Europe's Herschel space telescope has imaged one of the most popular subjects in the sky, the Horsehead Nebula, and its environs. The distinctively shaped molecular gas cloud is sited some 1,300 light-years from Earth in the Constellation Orion. It is in a region of space undergoing active star formation, something Herschel has been most keen to study.
The Hubble space observatory has also returned to the Horsehead scene, to celebrate 23 years in orbit. Together, these two great facilities give scientists a much broader insight into what is taking place in this familiar patch of the heavens. "You need images at all scales and at all wavelengths in astronomy in order to understand the big picture and the small detail," said Prof Matt Griffin, the principal investigator on Herschel's SPIRE instrument.
Hubble's new view of the Horsehead Nebula, a large cloud of hydrogen laced with dust
"In this new Herschel view, the Horsehead looks like a little feature, a pimple. In reality, of course, it is a very large entity in its own right, but in this great sweep of a picture from Herschel you can see that the nebula is set within an even larger, molecular-cloud complex where there is a huge amount of material and a great range of conditions," the Cardiff University, UK, said. To provide a sense of scale, the Horsehead Nebula, also known in the catalogues as "Barnard 33", is about five light-years "tall".
Hubble sees the Horsehead in near-infrared light. Herschel, on the other hand, goes to much longer wavelengths. This allows it to see the glow coming directly from cold gas and dust, the material that will eventually collapse under gravity to form the next generation of stars. Scientists are particularly keen to understand the mechanisms that drive the production of the biggest stars, objects much more massive than our own Sun that form relatively fast, burn bright but brief lives and interact strongly with their environment.
The Orion Molecular Cloud Complex is one of the best and nearest regions in space to study this activity. Prof Griffin explained, "You can see all the things we look for in Herschel images, the filaments, the bubbles; the wispy material, the reddish material that hasn't yet actually started to form stars. You can also see nebulosity where material has been lit up from inside by stars; and features like the Horsehead Nebula where that star formation has yet to really get going."
Hubble's new view was acquired by its Wide Field Camera-3 instrument, which was installed by astronauts on the last shuttle servicing mission in 2009. The image was taken to celebrate its 23rd birthday in orbit. It was launched on 24 April 1990. The much shorter wavelengths at which Hubble works means it can produce finer, sharper detail than Herschel. It illustrates particularly well the way the ultraviolet glare and stellar winds from nearby stars are sculpting the dusty stellar nursery.
Hubble hopefully has quite a few years of operations left in it. Herschel does not. Scientists are expecting to lose the telescope any day now. The superfluid helium it uses to cool its instruments and their detectors is all but gone. When the supply runs completely dry, Herschel will warm from its ultra-low functioning temperature and go blind.
Herschel is one of the largest space telescopes ever launched; its 3.5m diameter mirror perfectly captures infrared light
Infrared shines through gas and dust clouds that can block visible light, Herschel can see deep into dusty star-forming regions
The telescope is named after the astronomer William Herschel who discovered infrared radiation while studying the Sun in 1800
The Earth's atmosphere absorbs infrared, so Herschel was launched into space in 2009 to get a clear view of the infrared Universe