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NGC 6520 and the Bok Globule Barnard 86 - a widefield
view |
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Object description:
NGC 6520 and the
Bok Globule Barnard 86 Our image shows the
bright open star cluster NGC 6520 and its strange neighbor, the dark cloud
Barnard 86, distantly reminiscent of the silhouette of a gecko. The cosmic pair
stands against the background of millions of shining stars in the brightest
part of our Milky Way. The stars of the Milky Way in this region of the sky are
so densely packed that hardly any dark sky can be seen between
them.
This part of the constellation Sagittarius hosts one of the most
densely populated star fields in the entire sky - the great Sagittarius star
cloud. The multitude of bright stars creates a dramatic contrast with black
dark clouds like Barnard 86 in the center of this image, in a wide-angle view
taken with our 5-inch EDFS refractor.
Barnard
86, a so-called Bok globule, was described by its discoverer Edward
Emerson Barnard as a "drop of ink on a bright stellar background." Barnard was
an American astronomer who discovered and photographed a large number of
comets, dark clouds, and a moon of Jupiter, among numerous other contributions
to science. An exceptionally gifted visual observer and astrophotographer,
Barnard was the first to make long exposures to study dark clouds. Barnard
included the object in his 1905
catalog of dark nebulae.
Barnard 86 lies in front of the star field as seen
from Earth. The small dust grains that make up the cold and dense dark cloud
absorb the light from the stars behind it, making the cloud opaque. Presumably
we see here the remnants of the molecular cloud whose collapse created the
nearby star cluster NGC 6520. The few stars that appear to be in the middle of
Barnard 86 in the image are actually in the foreground, between us and the dark
cloud.
As a young open cluster NGC 6520
contains many hot stars. Their blue-white glow reveals their young age. Usually
open clusters consist of a few thousand stars that formed at the same time and
are therefore of the same age. The clusters themselves usually stay together
for a comparatively short time - a few hundred million years - before slowly
drifting apart. The age of the stars in NGC 6520 is estimated to be "only" 150
million years. It is assigned to class I 2 m
according to Trumpler. Together with Barnard 86 the star cluster is located in
a distance of nearly 2000 light-years to our solar system. NGC 6520 was
discovered by Friedrich Wilhelm Herschel on May 24, 1784. |
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In
addition, our wide field view shows NGC 6540, a
globular cluster only 0.8 arc minutes in size. Long classified as an open
cluster, NGC 6540 is now classified as a globular cluster of concentration
class XI according to Shapley. The cluster lies at a distance of about 17,000
light-years from the solar system and was discovered by Wilhelm Herschel on May
4, 1784.
West of Barnard 86 lies the globular cluster
Djorg 2 (ESO 456-38) about which no further
information can be found.
We also have a higher resolution image of NGC
6520 and Barnard 86, taken with the PlaneWave Astrograph,
which we show here.
« Hier the thumbnail to load a large annoted image and a size
comparison to the full moon. |
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A little curiosity
The animated image
section shows a field directly east of NGC 6520. Both images are adjusted in
imaging scale, one image is from 07/24/2017 and was taken with the PlaneWave
Astrograph. The second image was exposed to the Astro Physics EDF-S refractor
almost exactly 2 years later - on 08/30/2019.
What is it ? Had we
discovered a previously unknown near-sun fixed star with large proper motion ?
Unfortunately not, according to databases it is 2 variable stars of Mira
type.
If you look at the
animation of the
complete image field, it is amazing how many variable stars
are "blinking" there. There is always something new to discover. |
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All Images and all Content are ©
by Franz Hofmann + Wolfgang Paech |