Dark cloud in the Southern Cross not so dark

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Even the darkest patches of sky can shine brightly to Herschel. This previously bland cloud of dust reveals itself to be a place of intense star formation with filaments and condensations of dust cocooning newly forming stars. It hints that our Galaxy is a tireless place, actively forging new stars.

The swirling patterns of gas in this image came as a complete surprise to astronomers. They are located thousands of light years away from Earth in the constellation Crux, the Southern Cross. Crux is such a prominent constellation in the southern sky that it features on the flags of seven countries, including Australia and New Zealand.

Despite the distance of this gas pattern, it still covers a patch of sky large enough that four full moons would fit along each side of the picture. When astronomers look at this region of the Galaxy with an ordinary telescope they see nothing, just a black patch of sky.

Such dark nebulae were once thought to be ‘holes in the sky’, empty areas of space where there are no stars and so our view was out into the void beyond. We now know that this is not the case and the dark nebulae are dense, dusty clouds that obscure our view of the stars beyond.

This image was taken on 3 September 2009 and is composed of data taken by two of Herschel’s instruments, the Photodetector Array Camera and Spectrometer (PACS) and the Spectral and Photometric Imaging Receiver (SPIRE). Five wavelengths of infrared radiation have been colour-coded and put together to give this impression of what your eye would see – if it were sensitive to infrared light.

The surprise is that there is so much turbulence in the gas; clearly there are great forces at work. The first of these forces is gravity, pulling together the gas and dust to form new stars. The second is a combination of ‘winds’ and ‘jets’ of particles and radiation being generated by the young stars stirring up the surrounding gas. The turbulence indicates that there is a lot of hidden activity taking place in these dark clouds.

The image shows that cold interstellar gas and dust is not as smoothly distributed as previous data might have suggested. Instead it is an interconnected maze of filaments of different sizes, temperatures and concentrations, with strings of newly forming stellar embryos in all phases of development.

Star and planet formation is usually hidden from view by these great cocoons of dust. Now, Herschel allows us to see through the mask and inside to watch the process.

Credit: ESA and the SPIRE & PACS consortium

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