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The research teams that operate the Herschel and XMM-Newton space telescopes have celebrated Christmas by producing a very unusual and spectacular image of the famous spiral galaxy in Andromeda, M31.
Herschel has detected the far infrared light, that shows in extraordinary clarity the regions of gas and dust within which the very youngest stars are just taking shape, or have almost completed their formation. Located within cocoons, they heat the gas and dust from which they are forming via the energy released during gravitational collapse, making them visible at infrared wavelengths.
The most surprising structures highlighted by Herschel are five concentric rings of star formation. Until now, at optical wavelengths, only one ring, 75,000 light years wide, had been identified with certainty, but the new infrared image paints a dramatically different picture of this celebrated galaxy.
The rings may well have been formed by mergers with small galaxies, the gaseous and dusty components of which slowly dispersed within the disk of M31, provoking the proliferation of new stars.
XMM-Newton, on the other hand, has imaged the X-rays, tracing the very energetic processes that characterise the late evolutionary phases of stars. These include the super-high velocity gas expelled in supernova explosions, gas exchange in the symbiotic relationship between double stars in which one or more components are collapsed and the voracious activity of black holes of various sizes.
The clear concentration of X-ray emission from the central regions of M31 is symptomatic of the large number of evolved stars concentrated in a relatively small volume.
As can be seen from the above figure, the optical image of M31 shows us relatively little about the distribution of stellar populations of differing ages; only via the comparison of images that cover a much wider interval of the electromagnetic spectrum do we really begin to understand how our imposing neighbour is structured
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