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Visually Rating a Globular Cluster's Concentration Class

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#1 SNH

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Posted 10 July 2020 - 04:45 PM

Hello all,

 

I’ve found that some globular clusters don’t have a Shapley-Sawyer Concentration Class number. I know of two that are so bright that I’ve been able to see them in my 8x56 binoculars – NGC 6355 and NGC 6558. William Harris told me that it’s true those were never classified by Shapley/Sawyer and that the reason might have been because “They are in the Galactic bulge region, not very luminous, and obscured by quite a bit of foreground dust absorption.” Though that sounds reasonable, it still doesn’t quite answer it for me.

 

So I’m wondering if anyone has taken a stab at rating them? Late last year a friend bought me a copy of the Astronomical League’s Globular Cluster Guide and they show a way to determine that for yourself. So I’m thinking people have tried it and before I go and do it myself this summer, I’d like to hear what others have gotten!

 

Thanks,

Scott


Edited by SNH, 10 July 2020 - 04:46 PM.


#2 jeffreym

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Posted 10 July 2020 - 05:35 PM

Hi Scott,  I have looked at around 80 globular clusters, including these two.  I feel they both look un-collapsed and have very low concentrations.  When comparing these two to NGC 5897, which was classified by Harlow-Shapley, it is hard to think they would have been anything but Class XII.  I think 6558 looks to have a brighter center due to a few dim foreground stars clustered together and not a collapsed core.

 

I think part of the trouble I have had with the dimmer GCs I've looked at is I just don't think I'm using a larger enough telescope to make a fair comparison.  Correcting this is on my to-do list. smirk.gif

 

Have fun.

Jeff



#3 VariableBob

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Posted 10 July 2020 - 06:50 PM

This is an interesting question and one that may not have an easy answer as the Sawyer Shapley (It was Helen Sawyer's work so her name is first :-) ).  The classification scheme is pretty subjective and originally had Concentrations from I to X.  I do not  recall if the original paper had a class XI and until fairly recently there has not been an example of a class XII.  Sawyer and Shapley used photographic graphic plates to make there determination and they looked at many globular clusters.

 

There is a nice experiment than one can do with two globular clusters in Sag; M-22 and M-28.  I have used this demonstration at star parties using an 8" reflector at 80 X.  M-22 is a large globular cluster easily resolvable almost to the center; on the other hand M-28 is more condensed , stars are easily resolved in the halo; however the core does not resolve at 80x, it almost completely resolve at 120x.  Using the Sawyer Shapley scale  I tink that M-22 is eith XII or XIII while M-28 is IV or maybe a III.  THis is the best way making a determination is comparing the two clusters side by side.

 

Cheers,

 

Bob

 


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#4 Starman1

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Posted 10 July 2020 - 07:05 PM

Hello all,

 

I’ve found that some globular clusters don’t have a Shapley-Sawyer Concentration Class number. I know of two that are so bright that I’ve been able to see them in my 8x56 binoculars – NGC 6355 and NGC 6558. William Harris told me that it’s true those were never classified by Shapley/Sawyer and that the reason might have been because “They are in the Galactic bulge region, not very luminous, and obscured by quite a bit of foreground dust absorption.” Though that sounds reasonable, it still doesn’t quite answer it for me.

 

So I’m wondering if anyone has taken a stab at rating them? Late last year a friend bought me a copy of the Astronomical League’s Globular Cluster Guide and they show a way to determine that for yourself. So I’m thinking people have tried it and before I go and do it myself this summer, I’d like to hear what others have gotten!

 

Thanks,

Scott

https://www.messier....p/gc-class.html

https://en.wikipedia...entration_Class



#5 Starman1

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Posted 10 July 2020 - 07:11 PM

This is an interesting question and one that may not have an easy answer as the Sawyer Shapley (It was Helen Sawyer's work so her name is first :-) ).  The classification scheme is pretty subjective and originally had Concentrations from I to X.  I do not  recall if the original paper had a class XI and until fairly recently there has not been an example of a class XII.  Sawyer and Shapley used photographic graphic plates to make there determination and they looked at many globular clusters.

 

There is a nice experiment than one can do with two globular clusters in Sag; M-22 and M-28.  I have used this demonstration at star parties using an 8" reflector at 80 X.  M-22 is a large globular cluster easily resolvable almost to the center; on the other hand M-28 is more condensed , stars are easily resolved in the halo; however the core does not resolve at 80x, it almost completely resolve at 120x.  Using the Sawyer Shapley scale  I tink that M-22 is eith XII or XIII while M-28 is IV or maybe a III.  THis is the best way making a determination is comparing the two clusters side by side.

 

Cheers,

 

Bob

M28 is considered class IV.  M22 is considered class VII

M55 is considered class XI

See page 11 here for a list with classes:

http://articles.adsa...000012.000.html



#6 SNH

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Posted 17 July 2020 - 04:10 PM

Hi VariableBob,

You mentioned that "It was Helen Sawyer's work so her name is first". I have never heard it that way and have ever seen it written with "Shapley" first. So could you please link me to the journal paper or something to help me confirm what you are saying?

 

Thanks,

Scott



#7 SNH

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Posted 17 July 2020 - 04:39 PM

In this July 1927 paper titled "Photographic Magnitudes of Ninety-five Globular Clusters", Helen Sawyer signs off first at the very end. But in this August 1927 paper titled "A Classification of Globular Clusters", Helen Sawyer signs off second oddly enough. So I don't know where you would've seen that Sawyer should be listed first, unless it was precedent at the time to list the man first even though it was her work - which is very possible!

