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Infrared astronomy
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What is infrared astronomy?
Infrared astronomy studies astronomical objects by observing the infrared radiation they emit. Celestial objects like planets, stars, nebulae, and galaxies emit energy as infrared radiation, which lies on the electromagnetic spectrum between one micrometre and one millimetre.
Because dust particles block light at optical wavelengths, infrared astronomy is useful for examining objects that would otherwise be unobservable from Earth. The techniques of infrared astronomy enable investigators to examine many such objects that cannot otherwise be seen from Earth.
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How is infrared light different from visible light?
Infrared and visible light are both forms of electromagnetic radiation but differ in wavelength and frequency. Infrared radiation lies beyond the red end of the visible spectrum, with longer wavelengths (0.78 to 1,000 micrometres) and lower frequencies (10^12 to 5 × 10^14 Hz) compared to visible light.
Visible light, with wavelengths from 400 nm (violet) to 700 nm (red), is the portion of the spectrum the eye can see. Nearly 50% of the Sun's radiant energy is emitted as infrared radiation, with the rest primarily as visible light. Atmospheric haze and pollutants scatter visible light but are nearly transparent to parts of the infrared spectrum, which is why infrared astronomy enables observation of cosmic objects through interstellar dust clouds.
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Why do astronomers use infrared telescopes?
Astronomers use infrared telescopes to study celestial objects, such as planets, stars, nebulae, and galaxies, by observing the infrared radiation they emit. Many of these objects can't be seen from Earth using optical telescopes because dust particles block the light at optical wavelengths. Infrared radiation is readily absorbed by water vapor in Earth’s atmosphere, so infrared telescopes are often located on high mountaintops or in space to avoid this absorption.
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What kinds of objects can be studied with infrared astronomy?
Infrared astronomy studies celestial objects radiating energy in the infrared region of the electromagnetic spectrum. These objects include the Sun, planets, stars, nebulae, and galaxies.
Infrared radiation helps detect celestial objects that are otherwise difficult to observe. For example, dust in the interstellar medium blocks shorter wavelengths, making stars detectable only by their infrared emissions. Infrared emissions are also evident at the centers of many external galaxies, indicating the presence of supermassive objects like black holes.
Infrared telescopes, often located on high mountaintops or in space, are used to collect this radiation.
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How does Earth's atmosphere affect infrared observations?
Earth's atmosphere significantly affects infrared observations due to the absorption of infrared rays by water vapor, carbon dioxide, and other gases. This absorption restricts ground-based infrared astronomy to specific narrow windows within the electromagnetic spectrum.
To mitigate these atmospheric effects, infrared telescopes are strategically located on high mountaintops where atmospheric humidity is low, such as Mauna Kea in Hawaii, or in the Atacama Desert in Chile. Airborne observatories like the Stratospheric Observatory for Infrared Astronomy (SOFIA) fly at high altitudes to escape water vapor. Space-based telescopes, such as the Spitzer Space Telescope and the planned James Webb Space Telescope (JWST), offer the advantage of observing from above the atmosphere, thus avoiding atmospheric interference altogether.
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What are some famous infrared observatories or missions?
Several famous infrared observatories and missions have significantly advanced our understanding of the cosmos. These include:
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Infrared Astronomical Satellite (IRAS) The IRAS, launched in 1983, was the first space observatory to map the entire sky at infrared wavelengths. It discovered comets and revealed that some young stars have disks of dust particles, suggesting the formation of planetary systems.
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Spitzer Space Telescope The Spitzer Space Telescope, launched in 2003, gathered information on the origin, evolution, and composition of planets, stars, galaxies, and the universe. It determined the temperature, atmospheric structure, and composition of extrasolar planets.
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James Webb Space Telescope (JWST) The JWST is the largest space telescope, with a primary mirror 6.5 meters in diameter, and studies forming stars and galaxies.
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Stratospheric Observatory for Infrared Astronomy (SOFIA) Operated by NASA, the SOFIA consists of a 2.5-metre telescope flown in a special airplane above water vapour to collect infrared data.
