This is a most interesting cluster. It does indeed contain many bright red giants, and none of our well-known nearby clusters have so many bright red members. Compare Caroline's Rose with, say, the Hyades
(where Aldebaran is not a member), the Beehive Cluster
. Of the relatively well-known moderately nearby northern clusters, M37 in Auriga
is perhaps the one that most resembles NGC 7789. But the age of M37 is estimated to be no more than 550 million years
, according to Wikipedia. That is much younger than the age of Caroline's Rose. And a rich open cluster like M11
is clearly much younger than Caroline's Rose.
Of the southern clusters, NGC 3532
is impressive and contains many red giants, but it is clearly less dominated by them and therefore younger than Caroline's Rose.
An interesting cluster, comparable to Caroline's Rose, is NGC 2158
. It is a very rich cluster, and it contains many red giants. According to Wikipedia, the age of NGC 2158 is just over a billion years
, so again, this cluster is younger than Caroline's Rose.
A very interesting cluster is M67, whose age is estimated to be about 4 billion years
, which is very old indeed for an open cluster. M67 is therefore likely more than twice as old as Caroline's Rose. (Fascinatingly, although M67 is clearly an open cluster and not a globular one, which is obvious from its chemical composition, it does have a most interesting feature in common with globular clusters, namely, the presence of blue straggler stars. Take a look at this picture
to see several bluish stars inside M67, including a very blue star of spectral class B.)
M67 is located far from the plane of the Milky Way, which may be one of the reasons that this open cluster has existed for such a long time. Open clusters usually disperse when they are still quite young. The reason for their rapid "evaporation" is that they are typically relatively low in mass, and also they typically form in the thickly populated plane of the Milky Way, where there are all sorts of gravitational influences that can tear clusters apart.
NGC 7789 appears to be located in the plane of the Milky Way. Clearly it is also a rich and therefore a massive cluster. Its comparatively large mass must be the reason why it has managed to keep itself together and grow so old that it is full of bright red giants.