A different way of looking at globular cluster magnitudes

Large, bright globular clusters usually rank near the top of everyone's list of favorite objects for telescopic viewing. However, beyond the usual suspects, there are quite a few fainter globulars where detection can be very difficult or impossible, depending on aperture, seeing, and sky darkness. Because of the numbers involved, it is a fairly modest long term project for an observer to see a large fraction of the approximately 150 Milky Way globulars with a modest telescope.

Even if visible, the fainter globulars are usually not resolved for most observers, which makes them look a lot like round galaxies. In fact, the drop off in brightness away from the center behaves similarly to galaxies. Thus, as an adjunct to my galaxy photometry pages, I have put together this page on globular clusters. A big thanks to Tony Flanders for prompting me to actually undertake this logical extension.

What I have done is taken existing data on the brightness of a globular as a function of increasing circular aperture, and fit a function to that data. From that, I can assess the brightness of a globular at a set of apparent sizes. You can take a look at a sampling of fits if you like. The point is that if you are barely detecting a globular (or galaxy), you are only seeing the innermost core region, and not all globulars have the same light distribution. Thus, total magnitude may not be the best number for judging how likely you are to detect the globular. "Concentration class" can be a good indicator, when combined with total magnitude. But, it is a somewhat subjective index, and my technique gives magnitudes of what you might see, which can be directly compared among clusters in a single step.

When you can barely detect an object like a globular or a galaxy through a telescope, it will usually appear smaller than about 2 arcminutes. The higher the power, the smaller the visible portion can be for a detection. Thus, I have calculated visual magnitudes for assumed diameters of 0.5, 1, and 2 arcminutes. As a very rough guide, these correspond to typical powers of 200, 100, and 50, respectively. If you are trying to detect globulars with binoculars or the naked-eye, my derived numbers will probably not be very useful. I may work on that issue at some point. My derived quantities may not be very useful anyway, but I also give total magnitudes and positions for the globulars from William Harris' compilation of Milky Way Globular Cluster Parameters, and those numbers should be useful in any case.

As I see it, the main uses of my derived magnitudes would involve observing some globulars and then using your own visibility data combined with my data table to figure out if you have a realistic chance of seeing other objects under those conditions. I.e. maybe you are trying to see all the Herschel 400 objects with a small telescope under dark skies, or a large telescope under poor skies, etc. Note again that this list is geared toward the detection of the globular, and not with how easy it is to resolve stars. If you are resolving stars, you are way beyond mere detection. [Maybe not true for the poorest clusters?]

One important note: I have not (yet) included angular sizes for the globulars. It's a matter of choosing a particular criterion for assessing the size. The tidal radii of most globulars are huge, reflecting the fact that there can be gravitationally bound members of the cluster at very considerable distances away from the center. You can see for yourself that a cluster like M13 just sort of gradually fades away into the random background of stars. Where do you draw the line? Reasonable choices can involve things like a circle that encloses xx% of the total light, or picking a point where the surface brightness drops off to some level. I'll eventually commit to something. For now, you could check out the "Accurate Globular Cluster List", or grab Brian Skiff's compilation.

As you might guess, my data tables were put together in a mostly automated manner, so there may be a few stupid mistakes lurking.


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