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Infrared Astronomy

Infrared astronomy

Infrared astronomy involves the use of special telescopes that detect electromagnetic radiation (radiation that transmits energy through the inter-action of electricity and magnetism) at infrared wavelengths. The recent development of this technology has led to the discovery of many new stars, galaxies, asteroids, and quasars.

Electromagnetic spectrum

Light is a form of electromagnetic radiation. The different colors of light that our eyes can detect correspond to different wavelengths of light. Red light has the longest wavelength; violet has the shortest. Orange, yellow, green, blue, and indigo are in between. Infrared light, ultraviolet light, radio waves, microwaves, and gamma rays are all forms of electromagnetic radiation, but they differ in wavelength and frequency. Infrared light has slightly longer wavelengths than red light. Our eyes cannot detect infrared light, but we can feel it as heat.

Infrared telescopes

Two types of infrared telescopes exist: those on the ground and those carried into space by satellites. The use of ground-based telescopes is somewhat limited because carbon dioxide and water in the atmosphere absorb much of the incoming infrared radiation. The best observations are made at high altitudes in areas with dry climates. Since infrared telescopes are not affected by light, they can be used during the day as well as at night.

Words to Know

Dwarf galaxy: An unusually small, faint group of stars.

Electromagnetic radiation: Radiation that transmits energy through the interaction of electricity and magnetism.

Infrared detector: An electronic device for sensing infrared light.

Infrared light: Portion of the electromagnetic spectrum with wavelengths slightly longer than optical light that takes the form of heat.

Optical (visible) light: Portion of the electromagnetic spectrum that we can detect with our eyes.

Quasars: Extremely bright, starlike sources of radio waves that are the oldest known objects in the universe.

Redshift: Shift of an object's light spectrum toward the red-end of the visible light rangean indication that the object is moving away from the observer.

Stellar nurseries: Areas within glowing clouds of gas and dust where new stars are formed.

Space-based infrared telescopes pick up much of the infrared radiation that is blocked by Earth's atmosphere. In the early 1980s, an international group made up of the United States, England, and the Netherlands launched the Infrared Astronomical Satellite (IRAS). Before running out of liquid helium (which the satellite used to cool its infrared detectors) in 1983, IRAS uncovered never-before-seen parts of the Milky Way, the galaxy that's home to our solar system.

In 1995, the European Space Agency launched the Infrared Space Observatory (ISO), an astronomical satellite. Before it ran out of liquid helium in 1998, the ISO discovered protostars, planet-forming nebula around dying stars, and water throughout the universe (including in the gas giants like the planets Saturn and Uranus).

In mid-2002, the National Aeronautics and Space Administration (NASA) plans to launch the Space Infrared Telescope Facility (SIRTF), which will see infrared radiation and peer through the veil of gas and dust that obscures most of the universe from view. It will be the most sensitive instrument ever to look at the infrared spectrum in the universe. SIRTF researchers will study massive black holes, young dusty star systems, and the evolution of galaxies up to 12 billion light-years away.

Discoveries with infrared telescopes

Infrared telescopes have helped astronomers find where new stars are forming, areas known as stellar nurseries. A star forms from a collapsing cloud of gas and dust. Forming and newly formed stars are still enshrouded by a cocoon of dust that blocks optical light. Thus infrared astronomers can more easily probe these stellar nurseries than optical astronomers can. The view of the center of our galaxy is also blocked by large amounts of interstellar dust. The galactic center is more easily seen by infrared than by optical astronomers.

With the aid of infrared telescopes, astronomers have also located a number of new galaxies, many too far away to be seen by visible light. Some of these are dwarf galaxies, which are more plentifulbut contain fewer starsthan visible galaxies. The discovery of these infrared dwarf galaxies has led to the theory that they once dominated the universe and then came together over time to form visible galaxies, such as the Milky Way.

With the growing use of infrared astronomy, scientists have learned that galaxies contain many more stars than had ever been imagined. Infrared telescopes can detect radiation from relatively cool stars, which give off no visible light. Many of these stars are the size of the Sun. These discoveries have drastically changed scientists' calculations of the total mass in the universe.

