Infra-Red: Beyond Visible Light

Visible light never tells the whole story.

NGC 1499 in Hα/IR: Hα from Caradon Observatory, IR from WISE space telescope

The human eye is only sensitive to a small window in the electromagnetic spectrum. We can't perceive light that has a shorter wavelength than violet or a longer wavelength than red.

Credit: Wikimedia user Philip Ronan

Credit: Wikimedia user D-Kuru

Infra-red light was discovered by an astronomer, William Herschel, better known for discovering the planet Uranus with his home-built telescope. He simply used a prism, a sunbeam and a thermometer.

The prism splits the Sun's white light into its component colours as red light is refracted (or bent) less then blue light. When a thermometer is placed in the coloured beam its temperature rises, as the dispersed sunlight heats it. But Herschel was shocked to find that when he placed the thermometer just to the right of the red light the temperature went up even more. He concluded that the Sun was emitting a form of invisible light which we now call infra-red, literally "below red". This turned out to be a far more important discovery than finding a new planet. (It was also something of an accident, William moved the thermometer out of the beam to measure the ambient air temperature, but he deserves full credit for noticing and solving the puzzle.)

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Infra-red detection is somewhat misleadingly referred to as 'heat vision'. In fact, all objects emit light, it's just that objects at everyday temperatures radiate mostly infra-red. It's odd to think that we glow in infra-red light. Hotter objects shine in visible light, you can see this by turning on an electric hob. As it warms up it starts to glow a dull red, as peak emission moves up the electromagnetic spectrum from infra-red towards red. Our Sun, with a surface temperature of almost 6,000°C, shines most brightly in green light but we perceive it as white because blue and red light is also produced. Objects don't emit a single wavelength (or colour) of light but a range, the Sun also produces infra-red and ultra-violet light. There is a relationship between temperature and energy, higher frequency light-waves carry more energy so require higher temperatures to produce. Very hot objects emit x-rays, while cold ones produce microwaves or radio waves. This phenomenon is somewhat confusingly called 'black-body radiation', it matters to us because it's how the Sun warms our planet. It's also useful to blacksmiths, who use the colour of the heated metal to judge its temperature.

NGC 1499 in Hα and Hα/IR

Above are two images of NGC 1499, the California Nebula. The monochrome image on the left shows glowing hydrogen (actually a deep red colour, but the mono image shows more detail) and was taken at Caradon Observatory. The image on the right is a bi-colour composite of visible and infra-red light. Glowing hydrogen is shown as orange while 'warm' interstellar dust is blue. ('Warm', that is, compared to the near-void of space. The dust is at a temperature of somewhere between -170°C and -200°C.) Regions that emit both Hα and IR light are white.

The infra-red data reveals a much larger structure than can be seen in visible light. The blue patch on the left of the nebula appears be heated by stellar winds from the star Xi Persei, which is responsible for lighting up the nebula. The glowing hydrogen is a relatively thin skin on a large cloud of gas and dust. Concentrations of gas and dust block the UV light from XI Persei, providing shade.

The IR data has been raided from one of NASA's space telescopes, the Wide-field infrared Survey Explorer (WISE). WISE images are freely available for download but must be credited.

This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration.

Imaging rig shooting Hα image

WISE telescope, credit: NASA