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Sci-Tech Dictionary:

infrared astronomy

(¦in·frə¦red ə′strän·ə·mē)

(astrophysics) The study of electromagnetic radiation in the spectrum between 0.75 and 1000 micrometers emanating from astronomical sources.

 
 
Sci-Tech Encyclopedia: Infrared astronomy

The field of astronomical observations specializing in detecting photons from the infrared portion of the electromagnetic spectrum. The infrared portion of the spectrum spans the range from the red limit of human vision (approximately 0.7 micrometer) to the shortest wavelengths accessible to heterodyne radio receivers (several hundred micrometers). See also Electromagnetic radiation; Infrared radiation.

Astronomers observing the universe with infrared light encounter a number of fundamental differences relative to those observing with visible light:

1. Infrared observations are more sensitive to cooler objects than visible-wavelength observations. Blackbodies at a temperature cooler than 2000 kelvins (3100°F) radiate virtually all of their light in the infrared part of the spectrum. Infrared observations are particularly well suited to detect both forming stars and evolved stars (that is, stars in the final stages of their lives) since both classes of objects are cool. Since starlight often heats nearby dust grains to temperatures of tens or hundreds of degrees, reprocessing the visible starlight into exclusively infrared radiation, warm dust is also a common target for infrared observations. See also Heat radiation.

2. Interstellar dust is substantially more transparent at infrared wavelengths than at visible wavelengths. Infrared observations thus enable astronomers to view distant objects through the obscuring dust that permeates the Milky Way Galaxy. Forming and evolved stars often reside in dense clouds of interstellar dust grains and can be observed only at infrared and even longer radio wavelengths. See also Interstellar extinction; Interstellar matter; Scattering of electromagnetic radiation.

3. The quantized energies of molecular rotational and vibrational transitions, which give rise to molecular spectral lines, fall largely in the infrared part of the spectrum, as do many hyperfine lines of individual atoms. See also Infrared spectroscopy.

In addition to these astrophysical differences, the technology and practice of infrared astronomy differs from visible-wavelength astronomy in fundamental ways:

1. The Earth's atmosphere is opaque to infrared radiation through a substantial fraction of the spectrum. This opacity arises largely from water in the Earth's lower atmosphere.

2. Planck's law dictates that only photons with wavelength shorter than about 1 μm can induce chemical reactions or liberate free electrons. Thus physics limits photography and photon-counting photomultiplier tubes to operation mainly in the visible-wavelength domain. Infrared detection technology relies largely on either mimicking the photoelectric effect inside a crystalline semiconductor material or monitoring the temperature change of a semiconductor under the influence of infrared radiation. See also Photoemission.

3. Objects at room temperature (300 K or 80°F) emit radiation throughout most of the infrared spectrum. This glow interferes with the detection of faint astronomical sources, limiting the sensitivity of observations. Cooling of the entire telescope—impractical on the ground but possible in the vacuum of space—can substantially reduce the thermal glow of the telescope optics and result in unprecedented sensitivity to faint astronomical sources.

Infrared technology

Detectors of infrared radiation divide into two classes: bolometers and photovoltaic or photoconductive devices. Bolometer detectors have temperature-sensitive electrical conductivity. Incident radiation warms the detector, and the resulting subtle change in the electrical resistance of the detector is measured. Infrared photodetectors are crystalline semiconductors in which some electrons in the crystal lattice lie only a short distance in energy away from becoming unbound and behaving like metallic conducting electrons. Infrared light with energy in excess of the binding energy creates free charge carriers, either changing the bulk conductivity of the device (photoconductors) or charging or discharging a semiconductor junction (photovoltaics). See also Bolometer; Photoconductivity; Photovoltaic effect.

Since the mid-1980s, large-scale integration of semiconductor components has permitted the production of arrays of infrared detectors. These arrays now exist in formats as large as 2048 × 2048 elements (4 million detectors on a single device). Each detector on such an array is also substantially more sensitive than its 1980s counterpart.

Astronomical targets

The targets of infrared observations include ordinary stars and galaxies, planets, brown dwarfs, young stellar objects, evolved stars, starburst galaxies, and redshifted radiation.

Although popular interest in infrared astronomy focuses on exotic objects observable only using infrared light, infrared observations continue to play a fundamental role in understanding the more pedestrian stars and galaxies that constitute most of the visible-wavelength universe. At infrared wavelengths longer than 10 μm, infrared light emitted from dust warmed by stars in normal galaxies begins to dominate the infrared light from stars, making dusty spiral galaxies prime targets for mid-infrared observations. See also Galaxy, external; Hertzsprung-Russell diagram; Milky Way Galaxy; Star.

At infrared wavelengths longer than 3 μm, thermal radiation from the planets begins to dominate over reflected sunlight. Jupiter, still cooling from its initial formation, emits twice as much energy as it receives from the Sun—nearly all of it at infrared wavelengths. See also Asteroid; Jupiter; Planet; Planetary physics.

