SET AGAINST A BACKGROUND of millions of glowing stars from the brightest part of the Milky Way, a region so dense with stars that barely any dark sky can be seen, lies the bright star cluster NGC 6520 and its neighbour, the dark nebula Barnard 86.
This part of the constellation Sagittarius is one of the richest star fields in the whole sky – the Large Sagittarius Star Cloud. The huge number of stars dramatically emphasise the blackness of dark clouds like Barnard 86.
Known as a Bok globule, Barnard 86 was described as “a drop of ink on the luminous sky” by its discoverer Edward Emerson Barnard, an American astronomer who discovered and photographed numerous comets, dark nebulae, one of Jupiter’s moons, and made many other contributions. An exceptional visual observer and keen astrophotographer, Barnard was the first to use long-exposure photography to explore dark nebulae.
Through a small telescope Barnard 86 looks like a hole in the star fields, or a window onto a patch of distant, clearer sky. However, it is actually in the foreground of the star field – a cold, dark, dense cloud made up of small dust grains that block starlight and make the region appear black. It is thought to have formed from the remnants of an interstellar cloud that formed the star cluster NGC 6520, seen just to the left of Barnard 86.
NGC 6520 is an open star cluster that contains many hot stars that glow bright blue-white, a telltale sign of their youth. Open clusters usually contain a few thousand stars that all formed at the same time, giving them all the same age. Such clusters usually only live comparatively short lives, on the order of several hundred million years, before drifting apart.
Both NGC 6520 and Barnard 86 are thought to lie at a distance of around 6,000 light-years from our Sun. The stars that appear to be within Barnard 86 are actually in front of it, between us and the nebula.
The image was taken with the Wide Field Imager, an instrument mounted on the MPG/ESO 2.2-metre telescope at the ESO La Silla Observatory.
Adapted from information issued by ESO. Images courtesy ESO / Digitised Sky Survey 2. Acknowledgement: Davide De Martin
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