April 25, 2012
La Silla Observatory Eyes A Cluster Within A Cluster
The star cluster NGC 6604 is shown in this new image taken by the Wide Field Imager attached to the MPG/ESO 2.2-meter telescope at the La Silla Observatory in Chile. It is often overlooked in favor of its more prominent neighbor, the Eagle Nebula (also known as Messier 16), that lies a mere wingspan away. But the framing of this picture, which places the star cluster in a landscape of surrounding gas and dust clouds, shows what a beautiful object NGC 6604 is in its own right.
NGC 6604 is the bright grouping towards to the upper left of the image. It is a young star cluster that is the densest part of a more widely scattered association containing about one hundred brilliant blue-white stars . The picture also shows the cluster´s associated nebula – a cloud of glowing hydrogen gas that is called Sh2-54  – as well as dust clouds.
NGC 6604 lies about 5500 light-years away in the constellation of Serpens (The Serpent) and is located about two degrees north of the Eagle Nebula in the night sky. The bright stars are easily seen in a small telescope and were first catalogued by William Herschel in 1784. However, the faint gas cloud escaped attention until the 1950s when it was catalogued by Stewart Sharpless on photographs from the National Geographic—Palomar Sky Atlas.
The cluster´s hot young stars are helping a new generation of stars to form in NGC 6604, by collecting star-making material into a compact region with their strong stellar winds and radiation. This second generation of stars will quickly replace the older generation, as although the brightest young stars are massive, they consume their fuel copiously and live short lives.
Aside from aesthetics, NGC 6604 has other reasons to draw the gaze of astronomers, as it has a strange column of hot ionized gas emanating from it. Similar columns of hot gas, which channel outflowing material from young star clusters, have been found elsewhere in the Milky Way and other spiral galaxies, but the example in NGC 6604 is relatively nearby, allowing astronomers to study it in detail.
This particular column (often referred to by astronomers as a “chimney”) is perpendicular to the galactic plane and stretches an incredible 650 light-years in length. Astronomers think that the hot stars within NGC 6604 are responsible for producing the chimney, but more research is needed to fully understand these unusual structures.
 This stellar association is called Serpens OB. The first part of the name refers to the constellation in which it lies and the letters OB refer to the spectral type of the stars. O and B are the two hottest stellar classifications and most stars of these types are very brilliant blue-white stars, and relatively young.
 The name Sh2-54 means that the object is the 54th in the second Sharpless catalogue of HII regions, published in 1959.
The year 2012 marks the 50th anniversary of the founding of the European Southern Observatory (ESO). ESO is the foremost intergovernmental astronomy organization in Europe and the world´s most productive astronomical observatory. It is supported by 15 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious program focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organizing cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world´s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world´s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 40-meter-class European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world´s biggest eye on the sky”.
Image 1: The star cluster NGC 6604 is shown in this image taken by the Wide Field Imager attached to the 2.2-meter MPG/ESO telescope at the La Silla Observatory in Chile. NGC 6604 is the bright grouping towards to the upper left of the image. It is a young star cluster that is the densest part of a more widely scattered association containing about one hundred brilliant blue-white stars. The picture also shows the cluster´s associated nebula – a cloud of glowing hydrogen gas that is called Sh2-54 – as well as dust clouds. Credit: ESO
Image 2: This chart shows the location of the star cluster NGC 6604 in the constellation of Serpens (The Serpent). This map shows most of the stars visible to the unaided eye under good conditions and the location of the cluster itself is marked with a red circle. Although the star cluster itself is easily seen in a small telescope, the nebula is very faint and was only discovered photographically in the mid 20th century. Credit: ESO, IAU and Sky & Telescope
Image 3: This visible-light wide-field image of the region around the star cluster NGC 6604 was created from photographs taken through blue, red and infrared filters and forming part of the Digitized Sky Survey 2. The star cluster appears close to the center, as a knot of bright stars. This view shows the rich landscape of gas and dust clouds surrounding the cluster. The field of view is approximately 2.9 degrees across. Credit: ESO and Digitized Sky Survey 2
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