Infrared detectors have also been used to observe far-away objects such as quasars. Quasars have large redshifts, which indicate that they are moving away from Earth at high speeds. In a redshifted object, the waves of radiation are lengthened and shifted toward the red end of the spectrum. Since the redshift of quasars is so great, their visible light gets stretched into infrared wavelengths. While these infrared wavelengths are undetectable with optical telescopes, they are easily viewed with infrared telescopes.

[See also Electromagnetic spectrum; Galaxy; Spectroscopy; Star; Starburst galaxy ]

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infrared astronomy

infrared astronomy, study of celestial objects by means of the infrared radiation they emit, in the wavelength range from about 1 micrometer to about 1 millimeter. All objects, from trees and buildings on the earth to distant galaxies, emit infrared (IR) radiation. The study of such radiation from celestial objects is of particular importance for several reasons. Cosmic dust particles effectively obscure parts of the visible universe, such as the center of our galaxy, the Milky Way, but this dust is transparent in the IR wavelengths. Most of the energy radiated by objects ranging from interstellar matter to planets lies in the IR wavelengths; IR observations are therefore significant in studying asteroids, comets, planetary satellites, and interstellar dust clouds where stars are forming. Finally, because the expansion of the universe shifts energy to longer wavelengths, most of the visible radiation emitted by stars and galaxies during the early stages of the formation of the universe is now shifted to the IR range; studies of the most distant objects in the IR spectrum are necessary if astronomers are to understand how the universe was formed.

The beginnings of IR astronomy can be traced to the discovery of IR radiation in the spectrum of the sun by English astronomer Sir William Herschel about 1800. It is reported that Irish astronomer Lord William Rosse detected IR radiation from the moon about 1845. As early as 1878 the American inventor Thomas Alva Edison observed a solar eclipse from a site in Wyoming using a sensitive IR detector, and during the 1920s the first systematic IR observations of celestial objects were made by Seth B. Nicholson, Edison Pettit, and other American astronomers. However, modern IR astronomy did not begin until the 1950s because of the lack of appropriate instrumentation. Since then, special interference filters and cryogenic systems (to minimize IR interference from the radiation emitted by the equipment itself) have been introduced for ground-based observations, and aircraft, balloons, rockets, and orbiting satellites have been successively employed to carry the equipment above the water vapor in the earth's atmosphere.

The Kuiper Airborne Observatory (KAO), operated by the National Aeronautics and Space Administration (NASA), had its first flight in 1975. Named for the American astronomer Gerard P. Kuiper, the KAO was a C-141 jet transport that carried its 36-inch (91-cm) telescope to altitudes of up to 45,000 ft (13,720 m). Before it flew its last mission in 1995, the KAO was instrumental in the discovery of the rings of Uranus, the atmosphere around Pluto, and the definitive detection of water during the crash of comet Shoemaker-Levy 9 into Jupiter. Also sponsored by NASA is the Infrared Telescope Facility, a 10-ft (3-m) IR telescope located at an altitude of 14,000 ft (4,270 m) on the summit of Mauna Kea in Hawaii; established in 1979, it effectively is the U.S. national IR observatory. Also near the summit of Mauna Kea is the 12.5-ft (3.8-m) United Kingdom Infrared Telescope (UKIRT), the largest telescope in the world used solely for IR observations.

The first IR satellite to be launched (1983) was the Infrared Astronomical Satellite (IRAS), a joint venture of the United States, Great Britain, and the Netherlands. Orbiting the earth for 10 months, IRAS performed an all-sky survey that yielded catalogs of hundreds of thousands of IR sources, more than half of these previously unknown, including asteroids and comets; detected a new class of long-lived "cool" galaxies that are dim in the visible region of the spectrum; located a protoplanetary disk around a nearby star; and showed clearly for the first time the bulge near the center of the Milky Way. In 1989 the second IR satellite, the Cosmic Background Explorer (COBE), was launched by NASA. Operating through 1993, COBE detected small temperature variations in the cosmic microwave background radiation that provided vital clues to the nature of the early universe and its evolution since the "big bang." The European Space Agency (ESA) launched the Infrared Space Observatory (ISO) in 1995. Operating until May, 1998, ISO monitored nearby planets, asteroids, and comets. It found water vapor in the atmospheres of Saturn, Neptune, Uranus, and Titan, Saturn's largest moon; detected water vapor and fluorides in the interstellar medium; and studied the "cool" galaxies first seen by IRAS. The near-infrared camera multiobject spectrometer (NICMOS) was placed aboard the Hubble Space Telescope in 1997. Consisting of three cameras and three spectrometers, it has been used to study interstellar clouds where stars are being formed, young stars, and the atmospheres of Jupiter and Uranus.

