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Astrophoto: NGC 6755 and 6756 by Bernhard Hubl.
Stars arise at the heart of great interstellar clouds that have collapsed under their own weight. These free-floating clouds are comprised principally of hydrogen, the most common element in the universe, plus a smaller amount of dust from previous generations of suns. The size of a typical cloud is so enormous that it takes light many years to travel from one end to the other. The amount of material within an interstellar cloud is also staggering to imagine- so much material is brought together that multiple star births are common. Seen from the outside, a collapsed cloud can appear dark and foreboding. But, inside they are full of light from the hot, newly formed stars that have been incubated. Over time, the cloud will part or dissipate to reveal a new group of stars, similar to the two clusters seen in the accompanying picture.
Newly formed stars begin their existence in a gravitational embrace. They huddle together in a close but random formation that is called an open star cluster. Over time, the immense radiation produced by the cluster will blow back the clouds in which they formed and, at the same time, they will start to wander out of the cloud and from each other. Many star clusters are still enshrouded in faint whiffs of the cloud material that spawned them. For example, long exposure images of the Pleiades reveal faint nebulosity- the remains of the amniotic material present at their birth.
The number of constituents within a star cluster is based on the size of the cloud and the amount of time that has passed since the group was formed. This can range from as few as ten to over several thousand but many are numbered in the hundreds. Our view of open star clusters, therefore, is just a snapshot. Over time, the cluster will seem to diminish in size as each star begins to go its own way or meet its own fate.
But even as they part company, each star continues to travel more or less in the same general direction. Widely dispersed former star clusters are called stellar associations. These groups are more difficult to identify because the distance separating each star can become very large. Perhaps that’s why the first association was not identified until 1947, but today, several associations are now known. For example, most of the stars in the northern sky’s Big Dipper are actually former members of an open star cluster that have spread out to form an association of suns moving roughly in the same direction.
The photograph that accompanies this article is of an area in the sky within the southern constellation, Aquila. This constellation is easily visible during the summer in all but the most extreme northern latitudes. Far behind the stars that make up this constellation are the remote star clouds of our galaxy’s spiral arms.
The two star clusters seen in this picture, one in the middle and the other above and to the left of center, are about 5,000 light years in the distance. Although each cluster can be viewed through a modest telescope or pair of binoculars, neither appears very distinct- their distance from Earth, intervening interstellar dust and the riot of stars that lie further behind conspire to dim our view and reduce the contrast of these two groupings. As a result, images of either are few and far between. This picture is noteworthy because it is one of the only depictions that clearly show both.
This beautiful and deep picture was produced by Bernhard Hubl over two consecutive nights beginning July 17, 2006 from his imaging site in Schlierbach, Austria. Bernhard produced this image using a small four inch refractor and a 1.6 mega pixel astronomical camera. The total exposure time was 5.5 hours.
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