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BACKGROUND

Infrared Astronomy is the detection and study of the infrared radiation (heat energy) emitted from objects in the Universe. Every object that has a non-zero temperature radiates in the infrared. Thus, Infrared Astronomy involves the study of just about everything in the Universe. In the field of astronomy, the infrared region lies within the range of sensitivity of infrared detectors which is between about 1 and 300 microns (a micron is one millionth of a meter). The human eye detects only 1% of light at 0.69 microns, and 0.01% at 0.75 microns, and so effectively cannot see wavelengths longer than about 0.75 microns unless the light source is extremely bright.

Viewing the Invisible

The Universe sends us a tremendous amount of information in the form of electromagnetic radiation (or light). Much of this information is in the infrared, which we cannot see with our eyes or with visible light telescopes. Only a small amount of this infrared information reaches the Earth's surface, yet by studying this small range of infrared wavelengths, astronomers have uncovered a wealth of new information. Only since the early 1980's have we been able to send infrared telescopes into orbit around the Earth, above the atmosphere which hides most of the Universe's light from us. The new discoveries made by these infrared satellite missions has been astounding. The first of these satellites - IRAS (Infrared Astronomical Satellite) - detected about 350,000 infrared sources, increasing the number of cataloged astronomical sources by about 70%.

All Sky Map of IRAS Point Sources
The plane of our galaxy runs horizontally across the image

Exploring the Hidden Universe

In space, there are many regions which are hidden from optical telescopes because they are embedded in dense regions of gas and dust. However, infrared radiation, having wavelengths which are much longer than visible light, can pass through dusty regions of space without being scattered. This means that we can study objects hidden by gas and dust in the infrared, which we cannot see in visible light, such as the center of our galaxy and regions of newly forming stars.

The images below, of the central region of our own Milky Way Galaxy and of the Cygnus star-forming region, show how areas which cannot be seen in visible light can show up very brightly in the infrared. The top row shows these regions in visible red light. At this wavelength we are seeing the light from billions of stars, particularly the largest, brightest ones. Note the dark bands where vast clouds of dust block our view of more distant objects. The middle row shows the same regions in the near infrared (infrared wavelengths closest to visible light). Here the light we see is also generated by stars, but now it better traces the smaller, cooler ones. Notice how the the lanes of dust have become partially transparent, allowing us to see things that are hidden in visible light. Our view of the central bulge of stars in our own Milky Way galaxy is particularly striking since it is almost completely obscured at shorter wavelengths! The bottom images show these regions in the far infrared (infrared wavelengths farther from visible light). At these wavelengths, stars hardly emit any light at all. Instead almost everything we see is generated by the dust clouds themselves. The dust, which is colder than the coldest arctic night on earth, is still warm enough to emit the thermal infrared radiation seen here.

Detecting Cool Objects

Many objects in the universe which are much too cool and faint to be detected in visible light, can be detected in the infrared. These include cool stars, infrared galaxies, clouds of particles around stars, nebulae, interstellar molecules, brown dwarfs and planets. For example, the visible light from a planet is hidden by the brightness of the star that it orbits. In the infrared, where planets have their peak brightness, the brightness of the star is reduced, making it possible to detect a planet in the infrared. Some of the most exciting discoveries in infrared astronomy have been the detection of disks of material and possible planets around other stars. Recently, an infrared survey of the Trapezium star cluster in the Orion Nebula revealed over 100 low mass objects which are brown dwarf candidates. Click on the image below for details.

Exploring the Early Universe

In the infrared, astronomers can gather information about the universe as it was a very long time ago and study the early evolution of galaxies. As a result of the Big Bang (the tremendous explosion which marked the beginning of our Universe), the universe is expanding and most of the galaxies within it are moving away from each other. Astronomers have discovered that all distant galaxies are moving away from us and that the farther away they are, the faster they are moving. This recession of galaxies away from us has an interesting effect on the light emitted from these galaxies. When an object is moving away from us, the light that it emits is "redshifted". This means that the wavelengths get longer and thereby shifted towards the red part of the spectrum. This effect, called the Doppler effect, is similar to what happens to sound waves emitted from a moving object. For example, if you are standing next to a railroad track and a train passes you while blowing its horn, you will hear the sound change from a higher to a lower frequency as the train passes you by. As a result of this Doppler effect, at large redshifts, all of the ultraviolet and visible light from distant sources is shifted into the infrared part of the spectrum. This means that the only way to study this light is in the infrared. Infrared astronomy will provide a great deal of information on how and when the universe was formed and on what the early universe was like.

