13 February 2013

Bok Globule Barnard 86 - A Dark Cloud Against A Sea Of Stars

Star Cluster NGC 6520
The Wide Field Imager takes a picture of Barnard 86, a Bok Globule, set against a sea of stars from The Large Sagittarius Star Cloud.

Star clusters are groups of stars held together by their own gravitational field. These star clusters, formerly known as star clouds, are made up of more than a hundred thousand stars that are more than a million years old.

One prominent star cluster is the Large Sagittarius Star Cloud. It is about 600 light years wide and the multitude of stars in the area makes it the "most dense concentration of individual stars visible using binoculars". Within the Large Sagittarius Star Cloud is Barnard 86. It is a cold, dark cloud of gas and dust that absorbs background light making it almost opaque to visible light. These objects are known as Bok Globules.

It is this area of the sky that is imaged above, taken by the Wide Field Imager at the La Silla Observatory in Chile.

Barnard 86 Set Against the Large Sagittarius Star Cloud

This part of the constellation of 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 emphasise the blackness of dark clouds like Barnard 86, which appears at the centre of this new picture from the Wide Field Imager, an instrument mounted on the MPG/ESO 2.2-metre telescope at ESO's La Silla Observatory in Chile.

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, 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 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 formed 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.

Video: Vista Survey Telescope Image of Star cluster NGC 6520 and the dark cloud 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.

The incredible number of stars in this area of the sky muddles observations of this cluster, making it difficult to learn much about it. NGC 6520's age is thought to be around 150 million years, and both this star cluster and its dusty neighbour are thought to lie at a distance of around 6000 light-years from our Sun.

The stars that appear to be within Barnard 86 in the image above are in fact in front of it, lying between us and the dark cloud. Although it is not certain whether this is still happening within Barnard 86, many dark nebulae are known to have new stars forming in their centres -- as seen in the famous Horsehead Nebula, the striking object Lupus 3 and to a lesser extent in another of Barnard's discoveries, the Pipe Nebula. However, the light from the youngest stars is blocked by the surrounding dusty regions, and they can only be seen in infrared or longer-wavelength light.


European Southern Observatory
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