The Spitzer Space Telescope, a cryogenically cooled satellite observatory with a 2.8-ft (0.85-m) telescope, was launched in Aug., 2003, and placed in a solar orbit in which it trails the earth by 5.4 million mi (8.7 million km); the lifetime of its main instruments ended in 2009. In May, 2009, ESA launched the Herschel Space Telescope, with a 138-in. (3.5-m) mirror; it also was cryogenically cooled. Positioned some 930,000 mi (1.5 million km) from earth on a mission that lasted until 2013, it observed wavelengths from the infrared to the submillimeter. NASA's Wide-field Infrared Survey Explorer (WISE) was launched in Dec., 2009, on a six-month mission to survey the entire sky at infrared wavelengths. A KAO replacement, the Stratospheric Observatory for Infrared Astronomy (SOFIA), flew its first official science mission in 2010. Consisting of a Boeing 747-SP aircraft modified to accommodate a 8.2-ft (2.5-m) reflecting telescope (the largest airborne telescope in the world), it is a joint project of NASA and the German space agency, DLR.

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Infrared Astronomy

Infrared Astronomy

Electromagnetic spectrum

Utilizing infrared astronomy

Infrared view

Resources

Infrared astronomy is the study of infrared (IR) radiation, or that radiation between 750 and one million nanometers (where, one micrometer equals 1,000 nanometers) in wavelength. Throughout most of history astronomers were confined to using optical light, the light scientists can detect with their eyes. The advent of electronic detectors has, in the past few decades, opened up new vistas to astronomers, allowing them to utilize the entire electromagnetic spectrum. Infrared astronomers use traditional optical telescopes equipped with special detectors that can detect infrared light. Earths atmosphere is, for the most part, only mildly transparent to infrared light, so infrared astronomers work from high, dry mountain tops, airplanes, high altitude balloons, or space. The infrared spectral window allows astronomers to probe dusty regions of the universe that obscure optical light.

Electromagnetic spectrum

Light is a form of electromagnetic radiation. The electromagnetic waves that comprise electromagnetic radiation consist of oscillations in electric and magnetic fields, just as water waves consist of oscillations of the water in the ocean.

Certain properties describe all types of waves. One is the wavelength, which is the distance between two adjacent peaks in the wave. The frequency is the number of peaks that move past a stationary observer in one second. In the case of water waves at the beach, the frequency would be the number of incoming waves that hit a person in one second, and the wavelength would be the distance between two waves. A higher frequency corresponds to a shorter wavelength and vice versa.

The different colors of light that eyes can detect correspond to different wavelengthsor frequencies of light. Red light has a longer wavelength than violet light. Orange, yellow, green, and blue are in between. Infrared light, ultraviolet light, radio waves, microwaves, and gamma rays are all forms of electromagnetic radiation, but they differ in wavelength and frequency.

Infrared light has slightly longer wavelengths than red light. Human eyes cannot detect infrared light, but humans can feel it as heat. Infrared astronomy uses the wavelength range from about 0.75 to just less than one thousand micrometers. Wavelengths near 1,000 micrometers (1 millimeter) are considered radio waves and studied by radio astronomers using different techniques than infrared astronomers.

Infrared astronomers divide the infrared spectrum into near-, mid-, and far-infrared. The exact boundaries between these regions are indistinguishable, but near-infrared is generally considered to be from 0.75 to five micrometers. Wavelengths of five to 20 micrometers are considered mid-infrared. Wavelengths longer than about 20 micrometers are far-infrared.

Utilizing infrared astronomy

Special infrared detectors must be used to see the infrared universe. These detectors can be mounted on traditional optical telescopes either on the ground or above the atmosphere. The first infrared detector was a thermometer used by German-born English astronomer William Herschel (1738-1822) in 1800. He passed sunlight through a prism and placed the thermometer just beyond the red light to detect the heat from the infrared light. To detect the heat from distant stars and galaxies, modern infrared detectors must be considerably more sensitive. The infancy of infrared astronomy began with the advent of these detectors in the 1960s.