Adding To Our Knowledge Of Visible Objects

Objects which can be seen in visible light can also be studied in the infrared. Thus infrared astronomy can not only allow us to discover new objects and view previously unseen areas of the universe, but it can add to what we already know about visible objects. To get a complete picture of any object in the Universe we need to study all of the radiation that it emits. Infrared Astronomy has and will continue to add a great deal to our knowledge about the Universe and the origins of our Solar System.

Early Astronomy

After Sir William Herschel's discovery of infrared ,which showed that the Sun emits infrared radiation, astronomers tried to see if other objects in the universe gave out infrared waves. In 1856, astronomers used thermocouples (devices which convert heat into electric current) to detect infrared radiation from the Moon. Much later, in 1948 (decades before the first Moon landing), more sophisticated infrared studies of the Moon would show that its surface was covered with a fine powder. In the early 1900's, infrared radiation was successfully detected from the planets Jupiter and Saturn and from some bright stars such as Vega and Arcturus. However, the insensitivity of the early infrared instruments prevented the detection of other near-infrared sources. Work in infrared astronomy remained at a low level until breakthroughs in the development of new, sensitive infrared detectors were achieved in the 1960's.

New Technology

During the past few decades, infrared astronomy has become a major field of science due to the rapid advances in infrared detector technology. Many of these advances arose from US Department of Defense research into infrared array technology in the 1980's. Infrared radiation, having longer wavelengths and lower energy than visible light, does not have enough energy to interact with the photographic plates which are used in visible light astronomy. Instead infrared astronomers rely on electronic devices to detect radiation. Early infrared astronomers used thermocouples and thermopiles (a group of thermocouples combined in one cell).

In the 1950's astronomers started to use Lead-sulphide (PbS) detectors to study infrared radiation in the 1 to 4 micron range. When infrared radiation in this range falls on a PbS cell it changes the resistance of the cell. This change in resistance can be measured and is related to the amount of infrared radiation which falls upon the cell. To increase the sensitivity of the PbS cell it was cooled to a temperature of 77 degrees Kelvin by placing it in a flask filled with liquid nitrogen.

A major breakthrough came in 1961, with the development of the germanium bolometer. This instrument was hundreds of times more sensitive than previous detectors and was capable of detecting all infrared wavelengths. Basically, a cool thin strip of germanium is placed in a container which has a small opening in it. When infrared radiation comes through the opening and hits the germanium, it warms the metal and changes its conductivity (a measure of how much electrical current flows through an object). The change in conductivity can be measured and is directly proportional to the amount of infrared radiation entering the container. The germanium bolometer works best at an extremely low temperature (much lower than liquid nitrogen). The best way to cool the bolometer to such a low temperature is to surround it with liquid helium which cooled it to 4 degrees Kelvin. This is only a few degrees above absolute zero. To do this a metal Dewar (similar to a well insulated thermos flask) was developed which was able to hold the liquid helium in which the germanium bolometer was immersed. This type of infrared detector is sensitive to the entire range of infrared wavelengths. To study a particular wavelength of infrared emission from astronomical objects, astronomers place filters in front of the detectors, which filter out all but the desired wavelengths.

Infrared detector technology continues to advance at a rapid rate. Astronomers now use InSb and HgCdTe detectors for the 1 to 5 micron range. These operate in a way similar to the PbS detectors but use materials which are much more sensitive to the infrared. The development of infrared array detectors in the 1980's caused another giant leap in the sensitivity of infrared observations. Basically a detector array is a combination of several single detectors. These arrays allow astronomers to produce images containing tens of thousands of pixels at the same time. Infrared arrays have been used on several infrared satellite missions. In 1983 the IRAS mission used an array of 62 detectors. Astronomers now commonly use 256x256 arrays (that's 65,536 detectors!). Due to these breakthroughs in infrared technology, infrared astronomy has developed more rapidly than any other field of astronomy and continues to bring us exciting new views of the universe.