An object with a mass less than 8% that of the Sun (equivalent to 80 times the mass of Jupiter) is incapable of fusing hydrogen in its core to sustain its luminosity—the hallmark of being a star. At the time of their formation, the interiors of such substellar objects are warmed by their gravitational contraction from an initially diffuse interstellar cloud of gas. Even at this most luminous point in their evolution, most of the light emerges in the infrared part of the spectrum. Infrared surveys of the entire sky are beginning to reveal large numbers of these brown dwarfs, and they appear to be more common than ordinary stars in the Milky Way Galaxy. See also Brown dwarf.

Young stellar objects (YSOs) are newly formed or forming stars. These stars are being assembled by gravity out of dense (1000 atoms per cubic centimeter) interstellar clouds. The environment surrounding the forming star is naturally very dusty, and the slight rotation of the natal cloud combined with gravity drives the infalling material to form a thin flattened disk around the star. Virtually all of the disk emission emerges at infrared wavelengths. Planets accrete within these disks, and infrared observations provide the primary astrophysical insight into the process of planetary formation. See also Protostar.

After exhausting the initial supply of hydrogen in the stellar core, stars reconfigure themselves to liberate the energy of hydrogen shell burning around the core and helium burning within the core. Stellar physics dictates that the star must grow to large size and become cool at its surface in order to dissipate the energy being produced within. Dust grains condensing in the expanding envelope can completely enshroud the star. Under these circumstances, the observed radiation emerges entirely in the infrared part of the spectrum. These evolved stars are ideal tracers of the structure of the Galaxy. See also Giant star; Stellar evolution.

Starburst galaxies, which are undergoing active bursts of star formation, can produce most of their radiation at wavelengths of 10 μm or longer. The radiation emerges in this part of the spectrum because the star-forming regions are embedded in dust clouds which absorb the starlight and, having been warmed to temperatures of tens of kelvins, reradiate energy largely in the infrared portion of the spectrum. See also Starburst galaxy.

The gravitational interaction between two gas-rich galaxies can induce both objects to undergo an extensive burst of star formation. The resulting energy release can augment the flux of the galaxy by a factor of 10 or more with most of the radiation arising in the infrared part of the spectrum. Such “ultraluminous” infrared galaxies are among the most luminous galaxies in the universe.

The apparent Doppler shift due to the expansion of the universe causes the ultraviolet and visible light originally emitted by extremely distant stars and galaxies to be shifted into the infrared part of the spectrum. Redshifts this large originate from objects at distances of 1010 light-years or more. Since the light collected from these objects was emitted by them 1010 years ago, these observations probe the state of the universe at the earliest times. See also Big bang theory; Cosmology; Redshift.

Ground-based infrared astronomy

Telescopes dedicated to visible-wavelength astronomy are also effective collectors of infrared radiation. Equipped with infrared focal-plane-array imagers and spectrographs, these telescopes can observe the infrared universe through the accessible atmospheric windows. Such observations are particularly effective shortward of 2 μm, where thermal emission from the telescope is negligible relative to other backgrounds.

Infrared space missions

The opacity of the Earth's atmosphere at most infrared wavelengths and the need to place a telescope in an environment where the entire telescope structure can be cooled to cryogenic temperatures have motivated a number of extremely successful satellite missions largely devoted to infrared astronomy, including the Infrared Astronomy Satellite (IRAS), launched in 1983; the Infrared Space Observatory (ISO), launched in 1995; and the Space Infrared Telescope Facility (SIRTF), renamed the Spitzer Space Telescope, following its launch in 2003.

 
Britannica Concise Encyclopedia: infrared astronomy

Study of astronomical objects by observing the infrared radiation they emit. Its techniques enable examination of many celestial objects that give off energy at wavelengths in the infrared region of the electromagnetic spectrum but that cannot otherwise be seen from Earth because they do not emit much visible light or because that light is blocked by dust clouds, which infrared radiation can penetrate. Infrared astronomy originated in the early 19th century with the work of William Herschel (see Herschel family), who discovered infrared radiation while studying sunlight. The first systematic infrared observations of other stars were made in the 1920s; modern techniques, such as the use of interference filters for ground-based telescopes, were introduced in the early 1960s. Because atmospheric water vapour absorbs many infrared wavelengths, observations are carried out with telescopes sited on high mountaintops and from airborne and space-based observatories. Infrared astronomy allows studies of the dust-obscured core of the Milky Way Galaxy and the hearts of star-forming regions and has led to many discoveries including brown dwarf candidates and disks of matter around certain stars.

For more information on infrared astronomy, visit Britannica.com.

 
Columbia Encyclopedia: 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 Organization 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); it is expected to have a two-to-five-year operating lifetime. Future plans for IR astronomy include a KAO replacement, the Stratospheric Observatory for Infrared Astronomy (SOFIA), a joint project of NASA and the German space agency, DLR, that consists 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). SOFIA is expected to go into service in 2004 and have a 20-year lifetime.

 
Wikipedia: infrared astronomy

Infrared astronomy is the branch of astronomy and astrophysics which deals with objects visible in infrared (IR) radiation. Visible radiation ranges from 400 nm (blue) to 700 nm (red). Longer wavelengths than 700 nm but still shorter than microwaves are called infrared (or sometimes submillimeter waves).

