NGC 6604, the chimney cluster

The star cluster NGC 6604 is often overlooked in favour of its more prominent neighbour, 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.

The star cluster NGC 6604 is shown in this image taken by the Wide Field Imager attached to the 2.2-metre 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 (i.e. the 54th object in the second Sharpless catalogue of HII regions, published in 1959) — as well as dust clouds. Credit: ESO

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.

Aside from aesthetics, NGC 6604 has other reasons to draw the gaze of astronomers, as it has a strange column of hot ionised 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.

A paper published in late 2008 provides the following general information on NGC 6604:

NGC 6604 is a young cluster in Serpens with an age of 4-5 Myr and at a distance of about 1.7 kpc. It forms the densest part of the wider Ser OB2 association, which contains about 100 OB stars. NGC 6604 lies about 65 pc above the Galactic plane, and has attracted special interest since it has produced a thermal chimney stretching 200 pc out of the plane. The combined effect of winds and radiation from the many OB stars has produced a rim of dense molecular material in which second-generation star formation is currently taking place.

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 centre, 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

Sources:

• A Cluster Within a Cluster, eso1218 Photo Release – 25 April 2012
• Bo Reipurth, The Young Cluster NGC 6604 and the Serpens OB2 Association, Volume 5, Handbook of Star Forming Regions: Volume II, The Southern Sky.