Ground Based Infrared Observatory

Infrared detectors attached to ground based telescopes can detect the near-infrared wavelengths which make it through our atmosphere. The best location for ground based infrared observatories is on a high, dry mountain, above much of the water vapor which absorbs infrared. At these high altitudes, astronomers can study infrared wavelengths centered at 1.25, 1.65, 2.2, 3.5, 4.75, 10.5, 19.5 and 35 microns. Telescopes as well as our atmosphere emit infrared radiation which can complicate the observation of cosmic sources. Infrared telescopes are designed to limit the amount of this thermal emission from reaching the detectors. All ground based infrared detectors are cooled to extremely low temperatures to reduce their emission. In addition, astronomers making ground based observations measure both the emission from our atmosphere and from the object that they are observing. They then subtract the atmospheric emission from the infrared emission of a celestial object to get an accurate measurement.

In the mid-1960's, the first infrared survey of the sky was made at the Mount Wilson Observatory using liquid nitrogen cooled PbS detectors which were most sensitive at 2.2 microns. The survey covered approximately 75 percent of the sky and found about 20,000 infrared sources. Many of these sources were stars which had never been seen before in visible light. These stars were much cooler than our Sun and had surface temperatures of 1,000 degrees to 2,000 degrees Kelvin. Our Sun has a surface temperature of about 6,000 degrees Kelvin. The brightest 5,500 of these sources made up the first catalog of infrared stars. A partial infrared survey of the southern sky was also made in 1968 at the Mount John Observatory in New Zealand.

New observatories, specializing in infrared astronomy, became possible in the 1960's due to advances in infrared detectors. The largest group of infrared telescopes can be found on top of Mauna Kea (a dormant volcano) on the island of Hawaii. At an elevation of 13,796 ft., * the Mauna Kea Observatories , which were founded in 1967, are well above much of the infrared absorbing water vapor.

The Mauna Kea Observatories
Photo courtesy of Richard Wainscoat, Institute for Astronomy, University of Hawaii

By the early 1970s, it was found that the centers of most galaxies emit strongly in the infrared, including our own galaxy, the Milky Way. Quasars and other active galaxies were also found to be strong infrared emitters. All of this new information came from near-infrared observations which could be made from the ground. Today, most of the larger ground based telescopes have been modified to accommodate infrared detectors. Many ground based infrared telescopes are now using adaptive optics to create very sharp images. Adaptive optics removes the blurring of an astronomical image due to turbulence in earth's atmosphere.

In addition to absorbing most of the infrared radiation from cosmic sources, the Earth's atmosphere itself radiates in the infrared which interferes with infrared observations. This is why it is best to get above as much of the atmosphere as possible to observe in the infrared. To do this, infrared detectors have been placed on balloons, rockets and airplanes, allowing astronomers to study longer infrared wavelengths. Even though these methods can only observe a small part of the sky for short periods of time, they have contributed much to infrared astronomy.

The first cooled telescopes were those placed on rockets which could observe the sky for several minutes before reentry. The first infrared all sky map resulted from a series of rocket flights by the Air Force Cambridge Research Laboratory. This project, called Hi Star, surveyed the cosmos at wavelengths of 4, 10 and 20 microns. Although the total observation time accumulated by these flights was only about 30 minutes, they successfully detected 2363 reliable infrared sources which were published in the AFCRL Infrared Sky Survey. About 70% of these sources matched sources found by the Mount Wilson 2.2 micron survey. Rockets also found bright infrared emission from HII regions (regions of ionized hydrogen) and the center of our galaxy.

Helium filled, mylar balloons have carried infrared telescopes up to altitudes as high as 25 miles. In 1963, a germanium bolometer was attached to a balloon to make infrared observations of Mars. Beginning in 1966, the Goddard Institute of Space Sciences used balloons to survey the sky at 100 microns. Their program led to the discovery of about 120 bright infrared sources near the plane of our galaxy.

Infrared telescopes onboard aircraft such as the * Kuiper Airborne Observatory were used to discover the rings of Uranus in 1977. The KAO has been used to gather infrared astronomical data for over 20 years and can fly at an altitude of 41,000 feet which is above 99 percent of the Earth's water vapor. In addition to being able to study additional infrared wavelengths, airborne observatories can detect fainter infrared objects which cannot be observed well from the ground (such as interstellar clouds).

The Kuiper Airborne Observatory

Plans are being made by NASA for a new airborne observatory. * SOFIA - The Stratospheric Observatory For Infrared Astronomy will be an optical/infrared/sub-millimeter telescope mounted in a Boeing 747 and is expected to be fully operational by the year 2001.

