Article updated on 9 December 2020
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Image Credit: Dotted Yeti/Shutterstock.com (Elements of this image are furnished by NASA)
Around half of all starlight produced and emitted throughout the Universe’s 13.8-billion-year history has been absorbed and re-emitted as infrared light; most atoms and molecules - except hydrogen and helium – have their origins in the stars.
For obvious reasons, the sky was first studied at visible wavelengths, but as technology has become more sophisticated, we have been able to observe more of the electromagnetic spectrum, including the infrared region.
Throughout their lifetime, stars produce heavier elements, which are expelled into interstellar space as the star explodes during its final phase of life. Some of these atoms will join to form molecules; together these form the basic building blocks of all matter. Deducing what atoms and molecules are present, their distribution, abundance and environment is necessary to understanding the Universe.
Infrared spectroscopy originated in the 1830s after German-born British astronomer William Herschel discovered the existence of infrared radiation while studying sunlight. It wasn’t until the 1920s that the first systematic infrared observations of stellar objects other than the sun and moon were made by the American astronomers W.W. Coblentz, Edison Pettit, and Seth B. Nicholson.
What is Infrared Spectroscopy?
Ground-based infrared spectroscopy has a much longer history than space-based infrared spectroscopy, and as a result, many of the terms used relate to the windows in the Earth’s atmosphere where lower absorption spectroscopy makes astronomy feasible. Infrared spectroscopy is conducted in space because the Earth’s atmosphere blocks out most infrared wavelengths in addition to producing its own, which can overwhelm celestial sources.
The infrared part of the electromagnetic spectrum – which lies between 0.75 and 300 μm - is where the emission and absorption lines of virtually all molecules, as well as numerous atoms and ions, are found. It is divided into three regions, named for their position relative to the visible spectrum:
- The near-infrared: approximately 0.75 – 5 μm wavelength. This light behaves similarly to visible wavelengths due to its proximity (our eyes can see up to 0.75 μm).
- Mid-infrared: around 5 -25 μm.
- Far-infrared: approximately 25 - 350 μm.
Infrared spectroscopy involves detecting the absorption – and very occasional emission – of atoms as they change from one energy band to another within the infrared region of the electromagnetic spectrum. These absorptions are known as resonant frequencies, meaning the frequency of the absorbed radiation matches the frequency of the bond or group that vibrates.
The techniques used in infrared spectroscopy in astronomy are similar to those used in visible spectroscopy; it uses lenses, mirrors, dispersive media such as grating or prisms, and quantum detectors. As a consequence, they both often happen using the same telescope.
Advantages Over Optical Telescopes
Different wavelengths reveal different natural phenomena; only a small portion of the universe is visible to the naked eye. Objects such as interstellar gas, planets, asteroids, brown dwarfs and stars being born are all invisible to optical telescopes, but become visible with infrared spectroscopy.
Likewise, a star surrounded by dust is imperceptible with an optical telescope, but observable with near-infrared spectroscopy, since infrared can penetrate thick dust.
Spectrometers in Space
Spectrometers are found onboard many missions in space, such as the 2MASS and WISE astronomical surveys, which have revealed undiscovered star clusters.
The NASA Spitzer Space Telescope, launched in 2003, features the Infrared Array Camera, which operated in the mid- and near-infrared and used over 65,000 pixels across each of its four detectors until it ran out of coolant in 2017. It also features the Infrared Spectrograph, which provides high- and low-resolution spectroscopy at mid-infrared wavelengths; it has, for example, collected data that displays a strong signature of water vapor in the disk of gas and dust surrounding a young star.
Sources and Further Reading
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