Astronomy
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Infrared astronomy
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Infrared astronomy
Astronomers have found that infrared radiation is especially useful when trying to probe areas of our universe that are surrounded by clouds of gas and dust. Because of infrared’s longer wavelength, it can pass right through these clouds and reveal details invisible by observing other types of radiation. Especially interesting are areas were stars and planets are forming and the cores of galaxies where it is believed huge black holes might reside.
Infrared astronomy uses sensor-equipped telescopes to penetrate dusty regions of space, such as molecular clouds; detect objects such as planets, and view highly red-shifted objects from the early days of the universe. Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, and to detect overheating of electrical apparatus.
Infrared waves have longer wavelengths than visible light and can pass through dense regions of gas and dust in space with less scattering and absorption. Thus, infrared energy can also reveal objects in the universe that cannot be seen in visible light using optical telescopes. The James Webb Space Telescope (JWST) has three infrared instruments to help study the origins of the universe and the formation of galaxies, stars, and planets.
One of the advantages of infrared radiation observation is that it can detect objects that are too cool to emit visible light. This has led to the discovery of previously unknown objects, including comets, asteroids and wispy interstellar dust clouds that seem to be prevalent throughout the galaxy.
Infrared astronomy is particularly useful for observing cold molecules of gas and for determining the chemical makeup of dust particles in the interstellar medium, said Robert Patterson, professor of astronomy at Missouri State University. These observations are conducted using specialized CCD detectors that are sensitive to IR photons. Another advantage of infrared radiation is that its longer wavelength means it doesn’t scatter as much as visible light, according to NASA. Whereas visible light can be absorbed or reflected by gas and dust particles, the longer IR waves simply go around these small obstructions. Because of this property, IR can be used to observe objects whose light is obscured by gas and dust. Such objects include newly forming stars imbedded in nebulas or the center of Earth’s galaxy.
The birth of infrared astronomy came quite after the discovery of infrared radiation, by W. Herschel, who realized that the spectral region of the Sun located beyond the red is home to thermal radiation. The reasons of the delay are related to the chemical-physical characteristics of the infrared radiation itself. In fact, on the one hand the infrared radiation, coming from celestial sources, is usually absorbed by water vapor and carbon dioxide of the Earth’s atmosphere, on the other hand the thermal component, emanating from the same instrumentation, produces strong disturbances on the infrared signal to be detected.
Only in relatively recent times, the development of appropriate cooling techniques and the production of infrared-sensitive materials (lead sulfide cells, germanium optical parts, special photographic material), together with the realization of sophisticated electronic equipment for analysis, have allowed the start and progress of a field of research, proved to be very fertile in a short time, thanks also to space missions carried out with several probes equipped with telescopes and instrumentation for infrared.
In astronomical technology infrared bands are distinguished in thermal or photographic (up to 1.1 µm wavelength, region where detection techniques similar to those used in the visible are still possible), near (up to 4 µm), intermediate (up to 40 µm), extreme (up to 1 mm, practically in contiguity with the millimeter radio frequencies).
The sources of interest to infrared astronomy are characterized by the modest intrinsic temperature and therefore concern mainly the cold matter (gas and dust) that is diffused in the Galaxy, especially around its dynamic center. The galactic cold matter is articulated in large molecular formations, giving rise to those interstellar clouds from which is often re-emitted, by thermal dissipation, the incident radiation from surrounding stars or young associations of stars undergoing condensation in their interior.
Typical infrared sources are then almost all the long period variable stars (red giants of the Mira Ceti type) and the regions of star formation, whose optical observation is particularly hampered by absorption due to the presence of protostellar clouds themselves. Other infrared sources are the so-called black stars and brown dwarfs, celestial bodies too small to be able to radiate anything but thermal radiation, or too far in age to still emit visible light.
Another field of application for infrared astronomy is the search for extrasolar planetary systems, already formed or in gestation, which can be identified by that excess of thermal component, diffuse or concentrated, which is manifested for example in some stars close to us. In the near infrared the sky does not appear very different from usual, except for the fact that red stars (such as Betelgeuse, Antares) appear brighter than white-blue stars (such as Rigel). Moving into the intermediate infrared, the diffuse light of atmospheric origin is extinguished, so that the celestial sources now visible – the clusters of diffuse dust where the stars “nest”, and where are immersed very young stars acting as thermal exciters – become detectable even in the presence of the Sun.