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Infrared astronomy, study of astronomical objects through observations of the infrared radiation that they emit. Various types of celestial objects—including the planets of the solar system, stars, nebulae, and galaxies—give off energy at wavelengths in the infrared region of the electromagnetic spectrum (i.e., from about one micrometre to one millimetre). The techniques of infrared astronomy enable investigators to examine many such objects that cannot otherwise be seen from Earth because the light of optical wavelengths that they emit is blocked by intervening dust particles.
Infrared astronomy originated in the early 1800s with the work of the British astronomer Sir William Herschel, who discovered the existence of infrared radiation while studying sunlight. The first systematic infrared observations of stellar objects were made by the American astronomers W.W. Coblentz, Edison Pettit, and Seth B. Nicholson in the 1920s. Modern infrared techniques, such as the use of cryogenic detector systems (to eliminate obstruction by infrared radiation released by the detection equipment itself) and special interference filters for ground-based telescopes, were introduced during the early 1960s. By the end of the decade, Gerry Neugebauer and Robert Leighton of the United States had surveyed the sky at the relatively short infrared wavelength of 2.2 micrometres and identified approximately 20,000 sources in the northern hemispheric sky alone. Since that time, balloons, rockets, and spacecraft have been employed to make observations of infrared wavelengths from 35 to 350 micrometres. Radiation at such wavelengths is absorbed by water vapour in the atmosphere, and so telescopes and spectrographs have to be carried to high altitudes above most of the absorbing molecules. Specially instrumented high-flying aircraft such as the Kuiper Airborne Observatory and the Stratospheric Observatory for Infrared Astronomy have been designed to facilitate infrared observations near microwave frequencies.
In January 1983 the United States, in collaboration with the United Kingdom and the Netherlands, launched the Infrared Astronomical Satellite (IRAS), an unmanned orbiting observatory equipped with a 57-centimetre (22-inch) infrared telescope sensitive to wavelengths of 8 to 100 micrometres. IRAS made a number of unexpected discoveries in a brief period of service that ended in November 1983. The most significant of these were clouds of solid debris around Vega, Fomalhaut, and several other stars, the presence of which strongly suggests the formation of planetary systems similar to that of the Sun. Other important findings included various clouds of interstellar gas and dust where new stars are being formed and an object, Phaeton, thought to be the parent body for the swarm of meteoroids known as Geminids.
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IRAS was succeeded in 1995–98 by the European Space Agency’s Infrared Space Observatory, which had a 60-centimetre (24-inch) telescope with a camera sensitive to wavelengths in the range of 2.5–17 micrometres and a photometer and a pair of spectrometers that, between them, extended the range to 200 micrometres. It made significant observations of protoplanetary disks of dust and gas around young stars, with results suggesting that individual planets can form over periods as brief as 20 million years. It determined that these disks are rich in silicates, the minerals that form the basis of many common types of rock. It also discovered a large number of brown dwarfs—objects in interstellar space that are too small to become stars but too massive to be considered planets.
The next infrared space observatory was a U.S. satellite, the Spitzer Space Telescope, which was built around an all-beryllium 85-centimetre (33-inch) primary mirror that focused infrared light on three instruments—a general-purpose infrared camera, a spectrograph sensitive to mid-infrared wavelengths, and an imaging photometer taking measurements in three far-infrared bands. Together the instruments covered a wavelength range of 3.6 to 180 micrometres. The most striking results from the Spitzer’s observations concerned extrasolar planets; Spitzer determined the temperature and the atmospheric structure, composition, and dynamics of several extrasolar planets. The telescope operated from 2003 to 2020.
Two large space telescopes succeeded Spitzer. The James Webb Space Telescope (JWST) is the largest space telescope at any wavelength, with a primary mirror 6.5 metres (21.3 feet) in diameter. The JWST studies forming stars and galaxies and launched in 2021. The Nancy Grace Roman Space Telescope has a 2.4-metre (7.9-foot) mirror and launched in 2026.