Scientists classify infrared astronomy as part of optical astronomy because optical components (mirrors, lenses and solid state digital detectors) are usually used.

Discovery

After the use of prisms by Isaac Newton to split white light into a spectrum, it was found in 1800 by William Herschel that the hottest part of the band of light from the Sun was actually past the red end of the spectrum. These "heat rays" even displayed some spectral lines. Charles Piazzi Smyth in 1856 detected infrared radiation in the light of the Moon.

Modern infrared astronomy

Near infrared radiation (infrared radiation with wavelengths close to that of visible light) behaves in a very similar way to visible light, and can be detected using similar electronic devices. For this reason, the near infrared region of the spectrum is commonly incorporated as part of the "optical" spectrum, along with the near ultraviolet (most scientific instruments such as optical telescopes cover the near-infrared as well as the visible). The far infrared extends to submillimeter wavelengths, which are observed by telescopes such as the James Clerk Maxwell Telescope at Mauna Kea Observatory.

Like all other forms of electromagnetic radiation, infrared is utilised by astronomers to learn more about the universe. As infrared is essentially heat radiation, infrared telescopes (which include most major optical telescopes as well as a few dedicated infrared telescopes) need to have their detectors shielded from heat and chilled with liquid nitrogen in order to actually form images. This is particularly important in the mid infrared and far infrared regions of the spectrum. The principle limitation on infrared sensitivity from ground-based telescopes is the water vapour in the Earth's atmosphere, which absorbs a significant amount of infrared radiation. For this reason most infrared telescopes are built in very dry places at high altitude (above most of the water vapour in the atmosphere). Suitable locations on Earth include Mauna Kea Observatory at 4205 meters above sea level, the ALMA site at 5000 m in Chile and regions of high altitude ice-desert such as Dome C in Antarctic.

However, as with visible-light telescopes, space is the ideal place for their use and most optical telescopes launched into space (such as the Hubble Space Telescope) can also perform infrared observations. The recently launched Spitzer Space Telescope is dedicated solely to infrared observations.

Another way of doing infrared astronomy is by the use of airborne observatories sutch as SOFIA (Stratospheric Observatory for Infrared Astronomy) and the Kuiper Airborne Observatory.

By flying in hight altitude (Stratosphere) less water vapour will be between the telescope and space leading to a smaller IR absorption of the atmosphere.
The residual IR background (due to the absorption left) is statically removed by applying a chopping reduction technique of the observed field and a blank region.

The highest resolution infrared observations are performed by ground-based astronomical interferometers.

Infrared technology

The cheapest infrared detectors commonly used at research telescopes are HgCdTe arrays. These operate well between 1 and 5 micrometre wavelengths. For longer wavelength observations or higher sensitivity other detectors may be used, including other narrow gap semiconductor detectors, low temperature bolometer arrays or photon-counting Superconducting Tunnel Junction arrays.

Special requirements for infrared astronomy include: very low dark currents to allow long integration times, associated low noise readout circuits and sometimes very high pixel counts.

Astronomers' infrared spectrum

Infrared space telescopes such as Spitzer, IRAS, ISO and the forthcoming Herschel Space Observatory can observe across almost all of the infrared spectrum. However, most infrared astronomy is still done at ground-based telescopes, and these are limited to observations through a small number of spectral "windows", at wavelengths where the Earth's atmosphere is transparent. The main infrared windows are listed below:

Wavelength range Astronomical bands Telescopes
(micrometres)
0.65 to 1.0 R and I bands All major optical telescopes
1.25 J band Most major optical telescopes and most dedicated infrared telescopes
1.65 H band Most major optical telescopes and most dedicated infrared telescopes
2.2 K band Most major optical telescopes and most dedicated infrared telescopes
3.45 L band Most dedicated infrared telescopes and some optical telescopes
4.7 M band Most dedicated infrared telescopes and some optical telescopes
10 N band Most dedicated infrared telescopes and some optical telescopes
20 Q band Some dedicated infrared telescopes and some optical telescopes
450 submillimeter Submillimeter telescopes

Between these windows there are generally regions where infrared observations are more difficult or impossible from the ground due to the opacity of the atmosphere. Dedicated infrared and submillimeter telescopes are generally built at very high altitude sites like Mauna Kea Observatory, Hawaii and the ALMA site in Chile, or even flown on aircraft like SOFIA, providing the best sensitivity available from Earth based observatories. Data from space-based observatories like Spitzer, IRAS and ISO help fill in the gaps between the atmospheric windows listed above.

See also

External links


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Sci-Tech Dictionary. McGraw-Hill Dictionary of Scientific and Technical Terms. Copyright © 2003, 1994, 1989, 1984, 1978, 1976, 1974 by McGraw-Hill Companies, Inc. All rights reserved.  Read more
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Columbia Encyclopedia. The Columbia Electronic Encyclopedia, Sixth Edition Copyright © 2003, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/  Read more
Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Infrared astronomy".  Read more

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