Modern infrared detectors use exotic combinations of semiconductors that are cooled to either liquid nitrogen or liquid helium temperatures. Photovoltaic detectors utilize the photoelectric effect, the same principle as the solar cell in a solar powered calculator. Light strikes certain materials and kicks the electrons away from the atoms to produce an electric current as the electrons move. Because infrared light has less energy than ordinary optical light, photovoltaic infrared detectors must be made from materials that require little energy to force the electron from the atom.

Photoresistive thermal detectors work by measuring minute changes in the electrical resistance of the detector. The electrical resistance of a wire generally depends on its temperature. Infrared radiation striking a photoresistive detector will raise its temperature and, therefore, change its electrical resistance by a minute amount. A mixture of gallium and germanium is often used. These detectors must be cooled with liquid helium to get the extreme sensitivity required by infrared astronomers.

Early infrared detectors featured a single channel. Accordingly, they could measure the brightness of a single region of the sky seen by the detector, but could not produce pictures. Early infrared images or maps were quite tedious to make. Images were created by measuring the brightness of a single region of the sky, moving the telescope a bit, measuring the brightest of a second region, and so on.

In the 1980s, infrared arrays revolutionized infrared imaging. Arrays are essentially two-dimensional grids of very small, closely spaced individual detectors, or pixels. Infrared arrays as large as 256 x 256 pixels, or even larger, are now available, allowing astronomers to create infrared images in a reasonable amount of time.

In addition to images, astronomers can measure the brightness of an infrared source at various infrared wavelengths. Detectors record a range of wavelengths, so a filter must be used to select a specific wavelength.

This measurement of brightness is called photometry. Both optical and infrared astronomers break light up into its component colors, its spectrum. This can be done on a smaller scale by passing light through a prism. This process, spectroscopy, is useful for finding the compositions, motions, physical conditions, and many other properties of stars and other celestial objects. When light is polarized, the electromagnetic oscillations line up. Infrared polarimetry, measuring the amount of polarization, is useful in deducing optical properties of the dust grains in dusty infrared sources.

Ground-based infrared astronomy

Infrared light is heavily absorbed by both carbon dioxide and water vapor, major components of Earths atmosphere. Accordingly, the atmosphere is opaque to many infrared wavelengths. There are a few specific wavelength bands between one and five micrometers, around 10 micrometers, and, sometimes, near 20 micrometers at which the atmosphere is partially transparent. These bands make up the standard ground-based infrared bands. Still, astronomers must build infrared observatories at very dry, high-altitude sites to get above as much atmosphere as possible. One of the best infrared sites in the world is the 14,000-ft (4,200-m) summit of Mauna Kea in Hawaii. On a clear night, half a dozen large telescopes may probe the infrared sky, although some of the telescopes are used for optical astronomy. The high altitude at Mauna Kea makes observation at its summit very rigorous.

There are special difficulties to infrared astronomy, especially from the ground. The heat radiation from the telescope, telescope building, and atmosphere are all very bright in the infrared. They combine into an infrared background that is at least a million times brighter than strong astronomical infrared sources. To account for this strong background astronomers rapidly oscillate the telescope field of view from the star to a region of sky nearby. Taking the difference of the two intensities allows astronomers to subtract the background.

Airborne and space infrared astronomy

To conduct experiments in infrared astronomy at wavelengths other than those observable from the ground, astronomers must place their telescopes above the atmosphere. Options include mounting telescopes on high-altitude balloons, airplanes, rockets, or satellites. High-altitude balloons are less expensive than the other options, but astronomers cannot ride with the telescope and have little control over the flight path of the balloon. Today aircraft are more frequently used. Since 1974, NASA has operated the Gerald P. Kuiper Airborne Observatory (KAO), which is a 36 in (91 cm) infrared telescope in a military cargo plane. It flies at high altitudes in a controlled path with the astronomers along to operate the telescope. Astronomers can make observations at far-infrared wavelengths with more control than from a balloon. The KAO was retired in 1996. As of 2006, NASA is in the process of replacing the KAO with the Stratospheric Observatory for Infrared Astronomy (SOFIA), a 100-in (254 cm) telescope that will be flown on a Boeing 747SP. As of October 2006, the SOFIA was in its final test stage before being turned operational.

