Stellar Birth in a Dark Cloud

SPACE SCIENCE

A Unique, Wide-Field Infrared SOFI Image of RCW 108 This SOFI image of RCW 108 was obtained by ESO astronomer Fernando Comeron for one of his current research projects. He also performed the related, very extensive image processing. PR Photo 30a/00 was obtained in February 2000 by means of SOFI, the infrared camera and spectrograph at the ESO 3.58-m New Technology Telescope (NTT). The image is a mosaic that was prepared by combining 33 overlapping frames, taken in each of the J (wavelength 1.25 micron; here rendered as blue), H (1.65 micron; green) and K (2.2 micron; red) infrared filters. Each individual frame is in turn a stack of six frames totaling 12 sec exposure time, with the exception of the one covering the center of the region which is 15 times longer in order to increase the sensitivity around the central emission nebula (an HII region). The total area imaged is 12.8 x 12.8 arcmin^2, using SOFI in its normal resolution mode with 0.29 arcsec pixels and a field of about 5 arcmin. Thus the field shown here is about 6.6 times the field-of-view in a single SOFI frame. Five-sigma detection limits in the least-exposed areas of the image are typically J=18.6, H=17.7, K=17.2 mag. The image quality is between 0.6 and 0.7 arcsec for the different filters over most of the field. PR Photo 30b/00 is a smaller area near the centre of the field; it measures approx. 6.3 x 6.6 arcmin^2. North is up and East is left. Credit: European Southern Observatory.

Garching - Oct. 27, 2000
A remarkable, wide-field infrared image allows to see in unprecedented detail the inner regions of RCW 108, a large complex of stars and nebulae in the southern Milky Way. It is one of the widest, "deepest" and most spectacular infrared astronomical images ever obtained of a starforming region.

It is based on approx. 600 individual exposures with the SOFI multi-mode instrument on the ESO 3.58-m New Technology Telescope (NTT) at the ESO La Silla Observatory (Chile).

The above infrared view (PR Photo 30a/00) of an interesting region in the Southern Milky Way, full of stars and nebulae and known as the RCW 108 complex, was obtained in February 2000. It provides a deep look into a star-forming region, where stars are in the process of being born from dense clouds of dust and gas.

It is particularly interesting to compare the appearance of the area in this infrared view (SOFI) with that obtained earlier at visible wavelengths (WFI), cf. ESO PR Photos 21a-b/99. As they look quite different, such a comparison is greatly facilitated by referring to the positions of the same brighter objects in the two photos.

In visible light, the Eastern edge of the cloud appears as a bright rim; the same edge is present, although less prominent to the left in the infrared image. This is where the outer surface of the cloud is eroded and later completely dissolved by the strong ultraviolet light of a nearby cluster of stars, that lies beyond the edge of the infrared image.

Dust obscuration

The bright nebula IRAS 16362-4845 at the center of the infrared picture is much more prominent here (see PR Photo 30b/00) than in the visible one. This is because it is deeply embedded in the cloud and its visible light is heavily obscured by the dust particles. This effect is much smaller at longer (infrared) wavelengths, so the SOFI image provides a more detailed view of IRAS 16362-4845.

It is exactly this greater transparency of interstellar dust clouds in the infrared spectral region that is one of the main reasons why studies of very young stars are best carried out at infrared wavelengths. Such stars are often still surrounded by the remnants of their parental gas and dust clouds.

The colours of the stars

Dust not only dims the light of objects located behind the cloud, but also "reddens" it. This is because shorter-wavelength (e.g., blue) light is scattered out of the line-of-sight more efficiently than is longer-wavelength (e.g., red) light.

This effect is clearly seen in the SOFI image, where stars located within the cloud area on the average appear much redder than those that are outside. There are a few exceptions, however: some stars that are seen "in" the darkest part of the cloud appear to be blue. This is because they are located in front of the cloud or are only slightly embedded in it.

In fact, since the "real" (i.e., intrinsic) "infrared colours" of most stars are rather similar, the different colours we see in the SOFI picture are mostly due to different degrees of reddening by foreground dust. The colour of a star in this area is therefore mostly a measure of how deeply it is embedded in the nebula.

SPACE SCIENCE

Stephan's Quintet - A Mammoth Cosmic Collision

Garching - Oct. 25, 2000
A spectacular new image from the NASA/ESA Hubble Space Telescope of the group of galaxies called Stephan's Quintet provides a detailed view of one of the most exciting star forming regions in the local universe. Stephan's Quintet is a favoured object for amateur astronomers and has earned a reputation as a challenging target for good hobby telescopes. The quintet is a prototype of a class of objects known as compact groups of galaxies and has been studied intensively for decades.

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