Infrared astronomy

Infrared astronomy explores the universe by focusing on wavelengths of the electromagnetic spectrum that are longer than those of visible light. This region of the spectrum is useful for studying the process of star formation, for studying objects that are obscured by clouds of interstellar material, and for studying lower-temperature objects that do not radiate as prevalently in the visible portion of the spectrum.

Overview

Infrared astronomy focuses on wavelengths of electromagnetic radiation that are a little longer than those of visible light. The infrared region of the spectrum covers a wide range of wavelengths, from waves slightly longer than those of visible light (0.7 microns, or 0.7 millionth of a meter) to those as long as 1,000 microns. The longest infrared wavelengths are about one millimeter long and mark the boundary with the microwave radio spectrum.

Infrared radiation from distant sources is very difficult to detect. The Sun is so close that the infrared radiation it emits can be detected in the form of heat. The Moon also emits easily detected infrared radiation. However, very sensitive detectors are needed to detect emissions from other stars, planets, nebulae, or galaxies. The shortest infrared waves are known to astronomers as photographic infrared because they are very similar to visible light and can be detected with certain types of photographic emulsions and other types of optical detectors. At longer wavelengths of infrared radiation, objects that are not visible at optical wavelengths can be detected. Nevertheless, the detectors used for the photographic infrared are no longer helpful at these wavelengths.

Modern infrared detectors often use a substance called indium antimonide, which changes its electrical conductivity when exposed to infrared radiation. To be effective, it must be kept very cold. Solid nitrogen or liquid helium surrounds the material to bring its temperature from fifty kelvins to within a few kelvins of absolute zero. Another long-wavelength infrared detector uses a crystal of the semiconducting material germanium that contains traces of the rare metal gallium. This detector must be kept to a temperature of only two kelvins above absolute zero.

Earth’s atmosphere provides advantages and disadvantages to infrared astronomy. Some infrared observations can be done during the day and night, allowing infrared detectors to be mounted on large optical telescopes for daytime use. The disadvantage posed by the atmosphere is that water vapor and carbon dioxide absorb certain wavelengths of infrared radiation, making them invisible to astronomers. Infrared astronomers must choose particular wave bands where the atmosphere allows a clear window. Seeing through these windows is often a challenge, as everyday objects—such as telescopes and the sky—can radiate at these same wavelengths if they are at the appropriate temperature.

Infrared astronomers have designed ways of partially overcoming the problems the atmosphere poses. Infrared instruments are designed so that no stray radiation from the instrument can enter the detectors. To overcome sky brightness in the infrared, astronomers take measurements of the observational target. Measurements include the infrared brightness of both the object and the sky. The telescope is then moved slightly so that it no longer points at the source, where it only takes an infrared measurement from the background sky. When the second measurement is subtracted from the first, it is possible to determine the brightness of the object itself. This technique works well for stars but less for objects like nebulae, which cover a wider field of view. The technique can be modified to scan a wider portion of the sky, making images of larger areas possible. Detectors have been developed that can record such images in a single exposure.

The ideal earthbound infrared observatories are located at high altitudes and in arid atmospheric conditions. The best site is on Mauna Kea in Hawaii, 4,200 meters above sea level, where two of the world’s largest infrared telescopes reside: the National Aeronautics and Space Administration’s (NASA’s) three-meter-diameter telescope and the United Kingdom Infrared Telescope (UKIRT), with a 3.8-meter-diameter mirror.

At wavelengths longer than about thirty microns, the atmosphere begins to absorb so much infrared radiation that ground-based observation is impossible. Astronomers conduct their observations remotely to observe these longer (or far-infrared) wavelengths. In the 1970s, ten rocket flights carrying infrared detectors performed a survey of nine-tenths of the sky. During these early flights, it was discovered that the center of the Milky Way and other galaxies were strong sources of far-infrared radiation. High-altitude balloons have also been used to make observations; NASA converted a C-141 transporter plane into a flying infrared observatory, the Kuiper Airborne Observatory, complete with a 0.9-meter telescope. The observatory carries scientists to altitudes of about 12,500 meters, where they can make observations free of about 99 percent of the atmosphere’s water vapor.

