Very dark isolated interstellar clouds of very cold gas like black gaps have puzzled astronomers for more than a century.
Looking at the sky in the direction of the constellation Sagittarius, it is clear that there are extremely dark, opaque knots of
gas and dust especially in the region toward the center of our Milky Way.
This observation inspired Herschel in 1884 to say that there was undoubtedly a hole in the sky. In these areas no stars could be seen,
only featureless black. No obvious star formation is taking place in the vicinity of these isolated blank regions in the sky.
The phenomenon was named Bok globules, after the Dutch-American astronomer Bart Bok (1906 - 1983), who proposed their existence in the 1940's.
Today, they are even dubbed "holes in the heaven" because they appear like holes in the stellar background.
They can be studied with infrared and radio techniques .
Astronomers at the Harvard-Smithsonian Center for Astrophysics, studied thirty-two Bok globules using infrared, submillimeter,
and millimeter telescopes, including the Spitzer Space Telescope.
They found warm cores in twenty-six of these globules, and eighteen of them could be approximately age dated based on their
dust emission characteristics, and are from about one hundred thousand to two million years old.
Moreover, the team found that nearly two-thirds of these globules showed evidence of multiple stars, and in most of
these cases the several stars appeared to be of different ages, with only three cases of coeval young stars.
Bok Globule containing HH 46/47
Visible light view of a dark cloud (known as a 'Bok globule') which is illuminated by the nearby Gum Nebula.
Located at a distance of 1140 light-years and found in the constellation Vela, this cloud contains the protostar
HH46/47 which hidden from view in this visible-light image.
Molecular "Black Cloud" B68 toward the constellation Ophiuchus sillouetted against a region very rich in stars.
The eerily dark surroundings help make the interiors of molecular clouds some of the coldest and most isolated places in the universe.
That no stars are visible in the center indicates that Barnard 68 is relatively nearby, with measurements placing it about 500 light-years
away and half a light-year across. It is not known exactly how molecular clouds like Barnard 68 form, but it is known that these clouds are
themselves likely places for new stars to form. Credit: FORS Team, 8.2-meter VLT Antu, ESO
Bok globule - NGC 281
NASA's Hubble Space Telescope has photographed dense knots of dust and gas in our Milky Way Galaxy.
This cosmic dust is a concentration of elements that are responsible for the formation of stars in our galaxy
and throughout the universe. These dark, opaque knots of gas and dust are called "Bok globules," and they are
absorbing light in the center of the nearby emission nebula and star-forming region, NGC 281. These images were
taken with Hubble's Advanced Camera for Surveys in October 2005. NGC 281 is located nearly 9,500 light-years away
in the direction of the constellation Cassiopeia. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
An isolated cloud of dust called a Bok globule. No obvious star formation is taking place in its vicinity. They are absorbing light in
the center of the nearby star-forming region known as NGC 281, located nearly 9,500 light-years away in the direction of the constellation Cassiopeia.
It is thought that globules represent an early stage in the star formation process. For an interstellar cloud to produce a dark nebula,
it must lie between the Earth and either a dense star swarm or a bright nebula.
A closer observation of a dark nebula shows that stars are not completele missing in the direction of the nebula. In fact, there are
often bright stars that lie between Earth and the nebula. Also faint stars can be seen but in smaller numbers.
Reflection nebula NGC 1999
It lies about 1500 light-years away in the constellation Orion, just south of Orion's well known emission nebula, M42. Credit: Hubble Heritage Team (STScI) and NASA
Extending right of center, the ominous dark nebula is actually a condensation of cold molecular gas and dust so thick and
dense that it blocks light.
From our perspective it lies in front of the bright nebula, silhouetted against the ghostly nebular glow. New stars will likely
form within the dark cloud, called a Bok globule, as self-gravity continues to compress its dense gas and dust.
BHR 71 a well-isolated Bok globule spans about one light year and is located at a distance of approximately 200 parsecs from the Sun.
In visible light, BHR 71 is just a large black structure. The burst of yellow light toward the bottom of the cloud is the only indication that stars might be forming inside.
Most stars in our Galaxy are part of binary star systems, but few have ever been seen in formation.
Recent observations of dust-darkened Bok Globule BHR 71, however, show evidence for two young stars forming
deep in the cloud, likely close enough to form a binary. The brighter embedded star -- not visible here -- is
about 10 times as bright as the Sun and drives the jet that swept out the empty lane. The above four-color image
was taken with a Very Large Telescope in Chile. Credit & Copyright: J. Alves (ESO), E. Tolstoy (Groningen), R. Fosbury (ST-ECF), & R. Hook (ST-ECF), VLT
BHR 71 as seen by the Spitzer Space Telescope. Left: Blue is 3.6 ?m,
green is 4.5 ?m, and red is 8.0 ?m. Right: The Spitzer image combined with the optical
image of Figure 5. Courtesy NASA/JPL-Caltech/T. Bourke/c2d Legacy Team.
For most dark nebulae, about 75% of the starlight is lost. In some nebulae, however, more than 95% of the light is stopped by the dust.
The Coalsack in the constellation Crux
After observing the Coalsack, very dark large nebula, a French astronomer, Abbé Nicolas Louis de Lacaille (1713 -1762) said:
"One can add among the phenomena which strike the eye of anyone looking at the southern sky, a space of about 3 degrees in every
direction which seems intensely black in the eastern part of the Southern Cross. This is caused by the contrast with the
brightness of the Milky Way which surrounds this space on all sides."
The Coalsack, located 600 light years away from Earth, in the constellation Crux, is an easily observed absorption of
interstellar dust is the Coal Sack (or Coalsack). An inky-black cloud of dust and gas spans approximately 50 light years.
Stars are probably condensing deep inside the Coalsack, but their light has not yet broken through the cloud's very dense exterior.
It reflects most of the light from the starsbehind it, but a fraction is absorbed causing the Coalsack to glow feebly with about a
tenth of the brightness of the surrounding Milky Way
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