NGC 6520 & BARNARD 86 aka THE INK SPOT NEBULA

OPEN CLUSTER AND DARK NEBULA IN SAGITTARIUS (THE ARCHER)

(Image centered at: ra 18 h:04 m / dec - 27º 50')

September - 2024, Home Backyard in Martinez, Buenos Aires, Argentina

DATA

TYPE: OPEN CLUSTER AND DARK NEBULA

APPARENT DIAMETER: NGC 6520: 6 arc minutes / BARNARD 86: 4 arc minutes 

APPARENT MAGNITUDE (V): NGC 6520 7.6 / BARNARD 86: N/A   

DISTANCE: 5235 light years

IMAGE INFORMATION

INSTRUMENT: 6" ORION OPTICS UK (Ultra Grade Optics) w/Sky Watcher Coma Corrector (0.9x) working at at f4.5

CAMERA: QHY 183 MONO

MOUNT: VIXEN GDPX, OAG with QHY 5II Mono

FILTERS: BAADER LRGB Set Antlia HA 3nm filter

SKY CONDITIONS: urban skies - Bortle 8

EXPOSURES: LRGB (45,45,45,45)

OBJECT DESCRIPTION AND IMAGE SESSION

NGC 6520 is an open cluster of stars in the southern constellation of Sagittarius, about 4° to the east of the galactic center. It has an apparent visual magnitude of 7.6 and an angular size of 6.0′. NGC 6520c can be viewed with binoculars or small telescopes. Just to the west of this cluster is the dark nebula Barnard 86, aka as the "Ink Spot Nebula". Both objects are viewed against the dense starry background of the Large Sagittarius Star Cloud. This cluster is located at a distance of approximately 5,235 ly from the Sun.

This is a young open cluster of stars with age estimates yielding a values of 150 to 190 million years. However, the presence of stars with a spectral class of B4 and B5 suggest a much younger age of 60 million years. The estimated mass of this cluster is 364±54. The cluster and the nearby dark nebula Barnard 86 have radial velocities that differ by 30 km/s, and hence may be unrelated. The dust of Barnard 86 is visible only because it blocks out light from the myriads of stars beyond.

The majority of old stars in our Galaxy, as in most others, appear yellowish on photographs. This is because the hotter, bright blue stars have relatively short but spectacular lives. We see these old stars in vast numbers as the brightest patches of the Milky Way and a telescope reveals them to be as numerous and crowded as grains of sand, especially in this area close to the celestial equator.