middle, which is what you would expect
if the less massive ones have been stripped away. NGC 7492 also lies near two star streams
from the Sagittarius dwarf that is currently being devoured by the Milky Way but the
difference in the velocities of the streams and NGC 7492 suggest it was not part of
that galaxy. It is a very loose cluster, classified as class XII on the Shapley-Sawyer
system, and because of its distance it is difficult to resolve visually into stars.
Observationally this cluster is going to be a challenge, especially from northern latitudes,
because of both its faintness and lack of compact core. It also is not that well known
as a cluster. It is suggested in most of the deep sky references that it may require a
30 cm (12”) telescope to pick up, and even then it may be only faintly seen. Hartung
however suggests that it may show slight signs of granulation through a 30cm but it
needs a really transparent night for this, although of course for him observing from
Australia it would have been very high in the sky When trying for this object
choose a transparent night and try and observe it when it is within an hour of the meridian.
You are also going to need a good southern sky without too much light pollution. If you like
collecting objects on lists then NGC 7492 is also on the AL globular cluster list.
As always give a go and let us know what you see
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