Three open clusters and corresponding GAIA visualisations (NGC 559, NGC 609, NGC 743)

[Martin Meredith]

Another clear Monday night (why can't it be clear at the weekend?!) was too good an opportunity to continue my observation of Berkeley open clusters, this time in the Cassiopeia region. I may post some of the dozen I observed later, although frankly, like many Berkeleys, in some cases it was pretty hard to spot anything exciting, especially with a bright moon around. As in the last session I compensated for the faint Berkeleys by starting and ending with 3 more in-your-face (NGC) open clusters, which I'm posting here. 

The other reason for the post is to sing the praises of the latest GAIA data release (DR2) for the excellent magnitude reach and colour data it contains. For each of the 3 NGC clusters I'm showing GAIA data for comparison purposes. These reach down to magnitude 21 and are all 40' wide fields.

The shots all use StarlightLive as a capture engine (including control of  the electronic filterwheel), and Jocular for everything else. They were all 15s subs in L, R, G, B, live combined in LAB with adjustment of color stretch and saturation only (plus the usual L controls). No darks nor other calibration was done as I'm using an experimental version of the software and that aspect is currently not yet integrated. All shots have N at top to make comparison with the GAIA data easier. Usual kit: 8" f/4 Quattro Newt, Lodestar X2 mono camera, alt-az mounted using AzEQ6 mount.

 

First up is NGC 559. According to https://academic.oup.com/mnras/article/437/1/804/2907736, "NGC 559 is a moderately populated and heavily reddened intermediate-age open cluster, classified as type I2m by Trumpler (1930) and II2m by Ruprecht (1966). It is located in the direction of the second Galactic quadrant in the vicinity of the Perseus and Local arms ... " My shot shows many red stars, possibly a little more saturated than the GAIA data (which could perhaps usually have its saturation increased). The aforementioned article identified the 22 most likely cluster members, and it is notable that all of them have B-V colour indices in the range 0.63-1.73, suggesting orange-red colours.

 

 

 

 

Next, something quite different. NGC 743 is a sparse but fairly bright cluster of blue-white stars, with a formation that looks to me like a flock of migrating birds, or maybe a curved letter tau. I found very little information online about this cluster, although it is mentioned here https://iopscience.iop.org/article/10.3847/1538-3881/aaa45b which suggests a much large diameter -- 1 degree -- than my data. This means it is possible the cluster extends outside the zone shown in the GAIA plot also.

 

 

Finally, here is NGC 609. This is for me the best-looking of the three, with two squashed circlets (a pair of ear-rings, maybe?) containing nearly all reddish stars. The only paper I can find dealing explicitly with this cluster dates back to 1971. http://articles.adsabs.harvard.edu/pdf/1971AJ.....76..467L argues that it is similar to M11. Things have moved on since that time: it is interesting that it is described as "a conglomeration of very faint stars; a few of the brightest, of about mag 14, are discernable in a diffuse gray patch". As in NGC 559 the colour indices in this paper are at the redder end, with values up to 2.35. Most of those with index > 2 are very close to mag 15.

 

 

The GAIA data go much deeper. For a better comparison I produced an inverted grayscale image which I think indicates captures down to around mag 19 or so but no deeper. My stars are rather eggy (it was rather gusty) so most likely any further depth was limited by smearing the fainter captures into the background. 

 

(the odd appearance on my GAIA plot comes from processing artefacts when saving as .png, I suspect, resulting from the way I have mapped size/colour at the very faint end of the magnitude range -- this mapping is still a work in progress).

Thanks for looking

Martin

 

Edited February 4, 2020 by Martin Meredith

[Mike JW]

Hi Martin, impressive work. Happy memories for me of looking at these with the big Dob but of course not the colour. Mike

[RobertI]

Interesting captures Martin, NGC is reminiscent of the Coathanger Cluster to me,  but on a smaller scale of course. The blue/white stars are really striking against the background of redder stars.