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Herschel révèle la face cachée de la naissance des étoiles   RCW 120 is a galactic bubble with a large surprise. How large? At least 8 times the mass of the Sun. Nestled in the shell around this large bubble is an embryonic star that looks set to turn into one of the brightest stars in the Galaxy. The Galactic bubble is known as RCW 120. It lies about 4300 light-years away and has been formed by a star at its centre. The star is not visible at these infrared wavelengths but pushes on the surrounding dust and gas with nothing more than the power of its starlight. In the 2.5 million years the star has existed. It has raised the density of matter in the bubble wall so much that the quantity trapped there can now collapse to form new stars. The bright knot to the right of the base of the bubble is an unexpectedly large, embryonic star, triggered into formation by the power of the central star. Herschel’s observations have shown that it already contains between 8-10 times the mass of our Sun. The star can only get bigger because it is surrounded by a cloud containing an additional 2000 solar masses. Not all of that will fall onto the star, even the largest stars in the Galaxy do not exceed 150 solar masses. But the question of what stops the matter falling onto the star is a puzzle for modern astronomers. According to theory, stars should stop forming at about 8 solar masses. At that mass they should become so hot that they shine powerfully at ultraviolet wavelengths.
This light should push the surrounding matter away, much as the central star did to form this bubble. But clearly sometimes this mass limit is exceeded otherwise there would be no giant stars in the Galaxy. So astronomers would like to know how some stars can seem to defy physics and grow so large. Is this newly discovered stellar embryo destined to grow into a stellar monster? At the moment, nobody knows but further analysis of this Herschel image could give us invaluable clues.  This image is taken looking towards a region of the Galaxy in the Eagle constellation, closer to the Galactic centre than our Sun. Here, we see the outstanding end-products of the stellar assembly line. At the centre and the left of the image, the two massive star-forming regions G29.9 and W43 are clearly visible. These mini-starbursts are forming, as we speak, hundreds and hundreds of stars of all sizes: from those similar to our Sun, to monsters several tens of times heavier than our Sun.
These newborn large stars are catastrophically disrupting their original gas embryos by kicking away their surroundings and excavating giant cavities in the Galaxy. This is clearly visible in the 'fluffy chimney' below W43.  This image, in the constellation of Vulpecula, shows an entire assembly line of newborn stars. The diffuse glow reveals the widespread cold reservoir of raw material that our Galaxy has in stock for building stars.
Large-scale turbulence from the giant colliding Galactic flows causes this material to condense into the web of filaments that we see all over the image. These are the ‘pregnant’ entities where the material becomes colder and denser. At this point, gravitational forces take over and fragment these filaments into chains of stellar embryos that can finally collapse to form baby stars.  A picture of the first field observed in the H-ATLAS survey, made by combining the images made with the SPIRE camera at 250, 350 and 500 microns. The colours in the image are not real but have been used to represent the different infrared wavelengths. The faint blue whisps at the top of the image show dust in our own Galaxy and the bright object just above the centre of the picture is a 'Bok globule', a dense cloud of gas and dust, also in our Galaxy, in which a small star may be forming. The other objects in the picture are all galaxies, at distances up to 12 billion light-years. The image shows that the survey is detecting objects in our celestial ‘backyard’ and also other, further ones that we are seeing as they were not long after the Big Bang.  After only its first year in space, ESA's Herschel infrared space observatory is changing the way scientists view the Universe. From nearby celestial objects to distant ones, from the smallest molecules to the largest galaxies, Herschel is revealing new details about how the Universe behaves. Release date: 10 mai 2010 |