This image from the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s 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 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. Bok globules were first observed in
the 1940s by astronomer Bart Bok. They are very cold, dark clouds of
gas and dust that often have new stars forming at their centres. These
globules are rich in dust that scatters and absorbs background light, so
they are almost opaque to visible light.
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.
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 neighbor 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
(eso0202), the striking object Lupus 3 (image below) and to a lesser
extent in another of Barnard’s discoveries, the Pipe Nebula (eso1233).
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.
Source: The Daily Galaxy via ESO
No hay comentarios:
Publicar un comentario