This image from the Wide Field Imager on the MPG/ESO 2.2-meter telescope at the European Southern Observatory’s (ESO) La Silla Observatory in Chile shows the bright star cluster NGC 6520 and its neighbor, the strange gecko-shaped dark cloud Barnard 86. This cosmic pair is set against millions of glowing stars from the brightest part of the Milky Way — a region so dense with stars that barely any dark sky is seen across the picture.
This part of the constellation Sagittarius the Archer is one of the richest star fields in the whole sky — the Large Sagittarius Star Cloud. The huge number of stars that light up this region dramatically emphasizes the blackness of dark clouds like Barnard 86, which appears at the center of this new image.
This object, a small isolated dark nebula known as a Bok globule, 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, and one of Jupiter’s moons, among other achievements. 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 dearth of stars or a window onto a patch of distant, clearer sky. However, this object 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 opaque. It is thought to have descended from the remnants of a molecular cloud that collapsed to form the nearby star cluster NGC 6520, seen just to the left of Barnard 86 in this image.
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 live relatively short lives, on the order of several hundred million years, before drifting apart.
The incredible number of stars in this area of the sky muddles observations of this region, making it difficult to learn much about the cluster. Scientists think that NGC 6520’s age is around 150 million years and that both this star cluster and its dusty neighbor are some 6,000 light-years from the Sun.
The stars that appear to be within Barnard 86 are, in fact, in front of it between the dark cloud and us. Although it is not certain whether this is still happening within Barnard 86, many dark nebulae have new stars forming in their centers as seen in the famous Horsehead Nebula, the striking Lupus 3, and to a lesser extent in another of Barnard’s discoveries, the Pipe Nebula. However, the surrounding dusty regions block the light from the youngest stars, and they are only visible in infrared or longer-wavelength light.