 

Scott



#8 Starman1

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Posted 18 July 2020 - 10:04 AM

See:

https://iopscience.i...1086/128229/pdf


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#9 chemisted

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Posted 28 July 2020 - 12:36 PM

Hello all,

 

I’ve found that some globular clusters don’t have a Shapley-Sawyer Concentration Class number. I know of two that are so bright that I’ve been able to see them in my 8x56 binoculars – NGC 6355 and NGC 6558. William Harris told me that it’s true those were never classified by Shapley/Sawyer and that the reason might have been because “They are in the Galactic bulge region, not very luminous, and obscured by quite a bit of foreground dust absorption.” Though that sounds reasonable, it still doesn’t quite answer it for me.

 

So I’m wondering if anyone has taken a stab at rating them? Late last year a friend bought me a copy of the Astronomical League’s Globular Cluster Guide and they show a way to determine that for yourself. So I’m thinking people have tried it and before I go and do it myself this summer, I’d like to hear what others have gotten!

 

Thanks,

Scott

I have observed both of these clusters with my OGS RC-10 and Collins I3 image intensifier from a dark site in the mountains at 7800' elevation.  The intensifier eliminates some of the interstellar dust problem and the view of each cluster resolved to their cores.  For NGC 6355 my notes say: "Well Resolved.  Class VIII-IX?".  For NGC 6558 my notes say:  "Small & faint but fully resolved 2' ".  Unfortunately I did not make a classification try on this cluster.  These observations are from 2010 when I first got interested in doing a detailed study of globulars and I have not returned to either cluster since then.  A third cluster that I did make a try on is NGC 6256 which appeared to me to be Class X-XI.

 

For those interested in a few clusters that are classified XII check out NGC 5466, NGC 6496 and IC 1276 (also known as Pal 7).



#10 Tony Flanders

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Posted 28 July 2020 - 02:29 PM

There is a nice experiment than one can do with two globular clusters in Sag; M-22 and M-28.  I have used this demonstration at star parties using an 8" reflector at 80 X.  M-22 is a large globular cluster easily resolvable almost to the center; on the other hand M-28 is more condensed , stars are easily resolved in the halo; however the core does not resolve at 80x, it almost completely resolve at 120x.


Sorry for the delayed response, but I think this is largely due to the fact that M28 is twice as distant as M22; therefore its stars are (on average) just 1/4 as bright. Not a particularly good way to determine how concentrated the cluster is.


Edited by Tony Flanders, 28 July 2020 - 02:31 PM.


#11 Starman1

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Posted 28 July 2020 - 04:59 PM

Sorry for the delayed response, but I think this is largely due to the fact that M28 is twice as distant as M22; therefore its stars are (on average) just 1/4 as bright. Not a particularly good way to determine how concentrated the cluster is.

I think the Shapley-Sawyer classification is a visual or photographic one, not an intrinsic one.

I don't believe it describes a particular density of stars per cubic parsec.

So M22, at twice the distance, could have a different classification.



#12 SNH

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Posted 29 July 2020 - 12:06 PM

I'm working on the question of who should get more credit - Shapley or Sawyer - for the Concentration Class. Checked with William Harris and now asking Owen Gingerich. Will let you all know what I come up with!

 

Scott



#13 SNH

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Posted Yesterday, 07:03 AM

Haven't gotten anything back from Gingerich, but here is some of what Harris said to me:

 

"That is an odd little corner of astronomical history to be sure. In short, I know nothing more than you seem to have found already about the history of the concentration index. Who came up with it? I would tend to think Shapley given the broad basis of his work on globular clusters, but that is only a guess. And there is no one left alive now with any relevant direct personal knowledge."

 

"As far as I can tell she had a very good working relationship with Shapley in her early years and went much further in an academic
career of her own than most of his other research assistants and students. The fact that they wrote numerous publications together and switched first authorship now and then suggests they had some sort of professional agreement about the person having done the majority of work getting the first credit for it. That sort of agreement is conventional today in our field."

 

So as of right now, I'm not sure there is any more research I can perform on this matter! Thanks for all your thoughts on the matter.

 

Scott



#14 Feidb

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Posted Yesterday, 08:06 AM

Didn't even realize (okay, forgot) they had classes. Now that I think about it, I recall they, or some of them do. I decided not to add that to my database because I just enjoy the sight of them. A tempting statistic but beyond my limited interest. I DO notice concentration and even color though. I note that in my description, plus after first seeing the propeller in M13, I've also been looking for other patterns in globs and opens. However, it changes from night to night, it seems, depending on magnification and seeing, so that I'm not always seeing the same thing repeatedly, at least as distinctive as the propeller (off the top of my head). Hmmm. Have to think about that one. It's been so long since I've been out. Geez.



#15 Starman1

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Posted Yesterday, 09:29 AM

The class seems to be a visual classification, since long exposures of most globulars expose considerably more stars to view, which would significantly

change the classification.  Almost all have dense cores in photos.

I think 12 classes went overboard, and I see a lot of overlap or commonality between classes.

Just as is the case with the Trumpler classification of open star clusters, a lot of subjective evaluation determined the class,

and your subjective classification might not see things the same way.

In small scopes, Messier 4 is resolved and has a very loose concentration and not much core.  But in very large scopes, M4 is very rich and has a noticeable core of stars at the center.

So aperture would be a factor in determining the visual classification.

So, like the Trumpler classes for open clusters, take the class determination with a grain of salt--it might not be what you see.




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