The old open cluster NGC 6791 and bright carbon star U Lyr

[Martin Meredith]

NGC 6791 was thought in the 1980s [1] to be the oldest known open cluster, with an age exceeding 8 billion years. With new discoveries and more accurate distance estimates, it is now considered to be only the 7th oldest, with Berkeley 17 hitting a round 10 billion, and 3 ESO clusters reaching back further still. But by any measurement, NGC 6791 is ancient. 

Situated in Lyra at a similar declination to that of Vega and around 6 degrees in the direction of Cygnus, NGC 6791 is a rich collection of mainly faint stars with a moderate amount of reddening, an obvious cluster that is clearly distinguishable from the foreground star field. 

This is a 2m40s total exposure consisting of 8 x 5s subs in each of LRGB, live combined, observed with an 8" f4 reflector and Lodestar X2 mono CCD camera on 27th October 2021.

There are a range of stellar colours on display within the apparent radius of the cluster itself, with some peachy (presumably foreground) stars to the south, and a couple of almost blue stars forming a diagonal either side of the centre -- again, presumably foreground members. The B-V colour indices of these blue-ish stars are in the range 0.1-0.3. The lower of the two is a pulsating delta Scuti variable while the upper is a rotationally variable star.

However, the reason I've made the annulus transparent is to bring the deep red star at the upper right into the picture. While it might appear that this is merely a side-effect of over-saturated colours, in fact I turned saturation down to near minimum for this shot. This is the carbon star U Lyr with a spectacular colour index of over 4.6! The star varies from mag 8.3 to 13.5 in 451 days. It is hard to say where it is in the cycle without doing some proper photometry, but given that the aforementioned peachy stars are mag 10.5 I'd say it must be closer to max than min (how's that for caution). U Lyr is also an optical double star -- designated ES 2489 after its discoverer, T. E. Espin in 1930 -- with a separation of just over 10", a magnitude difference to secondary of nearly 3, and a position angle of 190 degrees [2]. It is visible nestling in the artefactual halo just below the carbon star.

Thanks for looking

Martin

 

[1] http://articles.adsabs.harvard.edu/pdf/1981AJ.....86.1332H

[2] https://www.stelledoppie.it/index2.php?iddoppia=80236

Edited November 1, 2021 by Martin Meredith
inserted a comma

[Paul M]

Another fascinating image and write up Martin. Every time I look at one of your findings I make a mental note to track the object down myself. Unfortunately the weather is causing a significant backlog

[Martin Meredith]

Thanks Paul. I heard about the weather you've been having in the UK. Here we've been pretty lucky so far (not to say unseasonably warm and dry until a few days back). Wishing you a clear spell to seek out these objects. Visually, this one might fit into a widefield view along with theta Lyr. 

Martin

[RobertI]

Excellent observation and write up Martin. A fascinating cluster with one very blue and super red star to boot! Definitely worth tracking down.

[Mike JW]

Hi Martin,

It amazes me that after all those billions of years the cluster is still together despite its many journeys around the galaxy. I looked at my notes from observing it visually with the 20" - faint loose cluster!. Great to see it in more detail. As to carbon stars - always pleasurable to track them down visually.

Mike