BNGC 4. For A Fistful Of Globulars

By Steve Mencinsky, 1992

When you observe 47 Tucanae - and why wouldn't you - how many globular clusters can you see in the field? If you telescope is larger than about 10" and the field of view is larger than about 1.5°, you should be able to see THREE:

  • Obviously, 47 Tucanae itself.
  • NGC 121, some 0.8° to the north.
  • Another, some 0.8° to the south, a little larger and fainter than NGC 121. This little-known globular cluster, Kron 3, apparently belongs to the Small Magellanic Cloud. It is not particularly difficult to observe - Andrew Murrell stumbled upon it with his 10" telescope from his backyard in Woy Woy, while Mark Blackford and I were able to see it very easily (from Ilford) in his beautiful 4 inch refractor.

Considering that this is a globular cluster residing in another galaxy, what CAN be seen is quite impressive. Tom Kaar's 17.5" telescope shows apparent partial resolution into "cotton-wool-like" threads. Try doing THAT with a globular in M31! However, Kron 3 is quite a teaser in this regard, because in the Murrell 20", boasting newly aluminised surfaces, it was even more fibrous-looking, but still not resolved. How large a telescope would one need to visually resolve this?

But there's more!!!
While observing Kron 3 with Andrew and David Frew from Ilford during the September new moon weekend, David made a casual remark, "There's another globular in the field", which was news to yours truly, since there were no more such marked on Mati's charts. Indeed there is: a few tens of minutes closer in to the SMC, there is the even fainter Kron 7 which, according to David's notes, is also intrinsic to the SMC. This globular was quite faint in my new 18" telescope appearing quite round uniformly bright and with a ragged edge. The below diagram shows the location of the two Kron globulars with respect to 47 Tucanae. The chart shows stars down to about 15th magnitude.