Object of the Week July 15, 2018 - NGC 7006

Object of the Week July 15, 2018 - NGC 7006

NGC 7006, C 42
Type: Globular Cluster
Constellation: Delphinus
RA: 21h 01m 29.0
Dec: +16° 11’ 18”
Size: 3.6’
Mag(v): 10.6


There are two cataloged globular clusters in the constellation Delphinus. The most prominent one is NGC 6934 which I wrote about as the Object of the Week three years ago here. The other, less prominent globular cluster is NGC 7006, this week’s Object of the Week.

NGC 7006 was discovered August 21, 1784 by William Herschel and later observed by his son, John Herschel on October 11, 1825. Per W Herschel, “Very bright, small, round, gradually much brighter in the middle. Resolvable.” Per Dreyer, “NGC 7006 (= GC 4625 = JH 2097 = WH I 52, 1860 RA 20 54 58, NPD 74 21.5) is "bright, pretty large, round, gradually brighter middle. The position precesses to RA 21 01 30.3, Dec +16 11 19, well within the outline of the cluster, the description fits and there is nothing else nearby, so the identification is certain.”

NGC 7006 is much smaller and fainter than the more prominent globular in Delphinus, NGC 6934. To a large extent this is due to NGC 7006 being the most distant object of its kind in the Milky Way galaxy, approximately 135,000 light-years from Earth. It was the work of Harlow Shapley that first established the distance of this globular cluster. His observations of 11 RR Lyrae stars in the NGC 7006 showed the cluster to be about five times as remote as M3 or M5, the most distant globulars known at that time (1920s).

Despite its small size of 3.6’ NGC 7006 should be detectable as something other than a star in a 6” telescope. Moving up to a 10” scope, I noticed a slight brightening towards the center of the globular, however there was not noticeable resolution of stars within the cluster. Moving up to a 15” scope, I’m able to discern a smattering of individual stars, however I don’t know whether they are part of the globular or just foreground stars.

Christian Luginbuhl and Brian Skiff in Observing Handbook and Catalogue of Deep Sky Objects provide the following description of the globular NGC 7006. “At a distance of nearly 40 kiloparsecs, this globular cluster is faintly visible in 15 cm. The faint glow is about 1’ diameter with a relatively bright core. With 25 cm it is 1.25’ diameter and shows no resolution even in good seeing. The haze is broadly brighter across the center, exhibiting no sharp central condensation. A mag. 14 par with 20” separation in pa 85° is visible 1.5’ S. In 30 cm the cluster is 1.5’ diameter. A few faint stars are resolved over a granular background at 475 x.”

Brent Archinal and Stephen James O’Meara in Deep-Sky Companions: The Caldwell Objects provide this description of NGC 7006 as observed through the 31” reflector at the Warren Rupp Observatory in Mansfield, Ohio. “The cluster was a scintillating mass of dim starlight. At high power we saw an unresolved, starlike inner core surrounded by a dark ring that defined the inner edge of a finely resolved, though jagged, outer core. With averted vision thin arms radiated out from the stellar ‘hole’ like the limbs of a starfish wrapped around a rock. The outer halo appeared loose and fractured, as if its shell were so old and brittle that pieces of it were flaking off into space.”

Attachment 3125
12 arcmin wide SDSS image

Attachment 3126
6 arcmin wide SDSS image

Attachment 3127
3.2 arcmin wide HST image

For those of you using large aperture you should try for three galaxies that lay less than 8” from the NGC 7006. They are PGC 65907 m15.69(v), PGC 1501723 m16.27(v), and PGC 65908 m16.08(v).

Interesting bit of trivia: NGC 7006 appears in the science fiction novel Beyond the Farthest Star by Edgar Rice Burroughs (written in 1940 but not published until 1964), where it is used as a point of reference by the inhabitants of the planet Poloda to determine the approximate location of Earth.

Now it is your turn. Give it a go and let us know.

I have an observation of this cluster from September 16, 2010, with my 20" telescope, which I only had for a few months back then:

A small globular cluster which is not resolved at 160x but partially at 366x.

I didn't know about the galaxies back then so I'll need to revisit this cluster.

105/610mm apo, 47x
Small, faint but obvious, some brightening toward the center.

130/819mm apo
37x: Fairly small and faint, but grows brighter toward the center.
117x: A pair of extremely faint stars lies just off the edge, approximately south by west of center. The cluster is about 2 arcminutes across and slightly elongated east-west. No resolution.

203/1791mm Newt, 172x
Round, a little fainter than NGC 6934, small, brightening toward the center.

254/1494mm Newt
43x: Fairly bright, small, round fuzzy spot with a large brighter core.
166x: Large, bright, mottled core with a faint halo that nearly doubles the diameter. About one and a half arcminutes. Lies inside a triangle of equally faint stars, mag 13 1/2, the southern of which is a wide, matched pair.

381/1727mm Newt
49x: Small but bright. Grows considerably brighter toward the center.
216x: Faint halo spans 3 arcminutes with a star on the south edge and a single star on the north edge. Star just off northwest edge and another just off east edge. Several extremely faint stars sparkle in the halo and some in the outer core. The core spans about one and a half arcminutes, is mottled, and grows brighter toward the center.

With my old 10 "a very compact and bright globular, mottled appearance, more obvious in the periphery.
It has an acceptable size even for small openings. With the traveler 4 "it is a compact and bright globular, it did not look like the mottled appearance as in more opening, I have to try the 16"
Attachment 3130

With a 10" at 86x, "Approximately 3-4 field stars surround some nebulosity. Quite faint. Burnham's = remotest globular cluster."
With a 20" at 272, "Very tight and no resolution."

I can add a 4" drawing from 2012 (67x, 48' field, "Ny" is West):
Attachment 3131
Attachment 3132

Obviously I saw no details with such a little scope.

I am however surprised about the difficulty of resolution with ~20" scopes that you mention above. We took a look at NGC 7006 after I finished my drawing with a 20" (I don't remember the enlargement, but certainly big enough). And I clearly remember that the globular was easily resolved into many many stars.

Transparency and Seeing were both very mediocre: 5/10 for both.

"I am however surprised about the difficulty of resolution with ~20" scopes that you mention above. We took a look at NGC 7006 after I finished my drawing with a 20" (I don't remember the enlargement, but certainly big enough). And I clearly remember that the globular was easily resolved into many many stars.[/QUOTE]

NGC 7006 is much smaller and fainter than the more prominent globular in Delphinus, NGC 6934. To a large extent this is due to NGC 7006 being the most distant object of its kind in the Milky Way galaxy, approximately 135,000 light-years from Earth. It was the work of Harlow Shapley that first established the distance of this globular cluster...

I wanted to add that although NGC 7006 may once have held the record as the most distant Milky Way GC, much more distant ones have been found in recent decades.

AM-1 in Horologium (confirmed as a globular in 1984) is generally stated as the most distant globular at 400,000 light-years. I've seen AM-1 from Australia in a 24-inch as a very faint, round glow, 20" diameter, with a low surface brightness. It required averted vision and appeared perhaps 15th magnitude with no resolution.

But the unusual "young" (age ~7.5 Gyr) globular Laevens 1 = Crater Cluster, discovered in 2014, is at a heliocentric distance of over 470,000 light-years! I observed it in Jimi's scope in 2016, but I know several people of this list observed it immediately after the discovery was announced in 2014!