Affiliation
American Association of Variable Star Observers (AAVSO)
Fri, 10/18/2013 - 14:39
Hello. I'm a relatively new member of the AAVSO. I've submitted a few observations - still very much a beginner. I've been a very active amateur astronomer for many years - mostly visual. Globular clusters are a particular interest of mine. Question: Are there any variable stars in globular clusters that are visually observable with a 12.5" Newtonian? Thanks, Lawrence Crary
Hi Lawrence,
Cretainly a number of Type 2 Cepheids in globular clusters come to mind. I am not sure what the VSX/AUID/AID or sequence situation is for these stars. Someone else will have to speak to that.
M2 V1, V5, V6, V11
M3 V154
M5 V42, V84
M12 V1
Cheers,
Doug
As Doug noted, a number of type II Cepheids in globular clusters might be within your reach if you are observing from a dark location. Some of these are in crowded fields while others are separated from near neighbors. Long term observations of these variables are useful for period change studies, but, at least since the days of E.E. Barnard, observations have generally been carried out photographically or, more recently, with the aid of CCDs. Still, it might be interesting to try one or two as an experiment. Of course, we are leaving the season when many of these clusters are well placed for evening viewing. If you are a beginner, you would have time before the clusters come around again for spring evening observing to practice your skills.
Horace
Indeed. But M2 is still pretty well-placed for evening observation at the moment.
A significant challenge for globular cluster variables is locating a finder chart or image which has even a passing resemblance to the view through an eyepiece!
Cheers,
Doug
Hello, Lawrence,
What a big coincidence! A couple of days ago an observer writing an article for S&T on M5 asked for information on V42 and V84, a couple of bright variables in that cluster.
Doug gave you some suggestions about targets and mentioned these two.
About the VSX point of view, the problem with globular cluster variables (and not only globular but also open clusters, although to a lesser extent) is that the coordinates need to be very accurate to properly identify the variables. There are really crowded areas and small differences in positions lead to potential identification mistakes. The literature on cluster variables usually don't give accurate positions. There are finding charts identifying the variables with sequential numbers but if we want to include all those variables in VSX safely, we should check the stars and confirm their IDs using current astrometric catalogues like UCAC4, 2MASS or PPMXL. This means direct import of lists with cluster variables is not "allowed" if we want to respect our standards. That is specially true for the old studies of GC variables.
Yesterday we were discussing this issue with Arne. There is no reason to reject submissions on globular cluster variables if they provide a finding chart that could be used to confirm the star's identification (and an accurate position).
The same way we will be able to add globular cluster variables if we consider their identifications are checked and their positions good enough to avoid identification problems.
On the observing side, you need to obtain good magnitudes for the comparison stars that you're going to use to estimate those variables. If they are bright enough maybe you can select some comparison stars outside the cluster, if they are very faint so that you need to use high powers, maybe you will need comp stars in the cluster itself. Those magnitudes could probably be found on the same papers where variability studies on the targets were published.
So we won't be making systematic efforts to have a complete list of cluster variables in VSX but if there are some interesting targets or new variables are found in those clusters, they could be accepted in VSX as long as a finding chart is provided and a good position from an astrometric catalogue is given.
Cheers,
Sebastian
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Sebastian Otero
VSX Team
American Association of Variable Star Observers