To record long-term images from space, astronomers must place infrared telescopes on orbiting satellites. Such experiments are quite expensive, but allow astronomers to record a large number of observations. Infrared observatories in space have a more limited lifetime than other space observatories because they run out of liquid helium. Space is cold, but not cold enough for infrared detectors, so they must still be cooled with liquid helium, which evaporates after a year or two. Astronomers must carefully plan their observations to get the most out of the limited lifetime.

In the early 1980s the Infrared Astronomical Satellite (IRAS) surveyed the entire sky at four infrared wavelengths not accessible from the ground (12, 25, 60, and 100 micrometers). The helium ran out in 1983 after a successful mission. Astronomers are still mining the vast amounts of data accumulated from that experiment. The satellite charted the positions of about 15,000 galaxies, allowing a sky survey team to produce a three-dimensional map that covers a sphere with a radius of 700 million light-years. Of particular interest to astronomers is the presence of massive superclusters, consisting of formed of galactic clusters containing dozens to thousands of galaxies like Earths own Milky Way galaxy. Between these super-clusters lie vast voids that are nearly galaxy-free, provoking great interest from scientists.

In 1995, the European Space Agency launched the Infrared Space Observatory (ISO), an astronomical satellite that operated at wavelengths from 2.5 to 240 micrometers. ISO allowed astronomers to study comet Hale-Bopp in detail. The satellite discovered protostars, planet-forming nebula around dying stars, and water throughout the universe, including in star-forming regions and in the atmospheres of the gas giants like Saturn and Uranus. The telescope survived until 1998, when it ran out of liquid helium.

On August 25, 2003, another infrared satellite was launched by NASA. Called the Spitzer Space Telescope, formerly called the Space Infrared Telescope Facility (SIRTF), it is a cryogenically-cooled infrared observatory that will be able to observe objects within the solar system and those near the far reaches of the universe. It is in a heliocentric orbit (about the sun). The satellite has a primary mirror that is made of beryllium, 33.5 in (65 cm) in diameter, and cooled to 5.5K (Kelvin). Its three instruments onboard will permit spectroscopy study from five to 40 micrometers, and spectrophotometry study from five to 100 micrometers.

Infrared view

Infrared light penetrates dust much more easily than optical light. For this reason infrared astronomy is most useful for learning about dusty regions of the universe.

One example is star-forming regions. A star forms from a collapsing cloud of gas and dust. Forming and newly formed stars are still enshrouded by a cocoon of dust that blocks optical light. Infrared astronomers can more easily probe these stellar nurseries than optical astronomers can. The view of the center of our galaxy is also blocked by large amounts of interstellar dust. The galactic center is more easily seen by infrared than by optical astronomers.

Many molecules emit primarily in the infrared and radio regions of the spectrum. One example is the hydrogenmolecule (H2) that emits in the infrared. Infrared astronomers can study the distribution of these different kinds of molecules to learn about the processes forming molecules in interstellar space and the clouds in which these molecules form.

In 1998, using data from the Cosmic Background Explorer (COBE), astronomers discovered a background infrared glow across the sky. Radiated by dust that absorbed heat from all the stars that have ever existed, the background glow puts a limit on the total amount of energy released by all the stars in the universe.

Astronomers began with data acquired by COBE, then modeled and subtracted the infrared glow from foreground objects in the solar system, the galaxys stars, and vast clouds of cold dust between the stars of the Milky Way. What remained was a smooth background of residual infrared light in the 240 and 140 micrometer wavelength bands in windows near the north and south poles of the Milky Way, which provide a relatively clear view across billions of light years.

KEY TERMS

Infrared detector An electronic device for detecting infrared light.

Infrared light Light with wavelengths longer than those of visible light, often used in astronomy to study dim objects.

Optical (visible) light The portion of the electromagnetic spectrum that humans can detect with eyes.

In 2006, American astrophysicists and cosmologists George Fitzgerald Smoot III (1945) and John Cromwell Mather (1946) received the Nobel Prize in physics for their work involving COBE. The two men, along with thousands of engineers and technicians, discovered the black body (any body that absorbs all electromagnetic radiation that falls upon it) form and anisotropy (property of being directionally dependent) of the cosmic microwave background radiation. Their discovery in infrared astronomy helps to further validate the big bang theory of the universe,.

