|Double Star Section Circulars|
Thank you to everyone that attended our Annual Meeting. As requested, Owen and David have given me their presentations which can be downloaded from the meeting archive page.
July 2016 - Picture of the Month
Looking towards the centre of the Milky Way
This image was provided courtesy of Robert Gendler, please click on it to see a larger version.
I've chosen another wide-field image for the Picture of the Month. This fantastic mosaic spans at least three constellations and is packed with deep-sky objects including open clusters, bright and dark nebulae and a globular cluster too. For us beginners, the Messier catalogue is well represented by, I think, 8 objects.
Most importantly this is the centre of our own galaxy and I find the view staggering. It's not easy to see from my northern latitude in the UK, and light pollution robs us of a great deal, but I find this image is mesmerising. What deep-sky observing is made of.
The image in this article doesn't to justice to Rob Gendler's work: it's just too small. Click on it to see a larger version, but better yet, visit Rob's website to learn more about the Galactic Center and view some really huge versions. There's one that's annotated too.
James Whinfrey - Website Administrator.
Object of the Season (Spring 2016)
Dark nebula B 86 and open cluster NGC 6520 in Sagittarius
Dark nebula B 86 and open cluster NGC 6520 in Sagittarius will be announced in DSO 171, and the results will be published in DSO 173.
This interactive image of the NGC 6520 and B86 was provided by the Digitised Sky Survey using Aladin Sky Atlas.
Wolfgang Steinicke - Nebulae and Clusters Section Director
July 2016 - Galaxy of the Month
NGC 6906 in Aquila
This interactive image of the galaxies around NGC 6906 was provided by the Sloan Digital Sky Survey using Aladin Sky Atlas. We have a widefield finder chart should help you locate these galaxies, and a close-up chart for the image above.
Summer is never the best time for galaxies but if you do get a clear moonless night in the semi dark then this month’s galaxy is NGC 6906 in Aquila.
First discovered in 1863 by Albert Marth using William Lassells 48” speculum mirror telescope in Malta this galaxy makes a nice pair with the un-related galaxy NGC 6901 (also discovered by Marth). Interestingly both galaxies are marked as part of a pair in NED but I think this is erroneous.
NGC 6906 itself is actually part of a sparse group of galaxies as listed in the Lyon Galaxy Group catalogue (LGG). Listed as number 435 in this catalogue the group consists of 3 galaxies, NGC 6906 along with UGC 11551 and UGC 11555. Both of the latter two are actually across the border in the constellation of Delphinius. The group is perhaps 1.6 degrees across on the sky. Interestingly all the galaxies are spirals. The distance is to NGC 6906 is perhaps 60 Mpc, so probably similar to the other two galaxies in the group.
NGC 6901 is perhaps at a distance of 69 Mpc, so 9 Mpc further away than NGC 6906. NGC 6901 is also incorrectly labelled as IC 5000 due to a bad set of co-ordinates published by Marth and Bigourdan rediscovering it and thinking it was a new galaxy. It is also listed as IC 1316 in SIMBAD but this is now recognised as an error and the designation has been deleted from NED. SIMBAD also lists NGC 6901 as being part of a group of galaxies but I have been unable to determine what other galaxies might be involved. NGC 6906 has also had an IC number assigned to it in IC 5006 but this is really a double star.
Both NGC 6901 and NGC 6906 are spiral galaxies with NGC 6901 being much further away. There is also a much fainter edge on spiral in the same field but at mag 16 it only has a LEDA number and will probably require a very large telescope to spot visually. NGC 6901 was host to SN 2004da which was discovered by the UK amateur Tom Boles.
Owen Brazell - Galaxy Section Director
July 2016 - Double Star of the Month
39 Dra (17 57 12.56 -30 22 24.80) can be found about 4 degrees following and slightly north of xi Dra in the 'head' of Draco. It is also known as b Dra.
The binocular user will see a pair of mag 5.1 and 8.0 stars separated by about 89". Any aperture larger than 6-cm will show the companion to A which William Herschel found in 1780 and allocated to his class 1 i.e. between 0 and 2". Since that time the pair has widened with increasing position angle and in 1993 was found at by the writer at 351° and 3".9. The AB stars are magnitudes 5.1 and 8.1 so it requires good seeing to see B clearly.
This is a quintuple system since all 3 bright stars are physically connected and both A and B are spectroscopic binaries. Surprisingly, although the motion in AB amounts to just 25 degrees in 230 years, a highly speculative orbit with a period of 3962 years appears in the USNO Sixth Orbit Catalogue.
In October 2011, John Nanson found
PZ 6 (17 59 05.28 -30 15 10.8) is a beautiful pair which lies about 1.5 degrees due west of gamma Sagittarii. In the 1st edition of the Cambridge Double Star Atlas it is given as PZ 4 but is corrected to PZ 6 in the second edition.
An observation of this pair on the evening of 5 September 2013 using the Johannesburg refractor showed the colours of the stars to be deep yellow and lilac.
The WDS gives spectral types M1Ib and G8II so this is a rare pairing of a supergiant and a giant. There is little motion between the two stars and the primary star sits a little over 1,000 light years away but with an uncertainty in the distance of 30%.
The stars are magnitudes 5.4 and 6.0, and the current separation of 5".7 appears to indicate that the stars have been slowly closing since the first measure in 1826. A 13.2 mag star at 25" was measured by the writer in September 2013.
By moving the telescope another 1.5 degrees further west, and crossing the border from Sagittarius into Scorpio, the observer will come across PZ 5 - another bright wide pair easily resolvable in a small aperture. The stars are mags 6.7 and 8.2 and both white - the spectral types are A3 and B9. Note, however, that PZ 5 is not labelled in CDSA 2 whilst it is in the first edition. The position in 2013 as determined by the writer was 104° and 5".7. On the same night a mag 11 companion was also noted at a distance of 49".
Bob Argyle - Double Star Section Director