image: This image, taken by OmegaCAM on the VLT Survey Telescope at Paranal Observatory, shows a section of the Ara OB1 stellar association. In the center of the image is the young open cluster NGC 6193, and to the right is the emission nebula NGC 6188, illuminated by the ionizing radiation emitted by the brightest nearby stars. view more
Credit: ESO
At the centre of the image is the open star cluster NGC 6193, containing around thirty bright stars and forming the heart of the Ara OB1 association. The two brightest stars are very hot giant stars. Together, they provide the main source of illumination for the nearby emission nebula, the Rim Nebula, or NGC 6188, which is visible to the right of the cluster.
A stellar association is a large grouping of loosely bound stars that have not yet completely drifted away from their initial formation site. OB associations consist largely of very young blue-white stars, which are about 100 000 times brighter than the Sun and between 10 and 50 times more massive.
The Rim Nebula is the prominent wall of dark and bright clouds marking the boundary between an active star-forming region within the molecular cloud, known as RCW 108, and the rest of the association [1]. The area around RCW 108 is made up of mostly hydrogen -- the primary ingredient in star formation. Such areas are also known as H II regions.
The ultraviolet radiation and intense stellar wind from the stars of NGC 6193 seem to be driving the next generation of star formation in the surrounding clouds of gas and dust. As cloud fragments collapse they heat up and eventually form new stars.
As the cloud creates new stars, it is simultaneously being eroded by the winds and radiation emitted by previous stars, and by violent supernova explosions. In this way, such star-forming H II regions tend to have a lifespan of just a few million years. Star formation is a very inefficient process, with only around 10% of the available material contributing to the process -- the rest is blown off into space.
The Rim Nebula also shows signs of being in the early phase of "pillar formation", meaning that in the future it could end up looking similar to other well-known star-forming regions, such as the Eagle Nebula (Messier 16, containing the famous Pillars of Creation) and the Cone Nebula (part of NGC 2264).
This single spectacular image was actually created from more than 500 individual pictures taken through four different colour filters with the VLT Survey Telescope. The total exposure time was more than 56 hours. It is the most detailed view of this region yet achieved.
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Notes
[1] Furthermore, this nebula has additional modest fame among astronomers, as a previous image was used as the cover of the DVD distribution of the collection of software for astronomers assembled by ESO: Scisoft, whose newest version was released a few weeks ago. It is therefore also known as the Scisoft Nebula.
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Links
Research papers on Ara OB1: http://adsabs.harvard.edu/cgi-bin/basic_connect?qsearch=Ara+OB1&version=1
Previous images of NGC 6193: http://www.eso.org/public/images/?search=NGC+6193
Photos of ESO's survey telescopes: http://www.eso.org/public/images/archive/category/surveytelescopes/
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Richard Hook
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Tel: +49 89 3200 6655
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Email: rhook@eso.org