Read more: Instant Expert: The unseen universe
Infrared was the first invisible radiation discovered – it has revealed asteroids, comets, interstellar dust and the birth of planets, stars and galaxies
As we look into a clear night sky, we see just a fraction of what the universe contains: mainly stars in our galaxy radiating in the narrow visible wavelength band between 390 and 750 nanometres.
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Optical telescopes extend that vision to far-off galaxies, but it is only in the past century or so, as we have begun to observe the broad sweep of invisible electromagnetic wavelengths, that the full drama of the cosmos has been unveiled.
The first invisible radiation to be detected was in the infrared range, at wavelengths from 750 nanometres up to a millimetre. It was discovered in 1800 when the British astronomer William Herschel used a prism to split sunlight and saw the mercury of a thermometer placed beyond the red end of the spectrum begin to rise.
Infrared astronomy took off in the 1960s. It studies objects in the universe at temperatures between 10 and 1000 kelvin: asteroids, comets, interstellar dust and newly forming stars and galaxies.
Dust to dust
The most significant source of the infrared light that reaches Earth is the interstellar medium. This mixture of gas and dust pervades the space between stars in galaxies and has a temperature of 10 to 50 kelvin. It radiates only in the infrared, and dims the visible light from distant stars, reddening their colour.
The first direct image of the interstellar dust came in 1983 courtesy of the , a space telescope funded by the US, the Netherlands and the UK. It was a signal moment in astronomy. Observing interstellar dust allows us to glimpse the full cycle of stellar life and death, including the formation of new stars and planetary systems from the dust – sometimes in violent bouts as distant galaxies collide – long before these stars become visible to optical telescopes. A striking example lies in the pair of merging galaxies known as , around 45 million light years from us: their brightest infrared regions (image left) are dark at visible wavelengths (image right).
Infrared observations also reveal dying stars blowing off clouds of dust and gas, replenishing the interstellar medium. The dust is mainly silicates and amorphous carbon – sand and soot. The production of this dust is crucial to our existence: every carbon atom in our bodies was created in the core of a star, was ejected as that star died, and drifted around in the interstellar medium before being sucked into our solar system.
Other worlds
The first dedicated infrared space telescope, IRAS, found discs of dust and other debris around some bright stars, pointing the way to searches for planetary systems. Infrared surveys have since detected many debris discs and planets in the process of forming.
Most fully-formed extrasolar planets are discovered by optical telescopes looking either at small changes in the star’s velocity as the planet orbits it, or tiny drops in brightness as the planet crosses the surface of the star. Infrared instruments, such as NASA’s (left), have an important complementary role to play. They look for “hot Jupiters”, close-orbiting massive planets, as they pass in front of their star.
An infrared instrument on the European Southern Observatory’s was the first to provide a direct image of an extrasolar planet. This body, in orbit around a brown dwarf star, is five times the mass of Jupiter.
Galactic origins
Because infrared observations spy out stars as they form and die, we can use them to look back in time, tracing how stars and galaxies formed throughout cosmic history almost as far back as the big bang.
When NASA’s space mission, launched in 1999, measured the total background radiation at millimetre and sub-millimetre wavelengths, it found a strong contribution from distant galaxies. It turns out that more than half of the energy emitted by far-off stars at optical and ultraviolet wavelengths is absorbed by dust and re-emitted in the infrared before it reaches us, making infrared essential for our understanding of the universe.
The infrared is also important for finding out how galaxies first arose. The universe is expanding, which means most galaxies are receding from us and the radiation they emit undergoes a Doppler shift to longer wavelengths. This “red shift” means visible light from the most distant galaxies known, emitted in the first billion years after the big bang, is stretched to infrared wavelengths by the time it reaches us.
Star instrument: Herschel
Most infrared wavelengths are absorbed by water and carbon dioxide in the atmosphere, with only a few narrow spectral “windows” of infrared reaching the ground. Infrared telescopes must therefore be situated at the top of mountains or, better still, in space.
The current top dog in the infrared pack is the European Space Agency’s , which started operating in 2009. It is the largest telescope ever launched into orbit, and carries a spectrometer and two cameras that cover wavelengths between 70 and 500 micrometres. All this equipment has to be cooled to temperatures close to absolute zero to prevent the telescope’s own infrared emissions affecting the measurements.
As interpretation of Herschel data gets under way, the telescope is already delivering some spectacular images of filamentary interstellar dust clouds in which stars may be forming, as well as galaxies with unexpectedly large amounts of very cold dust missed by earlier studies.