UCL Mathematical & Physical Sciences


Cold dust in the Crab Nebula

Crab Nebula seen by Herschel

16 December 2013

Friday saw the publication of an important new discovery. Scientists at UCL made the first ever detection of a noble gas molecule in space, finding argon hydride within the tendrils of gas that form the Crab Nebula.

But the discovery was a fluke. They were in fact looking for something else: cold dust.

Supernovae remnants like the Crab Nebula, pictured above using far infrared detectors onboard the Herschel Space Observatory, are the clouds of debris left over after very large stars explode. Made of cold gas and dust, they glow brightly and colourfully, and can even be seen with modest amateur telescopes. The light comes from gas energised by the neutron star at the nebula's heart, which begins to glow in a process very similar to neon signs.

However the dust within these nebulae can't be seen in visible light, as it is too cold. Cold dust only emits much longer wavelengths radiation, such as far-infrared and radio waves.

To observe the dusty component of the Crab Nebula, the team needed a telescope that can see far-infrared light.

The problem: these wavelengths, emitted by very cold objects, cannot be seen through Earth's warm atmosphere.

The solution: Herschel, a space telescope part-built by UCL's Mullard Space Science Laboratory, which completed its mission this year. Herschel was designed to observe the faint far-infrared signals coming from cold dust and gas.

Despite being the largest space telescope ever flown, with excellent optics and world-class cameras, Herschel's image is relatively fuzzy. Its picture is not even significantly sharper than visible light images made by the small telescopes at ULO, UCL's observatory, through the hazy and turbulent North London sky.

This is because longer wavelengths (such as far-infrared) are intrinsically harder to focus than short ones (such as visible light).

Even within the visible spectrum, blue light can be focused to a much sharper point than red can. And this is despite red light only having about twice the wavelength of blue light.

For Herschel, which observes far-infrared light with wavelengths 100 to 1000 times longer than visible light, even getting this level of sharpness is a remarkable technical achievement. And with Herschel's retirement in Spring 2013, astronomers now have no functioning observatory which can make this type of observation.

Herschel image credit: ESA, PACS Consortium, Mike Barlow (UCL Physics & Astronomy)

ULO image credit: University of London Observatory/UCL Physics & Astronomy/Bob Winter/Dominic Reeve


High resolution image

Herschel image

ULO image

These images can be reproduced freely providing the source is credited