Infrared (IR) astronomy is the branch of modern astronomy that studies and analyzes infrared (IR) radiation from celestial objects. Most celestial objects emit some quantity of infrared radiation. However, when a star is not quite hot enough to shine in the visible portion of the electromagnetic spectrum, it emits the bulk of its energy in the infrared. IR astronomy, consequently, involves the study of relatively cool celestial objects, such as interstellar clouds of dust and gas (typically about -280°F [100 kelvins]) and stars with surface temperatures below temperatures below about 10,340°F (6,000 kelvins).
Many interstellar dust and gas molecules emit characteristic infrared signatures that astronomers use to study chemical processes occurring in interstellar space. This same interstellar dust also prevents astronomers from viewing visible light coming from the center of our Milky Way galaxy. However, IR radiation from the galactic nucleus is absorbed not as severely as radiation in the visible portion of the electromagnetic spectrum, and IR astronomy enables scientists to study the dense core of the Milky Way.
Infrared astronomy also allows astrophysicists to observe stars as they are being formed (they call these objects protostars) in giant clouds of dust and gas (called nebulae), long before their thermonuclear furnaces have ignited and they have "turned on" their visible emission.
Unfortunately, water and carbon dioxide in Earth's atmosphere absorb most of the interesting IR radiation arriving from celestial objects. Earth-based astronomers can use only a few narrow IR spectral bands or windows in observing the universe, and even these IR windows are distorted by "sky noise" (undesirable IR radiation from atmospheric molecules). With the arrival of the space age, however, astronomers have placed sophisticated IR telescopes (such as the Spitzer Space Telescope) in space, above the limiting and disturbing effects of Earth's atmosphere, and have produced comprehensive catalogs and maps of significant infrared sources in the observable universe.
The challenge of finding an Earth-size (terrestrial) planet orbiting even the closest stars can be compared to finding a tiny firefly next to a blazing searchlight when both are thousands of miles (km) away. Quite similarly, the infrared emissions of a parent star are a million times brighter than the infrared emissions of any companion planets that might orbit around it. Beyond the year 2020, data from the Terrestrial Planet Finder mission should allow astronomers to analyze the infrared emissions of extrasolar planets in star systems up to about 100 light-years away. They will then use these data to search for signs of atmospheric gases, such as carbon dioxide, water vapor, and ozone. Together with the temperature and radius of any detected planets, these atmospheric gas data will enable scientists to determine which extrasolar planets are potentially habitable or even if they may be inhabited by rudimentary forms of life.
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