Infrared Astronomy

Infrared Astronomy is the study of astronomical objects at infrared wavelengths.  Infrared radiation can penetrate dust clouds more easily than that at optical wavelengths, so astronomers using the infrared can study stars forming deep inside dense dust clouds, the centre of the Galaxy, and other galaxies (both normal and peculiar). Astronomical sources of infrared radiation include the planets and other Solar System bodies, stars and the dusty regions themselves. Wien’s law shows that comparatively low-temperature objects (less than 3000 K) emit most strongly, and so are easiest to observe, in the infrared.

Astronomical sources emit infrared radiation either throughout the electromagnetic spectrum (according to the Planck distribution of black body radiation) or in one or more spectral lines or bands, depending upon the temperature and composition of the emitting gas. The emission in the infrared lines is often the main mechanism for cooling gas/dust clouds (such as the dense clouds where new stars are forming). Among the most significant infrared atomic lines are the forbidden lines of carbon ([C II] at 157 um) and oxygen ([O III] at 88.35 m). Infrared observations from space, with ISO, have proved that water is very common in the Solar System, in the Galaxy and in other galaxies. The IR has revealed the existence of another kind of molecule, polycyclic aromatic hydrocarbons (PAHs), the emission bands of which were originally known as the Unidentified Infrared Bands (UIBs). The molecular bands are not only indicative of chemical composition but also of the physical processes in progress, such as the passage of shock waves through regions in which star formation is occurring.

Astronomers on the ground, using the IR, must limit their observations to the atmospheric ‘windows’ where there is very little absorption by water vapor or carbon dioxide, and infrared telescopes are usually sited on mountain tops (above about 3000 m/l0,000 ft) to take advantage of the windows at 0.75–2.5 um, 3–5 um,7.5–14 um and 16–21 um. Mauna Kea in Hawaii and Atacama in Chile are examples of good sites. To avoid the Earth’s atmosphere which hinders the process, infrared observations have been made from high-flying aircraft, notably the kuiper airborne observatory (KAO) and the new stratospheric observatory for infrared astronomy (SOFIA), from unmanned balloons, rockets, and from Earth-orbiting satellites.

The infrared astronomical satellite (IRAS) has surveyed the entire sky at several infrared wavelengths cataloguing almost a quarter of a million objects. More recently the infrared space observatory (ISO) pointed over 30,000 astronomical objects for detailed study.

The huge infrared emission output when stars form means that these regions can be detected at large distances; the starburst galaxies are one example. Closer to our home, Solar System scientists have used the techniques of infrared astronomy to work out the basic chemistry of the planets, their satellites and the Asteroids. In the near-infrared region, molecules in planetary atmospheres exhibit a rich absorption spectrum, the analysis of which permits the composition and temperature of the atmospheres to be established. By careful selection of a precise IR wavelength, different layers in the atmosphere can be probed in detail because of the variations in temperature.

Questions to Ponder:

• What kinds of astronomical objects emit infrared light?
• How does interstellar dust produce infrared light?
• How does our atmosphere block infrared radiation from space?