At longer wavelengths in the image of the firmament prevail molecular clouds (at the temperature of a few tens of kelvin). These hide the direct vision already beyond 2-3 thousand light years away from the Sun, but their emission in the infrared is a sign of the occurrence of an impressive complex of violent phenomenologies: vortices of matter flowing in intense gravitational centers, such as galactic bulbs, neutron stars, black holes; corpuscular winds projected by stars in phase of gigantism; dispersion of matter in the mechanisms of mutual phagocytosis established within extraordinary thickenings of stars; and so on.
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Infrared astronomy from earth
The beginnings of astronomical investigation in the infrared can be traced back to S.P. Langley (1881) with the invention of the bolometer. In 1922 decisive improvements were introduced by the use, due to E. Pettit and S.B. Nicholson, of the thermocouple. In the forties, G.P. Kuiper adopted lead sulfide cells for the detectors to be sent to high altitudes, aboard balloons; detectors of which G. Neugebauer and B. Leighton, ten years later, will be the first. Leighton, ten years later, improved the performance with cooling, managing to compile a first catalog of 5612 sources (the Two Micron Survey).
The modern sensors, cooled with nitrogen and liquid helium at a few Kelvin, are ten thousand times more sensitive than their ancestors, and are able to work up to 20 µm. Mounted on rockets, balloons and airplanes (e.g. KAO, Kuiper Airborne Observatory, an aircraft equipped with a 90 cm aperture telescope), during the seventies they already allowed the scanning of about nine tenths of the sky. In this technological phase, we should not forget the TIRGO, a telescope of Italian design installed on the top of the Gornergrat, in Swiss territory.
At the end of the 20th century and at the beginning of the 21st century, the awareness of the importance of infrared astronomy has assumed such general and unconditional connotations that the scientific community has done its best to realize and propose new and increasingly sophisticated designs. For the ground survey, it is worth mentioning the installation on Mauna Kea – over 4000 m high – of 4 powerful structures: the British UKIRT telescope with a 3.8 m mirror; the NASA IRTF telescope, with a 3 m mirror; the twin KECK telescopes, with multiple mirrors for a 10 m aperture. For its part, NASA has built, as a replacement for the Kuiper airborne observatory, a second generation airborne observing facility, SOFIA (Stratospheric Observatory For Infrared Astronomy), to explore the sky over the full range of infrared frequencies thanks to its giant 2.5 m eye and the availability of a vast array of receptors.
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Infrared astronomy from space
IRAS (InfraRed Astronomy Satellite) was the first orbiting celestial infrared observatory in which most of the serious technological problems that opposed a systematic investigation from space were solved. Similar to a giant cryostat orbiting at an altitude of 900 km, IRAS – with its 60 cm objective – in the late eighties produced a map containing 300,000 sources of different types, including galaxies thousands of times brighter than the Milky Way, but radiating exclusively in the infrared. It revealed the presence of protoplanetary disks around numerous stars (β Pictoris, Vega) and that of cirriform formations scattered throughout the galactic space.
Subsequently, it was the turn of COBE (Cosmic Background Explorer), a cosmological probe, to investigate in the infrared the fossil radiation in order to capture the thermal inhomogeneities related to the primitive genetic centers of the Universe (protogalaxies and early generations of stars). In the investigation, the probe has also provided a general view of the dust that thickens in the disk and in the bulb of our Galaxy, thus improving the knowledge of its evolutionary mechanisms.
Equally and perhaps more important than IRAS was the orbiting infrared observatory of the European Space Agency (ESA), namely ISO, (Infrared Space Observatory) with 60 cm optics, which worked in space from 17 November 1996 to 8 April 1998. In 2003, NASA has completed the complex of the 4 large space observatories of its scientific program with the infrared observatory SIRTF (Space InfraRed Telescope Facility, renamed Spitzer Space Telescope after the launch), equipped with a 90 cm mirror and a spectrograph, refrigerated with liquid helium.
On the European side, always in the field of infrared space astronomy, the European Space Agency has defined the projects for the analogue of the Spitzer Space Telescope, that is for FIRST (already renamed Herschel), which will operate in the far infrared and will be put into orbit in 2007. Thanks to their extraordinary qualities of sensitivity and spatial resolution, with these infrared space observatories astronomers promise to observe celestial infrared sources a thousand times fainter than those known so far, and to discover the invisible component – dead stars, brown dwarfs, cometary reservoirs, extra-solar planetary bodies, intergalactic halos, etc. – that could be a not negligible part of the universe. – that could constitute a non-negligible part of the so-called dark matter.