The above examples are just a few of the observations made by infrared astronomers. In the past few decades, the new vistas opened in the infrared and other spectral regions have revolutionized astronomy. As such, telescopes in outer space have been found to be ideal places to apply infrared astronomy. Consequently, many telescopes are launched that include the ability to view infrared radiation. In fact, the Herschel Space Observatory, formerly called the Far Infrared and Sub-millimeter Telescope, is planned to be launched in 2008 by the European Space Agency. As its former name states, it will study the far infrared and sub-millimeter wavelengths of the infrared radiation bandwidth. In addition, it will also contain the largest mirror ever launched into space: with a diameter of 11.5 ft (3.5 m).

See also Stellar evolution.

Resources

BOOKS

Arny, Thomas. Explorations: An Introduction to Astronomy. Boston, MA: McGraw-Hill, 2006.

Bacon, Dennis Henry, and Percy Seymour. A Mechanical History of the Universe. London: Philip Wilson Publishing, Ltd., 2003.

Chaisson, Eric. Astronomy: A Beginners Guide to the Universe. Upper Saddle River, NJ: Pearson/Prentice Hall, 2004.

Hanel, R.A., et al. Exploration of the Solar System by Infrared Remote Sensing. Cambridge, UK, and New York: Cambridge University Press, 2003.

Kundt, Wolfgang. Astrophysics: A New Approach. Berlin and New York: Springer, 2005.

Smolin, Lee. The Life of the Cosmos. Oxford: Oxford University Press, 1999.

OTHER

Smoot Group, Lawrence Berkeley Laboratory, University of California at Berkeley. Cosmic Background Explorer. <http://aether.lbl.gov/www/projects/cobe/> (accessed October 12, 2006).

Spitzer Science Center, Jet Propulsion Laboratory, NASA. Spitzer Space Telescope. <http://www.spitzer.caltech.edu/> (accessed October 12, 2006).

Paul A. Heckert

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Infrared Astronomy

Infrared astronomy

Throughout most of history astronomers were confined to using optical light , the light we can detect with our eyes. The advent of electronic detectors has, in the past few decades, opened up new vistas to astronomers, allowing them to utilize the entire electromagnetic spectrum . Infrared astronomers use traditional optical telescopes equipped with special detectors that can detect infrared light. Earth's atmosphere is, for the most part, only mildly transparent to infrared light, so infrared astronomers work from high, dry mountain tops, airplanes, high altitude balloons, or space . The infrared spectral window allows astronomers to probe dusty regions of the universe that obscure optical light.


Electromagnetic spectrum

Light is a form of electromagnetic radiation . The electromagnetic waves that comprise electromagnetic radiation consist of oscillations in electric and magnetic fields, just as water waves consist of oscillations of the water in the ocean .

Certain properties describe all types of waves. One is the wavelength, which is the distance between two adjacent peaks in the wave. The frequency is the number of peaks that move past a stationary observer in one second. In the case of water waves at the beach, the frequency would be the number of incoming waves that hit a person in one second, and the wavelength would be the distance between two waves. A higher frequency corresponds to a shorter wavelength and vice versa.

The different colors of light that our eyes can detect correspond to different wavelengths—or frequencies—of light. Red light has a longer wavelength than violet light. Orange, yellow, green, and blue are in between. Infrared light, ultraviolet light, radio waves , microwaves, and gamma rays are all forms of electromagnetic radiation, but they differ in wavelength and frequency.

Infrared light has slightly longer wavelengths than red light. Our eyes can not detect infrared light, but we can feel it as heat . Infrared astronomy uses the wavelength range from about 1 micrometer to a few hundred micrometers. Wavelengths near 1,000 micrometers (1 millimeter) are considered radio waves and studied by radio astronomers using different techniques than infrared astronomers.

Infrared astronomers divide the infrared spectrum into near-, mid-, and far-infrared. The exact boundaries between these regions are indistinguishable, but near-infrared is generally considered to be from one to five micrometers. Wavelengths of 5-20 micrometers are considered mid-infrared. Wavelengths longer than about 20 mircrometers are far-infrared.

