PDA

View Full Version : Object of the Week, September 15, 2019 – NGC 6717/Palomar 9 - the forgotten globular cluster



Uwe Glahn
September 26th, 2019, 07:26 PM
Palomar 9
= NGC 6717, ESO 523-14
Type: Globular Cluster
Class: VIII

RA: 18h 55m 06.2s
Dec: -22° 42' 01"
Constellation: Sgr

Size: 5.4'
Mag: 8.4V
Brightest Star: 15.0V Skiff (14.0V Archinal)
Distance (Galactic center): 7800 l.y.
Distance (sun): 23000 l.y.

The history of the obscure sounding Globular Cluster began simple - with the discovery of William Herschel in 1784. He described the object as "...nebulosity" but saw similar to his son John stars in it.
The confusion began 1901 when Bigourdan published his results with the 12,4-inch refractor of Paris Observatory and wrote: "nebulous star mag 13, situated near NGC 6717 in PA 35° [NE], d = 15"" Did he saw the knot in the NE as a individual object or simply the same object NGC 6717? He got an IC entry and the number 4802.
But what type of object was NGC 6717? 1931 Collinder accepted NGC 6717 rightly as a globular star cluster. But the story was not over.
Unfortunately (or not) George O. Abell missed the entry and cataloged this star cluster under number "9" in his compilation of 15 "Globular Clusters...discovered on the National Geographic Society-Palomar Observatory Sky Survey" in 1955.

Physical the metal-poor cluster lies near the Galactic center in the bulge. Astronomers observed a very concentrated structure and suspect a so called "post-core-collapse morphology".

In the telescope NGC 6717 is in my mind one of only a few clusters which are worth a visit for nearly all apertures. In small apertures to see the cluster at all and in big telescopes to resolve as many stars as possible. I visited the cluster often and got results from 4-inch to 27-inch, see my Palomar project (http://www.deepsky-visuell.de/Projekte/PalomarGC_E.htm).

DSS red 10'x10'
3640

PanSTARRS 4'x4'
3641

HST
3642

sketch: 16", 515x, NELM 7m0+ (Austria, 47°N)
3643

But now it is your turn.
Give it a go and let us know!

Raul Leon
September 26th, 2019, 11:35 PM
Hi here's my observation from 9/18/2009 : Palomar 9 aka ngc 6717 is a globular cluster in the constellation Sagittarius; magnitude 8.4 size: 5.4' it is small, so high magnification is needed to get a good look ; it resolved into about 6 dim stars. I used a 7mm Nagler at 250x magnification with my 14.5 Starstructure Dob f/4.3 3644

wvreeven
September 27th, 2019, 09:27 AM
My notes from 10 July 2008 in southern France using my old home made 15 cm Newton telescope:

"This globular cluster can be found a few arc minutes due south of a strikingly yellow, very bright star. To the north east a star is visible attached to it. The globular cluster itself is visible as a faint smudge without a clear core."

Judging from the photos posted by Uwe, the star to the north east actually is the double star which I wasn't able to split.


Wouter

Ivan Maly
September 27th, 2019, 08:33 PM
I have a sketch from this summer, D=508 mm, F=2000 mm, ocular 5.5 mm, sky m21.74 /sq. arcsec, turbulence 1-2 arcsec.

3645

lamperti
September 27th, 2019, 09:59 PM
Ivan: your sketch reminds me of my description from 1993, with a 13" at 240x: "A globular cluster, fairly bright. Just about resolvable. Reminded me of a galaxy."

kisspeter
September 28th, 2019, 04:36 PM
I have a 16" drawing from 2006 (196x, "Ny" stands for West). This was actually the first Palomar globular I saw. It's a little bit strange cluster. It has a few bright stars and a lot of very faint ones (which I didn't see). But not much in between. It looked more like an open cluster in the 16" scope.
3646

MikeWiles
September 30th, 2019, 05:10 PM
My observation from this past Saturday night with a 20" f/3 Spica Eyes dob from Portal, Arizona.

10mm Ethos, 177x, 33.9' - Small, round cluster, brighter than I expected it would be. Hard to tell if it's an open or a globlular. Cluster has a grainy appearace, and 6 stars are visible across the 2' face.

6mm Ethos, 295x, 20.3' - At this magnification the cluster appears slightly elongate E/W - about 1/3 again as wide in the Ease/West. More granular appearance and about a dozen stars are resolved across the face. 5th magnitude Nu Sag is distracting just a few arc minutes to the north. I could be imagining, but is seems to be darker down in a north/south direction just off the west side....there are fewer stars than elsewhere in the field.

Bertrand Laville
September 30th, 2019, 06:23 PM
Hi All,

Two drawings with 10" and 20".
No stars with the 10" but IC 4802 well seen, 14 stars with the 20" Obsession in a perfect Namibian sky.
Reports at : http://www.deepsky-drawings.com/pal-09-ngc-6717-t254/dsdlang/fr

Clear skies
Bertrand
http://www.deepsky-drawings.com

3649

3650