Infrared Astronomy from Earth's Atmosphere

In the 1970s, astronomers around the world began to consider the possibility of placing an infrared telescope on a satellite in orbit around the Earth. This telescope would be above the Earth's atmosphere and could view the sky at the far-infrared wavelengths which were difficult to detect on Earth. It could view a large area of the sky and observe regions for a longer period of time.

By 1977, an international collaboration was formed by the Netherlands, United States and Great Britain to develop IRAS - The Infrared Astronomical Satellite. The American team built the telescope, detectors and cooling system. The British built the satellite ground station and control center and the Dutch team built the spacecraft which included the on-board computers and pointing system.

It takes a great deal of effort to build an infrared space telescope. After many years of hard work and after overcoming several complications, IRAS was successfully launched on January 25, 1983. The telescope was housed in a dewar, filled with 127 gallons of liquid helium and contained 62 detectors. The entire telescope was cooled to a temperature of just a few degrees above absolute zero because otherwise the telescope itself would emit infrared radiation (heat) which would interfere with the observations. A space infrared telescope must be cooler than the objects in space that it will observe.

IRAS in orbit - Artist Rendition

The IRAS mission would last as long as the liquid helium did. For the next ten months, IRAS scanned more than 96 percent of the sky four times, providing the first high sensitivity all sky map at wavelengths of 12, 25, 60 and 100 microns. IRAS increased the number of cataloged astronomical sources by about 70%, detecting about 350,000 infrared sources. IRAS discoveries included a disk of dust grains around the star Vega, six new comets, and very strong infrared emission from interacting galaxies as well as wisps of warm dust called infrared cirrus which could be found in almost every direction of space. IRAS also revealed for the first time the core of our galaxy, the Milky Way.

Several successful infrared satellite missions were launched after IRAS. During July and August of 1985, an infrared telescope was flown onboard the Space Shuttle's Spacelab 2 to complement observations made by the IRAS mission. This mission produced a high quality map of about 60% of the plane of our galaxy.

The Space Shuttle With Skylab 2

In November 1989, NASA launched the * COBE satellite to study both infrared and microwave characteristics of the cosmic background radiation (the remains of the extreme heat that was created by the Big Bang). Over a ten month period, COBE mapped the brightness of the entire sky at several infrared wavelengths and discovered that the cosmic background radiation is not entirely smooth, showing extremely small variations in temperature. These variations may have led to the formation of galaxies.

* The Infrared Telescope in Space (IRTS), launched in March 1995, was Japan's first infrared satellite mission. During its 28 day mission, IRTS surveyed about 7% of the sky with four instruments: A Near and Mid Infrared Spectrometer which covered wavelengths of 1.4 to 4 microns and 4.5 to 11 microns respectively, a Far Infrared Line Mapper which studied Oxygen and Carbon spectral lines at 63 and 158 microns, and a Far infrared Photometer which studied the sky at four bands centered at 150, 250, 400, and 700 microns. This data should add to our knowledge of cosmology, interstellar matter, late type stars and interplanetary dust.

The European Space Agency launched the * Infrared Space Observatory (ISO) in November 1995. ISO, which observed at wavelengths between 2.5 and 240 microns, not only covered a much wider wavelength range than IRAS but was also thousands of times more sensitive than IRAS and viewed infrared sources with much better resolution. ISO took data for about 2.5 years (3 times times longer than IRAS). It ceased operations in April 1998 when its supply of liquid helium ran out. ISO contained instruments which measured details of both the shorter and longer wavelength regions of the infrared spectrum, an infrared camera which had two infrared arrays, and a photometer. Unlike IRAS, which was an infrared survey mission, ISO is operated like a ground based observatory, having astronomers submit observing proposals to study specific astronomical objects in detail. As hundreds of astronomers from several countries study the data from ISO, important * new discoveries about our universe are expected to emerge. ISO has already detected dry ice in interstellar dust and hydrocarbons in some nebulae.

The Midcourse Space Experiment (MSX) was launched in April 1996 and lasted until its liquid helium coolant ran out in Feb 1997. During its 10 months of operation, MSX gathered a vast amount of data at 4.2 - 26 microns. MSX studied the infrared emission from the gas and dust which permeates the universe. MSX had 30 times the spatial resolution as IRAS and surveyed areas of the sky which were missed by IRAS.