Utilizing infrared astronomy

Special infrared detectors must be used to see the infrared universe. These detectors can be mounted on traditional optical telescopes either on the ground or above the atmosphere. The first infrared detector was a thermometer used by William Herschel in 1800. He passed sunlight through a prism and placed the thermometer just beyond the red light to detect the heat from the infrared light. To detect the heat from distant stars and galaxies, modern infrared detectors must be considerably more sensitive. The infancy of infrared astronomy began with the advent of these detectors in the 1960s.

Modern infrared detectors use exotic combinations of semiconductors that are cooled to either liquid nitrogen or liquid helium temperatures. Photovoltaic detectors utilize the photoelectric effect , the same principle as the solar cell in a solar powered calculator . Light strikes certain materials and kicks the electrons away from the atoms to produce an electric current as the electrons move. Because infrared light has less energy than ordinary optical light, photovoltaic infrared detectors must be made from materials that require little energy to force the electron from the atom.

Photoresistive thermal detectors work by measuring minute changes in the electrical resistance of the detector. The electrical resistance of a wire generally depends on its temperature . Infrared radiation striking a photoresistive detector will raise its temperature and therefore change its electrical resistance by a minute amount. A mixture of gallium and germanium is often used. These detectors must be cooled with liquid helium to get the extreme sensitivity required by infrared astronomers.

Early infrared detectors featured a single channel. Accordingly, they could measure the brightness of a single region of the sky seen by the detector, but could not produce pictures. Early infrared images or maps were quite tedious to make. Images were created by measuring the brightness of a single region of the sky, moving the telescope a bit, measuring the brightest of a second region, and so on.

In the 1980s infrared arrays revolutionized infrared imaging. Arrays are essentially two dimensional grids of very small, closely spaced individual detectors, or pixels. Infrared arrays as large as 256 × 256 pixels are now available, allowing astronomers to create infrared images in a reasonable amount of time.

In addition to images, astronomers can measure the brightness of an infrared source at various infrared wavelengths. Detectors record a range of wavelengths, so a filter must be used to select a specific wavelength. This measurement of brightness is called photometry. Both optical and infrared astronomers break light up into its component colors, its spectrum. This can be done on a smaller scale by passing light through a prism. This process, spectroscopy , is useful for finding the compositions, motions, physical conditions, and many other properties of stars and other celestial objects. When light is polarized, the electromagnetic oscillations line up. Infrared polarimetry, measuring the amount of polarization, is useful in deducing optical properties of the dust grains in dusty infrared sources.


Ground-based infrared astronomy

Infrared light is heavily absorbed by both carbon dioxide and water vapor, major components of Earth's atmosphere. Accordingly, the atmosphere is opaque to many infrared wavelengths. There are a few specific wavelength bands between one and five micrometers, around 10 micrometers, and sometimes near 20 micrometers at which the atmosphere is partially transparent. These bands make up the standard ground based infrared bands. Still, astronomers must build infrared observatories at very dry, high-altitude sites to get above as much atmosphere as possible. One of the best infrared sites in the world is the 14,000-ft (4,200-m) summit of Mauna Kea in Hawaii. On a clear night half a dozen large telescopes may probe the infrared sky, although some of the telescopes are used for optical astronomy. The high altitude at Mauna Kea makes observation at its summit very rigorous.

There are special difficulties to infrared astronomy, especially from the ground. The heat radiation from the telescope, telescope building, and atmosphere are all very bright in the infrared. They combine into an infrared background that is at least a million times brighter than strong astronomical infrared sources. To account for this strong background astronomers rapidly oscillate the telescope field of view from the star to a region of sky nearby. Taking the difference of the two intensities allows astronomers to subtract the background.


Airborne and space infrared astronomy

To conduct experiments in infrared astronomy at wavelengths other than those observable from the ground, astronomers must place their telescopes above the atmosphere. Options include mounting telescopes on high-altitude balloons, airplanes, rockets, or satellites. High-altitude balloons are less expensive than the other options, but astronomers cannot ride with the telescope and have little control over the flight path of the balloon . Today aircraft are more frequently used. Since 1974, NASA has operated the Kuiper Airborne Observatory (KAO), which is a 36 in (91 cm) infrared telescope in a military cargo plane. It flies at high altitudes in a controlled path with the astronomers along to operate the telescope. Astronomers can make observations at far-infrared wavelengths with more control than from a balloon. Beginning in 2001, NASA is replacing the KAO with the Stratospheric Observatory for Infrared Astronomy (SOFIA), a 100 in (254 cm) telescope that will be flown on a 747.

To record long-term images from space, astronomers must place infrared telescopes on orbiting satellites. Such experiments are quite expensive, but allow astronomers to record a large number of observations. Infrared observatories in space have a more limited lifetime than other space observatories because they run out of liquid helium. Space is cold, but not cold enough for infrared detectors, so they must still be cooled with liquid helium, which evaporates after a year or two. Astronomers must carefully plan their observations to get the most out of the limited lifetime.

In the early 1980s the Infrared Astronomical Satellite (IRAS) surveyed the entire sky at four infrared wavelengths not accessible from the ground (12, 25, 60, and 100 micrometers). The helium ran out in 1983 after a successful mission. Astronomers are still mining the vast amounts of data accumulated from that experiment. The satellite charted the positions of 15,000 galaxies, allowing a sky survey team to produce a three-dimensional map that covers a sphere with a radius of 700 million lightyears. Of particular interest to astronomers is the presence of massive superclusters , consisting of formed of galactic clusters containing dozens to thousands of galaxies like our own. Between these superclusters lie vast voids that are nearly galaxy-free, provoking great interest from scientists.

In 1995, the European Space Agency launched the Infrared Space Observatory (ISO), an astronomical satellite that operated at wavelengths from 2.5 to 240 micrometers. ISO allowed astronomers to study comet Hale-Bopp in detail. The satellite discovered protostars, planet-forming nebula around dying stars, and water throughout the universe, including in star-forming regions and in the atmospheres of the gas giants like Saturn and Uranus . The telescope was live until 1998, when it ran out of liquid helium.

Future infrared satellites planned include the NASA's Space Infrared Telescope Facility (SIRTF), slated for launch in late 2001.


Infrared view

Infrared light penetrates dust much more easily than optical light. For this reason infrared astronomy is most useful for learning about dusty regions of the universe.

One example is star-forming regions. A star forms from a collapsing cloud of gas and dust. Forming and newly formed stars are still enshrouded by a cocoon of dust that blocks optical light. Infrared astronomers can more easily probe these stellar nurseries than optical astronomers can. The view of the center of our galaxy is also blocked by large amounts of interstellar dust. The galactic center is more easily seen by infrared than by optical astronomers.

Many molecules emit primarily in the infrared and radio regions of the spectrum. One example is the hydrogen molecule (H2) which emits in the infrared. Infrared astronomers can study the distribution of these different kinds of molecules to learn about the processes forming molecules in interstellar space and the clouds in which these molecules form.

In 1998, using data from the Cosmic Background Explorer (COBE), astronomers discovered a background infrared glow across the sky. Radiated by dust that absorbed heat from all the stars that have ever existed, the background glow puts a limit on the total amount of energy released by all the stars in the universe.

Astronomers began with data acquired by COBE, then modeled and subtracted the infrared glow from foreground objects in our solar system , our galaxy's stars, and vast clouds of cold dust between the stars of our Milky Way . What remained was a smooth background of residual infrared light in the 240 and 140 micrometer wavelength bands in "windows" near the north and south poles of the Milky Way, which provide a relatively clear view across billions of light years.

The above examples are just a few of the observations made by infrared astronomers. In the past few decades, the new vistas opened in the infrared and other spectral regions have revolutionized astronomy.

See also Stellar evolution.

Resources

books

Bacon, Dennis Henry, and Percy Seymour. A Mechanical History of the Universe. London: Philip Wilson Publishing, Ltd., 2003.

Smolin, Lee. The Life of the Cosmos. Oxford: Oxford University Press, 1999.

periodicals

Gatley, Ian. "An Infrared View of our Universe." Astronomy (April 1994): 40-43.

Stephens, Sally. "Telescopes That Fly." Astronomy (November 1994): 46-53.


Paul A. Heckert

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Infrared detector

—An electronic device for detecting infrared light.

Infrared light

—Light with wavelengths longer than those of visible light, often used in astronomy to study dim objects.

Optical (visible) light

—The portion of the electro-magnetic spectrum that we can detect with our eyes.

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