These types of observations are valuable, but the best way to solve the observational problems posed by the atmosphere is to observe outside the atmosphere. Although astronomers have flown many satellites to measure other types of radiation from space, the infrared band has presented difficulties because of the necessity of keeping the detectors at extremely low temperatures. In 1983, a fully dedicated infrared satellite was finally launched. The Infrared Astronomical Satellite (IRAS) was a joint project by the United States, the Netherlands, and England. IRAS investigated the sky from an orbital altitude of 900 kilometers. Throughout its development, this mission proved one of the most challenging attempts ever. The infrared detectors had to be designed so that even in orbit, they were cooled to within a few degrees of absolute zero with nearly 90.7 kilograms of liquid helium. The lifetime of the satellites was limited because the liquid helium slowly boils away. IRAS was able to function efficiently for a total of ten months. The principal instrument aboard IRAS was an array of sixty-two semiconductors sensitive to most infrared spectrums. The satellite was roughly the size of a small automobile and weighed 1,076 kilograms.

Despite the complexity of keeping the instruments cold, the mission was highly successful. IRAS scanned 95 percent of the sky four times at the middle and far-infrared wavelengths. It detected and cataloged about 250,000 celestial sources of infrared radiation. IRAS produced an enormous catalog of infrared sources, significantly expanding the field of infrared astronomy. Numerous space-based detectors and specially dedicated observatories followed. One example of an instrument on an observatory primarily taking data in the visible was the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) installed on the Hubble Space Telescope (HST) by shuttle astronauts in 1997 during the STS-82 servicing mission. Two examples of dedicated observatories that went far beyond the early discoveries of IRAS were the Space Infrared Telescope Facility (later renamed the Spitzer Space Telescope), a member of NASA’s Great Observatories program, and ESA’s Infrared Space Observatory (ISO).

The European Space Agency (ESA) launched the Infrared Space Observatory (ISO) on November 17, 1995, and placed it in an elliptical orbit ranging from as close to the surface of the Earth as 1,000 kilometers to as high above the surface as 70,500 kilometers; this gave ISO a twenty-four-hour orbital period. ISO was designed to detect infrared radiation ranging from 2.5 to 240 microns in wavelength. To achieve that, ISO was equipped with four separate scientific instruments cooled by liquid helium. The instruments were disturbed by energetic particles when ISO dropped through and rose out of Earth’s Van Allen radiation belts, but the observatory spent 70 percent of each orbit well beyond those disruptive belts. The ISO mission concluded on May 16, 1998, after the cryostat’s helium had boiled off, thereby raising the temperature of the instruments sufficiently high to render them useless for infrared measurements.

The Hubble Space Telescope was placed into orbit from the space shuttleDiscovery on mission STS-31 on April 25, 1990. One of NASA’s four Great Observatories, Hubble ,was designed to be adaptable and have rotating instruments in its science bay to conduct investigations from part of the infrared through the entire visible portion of the electromagnetic spectrum to portions of the ultraviolet. As first deployed, Hubble suffered from a precise but inaccurate optical prescription for its main mirror. Astronauts on STS-61 in December 1993 installed a corrective device called Corrective Optics Space Telescope Axial Replacement (COSTAR), which brought incoming light into proper focus, saving Hubble from an otherwise dismal outcome. On a second servicing mission, STS-82, in February 1997, astronauts removed an instrument from Hubble and replaced it with the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), designed to perform infrared studies involving wavelengths of 0.8 to 2.4 microns.

NICMOS was outfitted with a unique thermal management system, making use of a block of solid nitrogen to keep the instruments cooled to about forty kelvins. Unfortunately, after NICMOS was incorporated into HST in space and afterward put through early commissioning activities and subsequent science observations, it was clear that a heat leak had developed that would raise the solid nitrogen’s temperature to a point after two years or less where it would no longer provide sufficient cooling for the sensors in the instruments to produce accurate results. Meanwhile, scientists and engineers developed a mechanical cryocooler that would, in turn, be installed on NICMOS during the next shuttle servicing mission to HST. It was capable of monitoring temperatures between seventy-five to eighty-six kelvins, low enough for the instruments to function. In March 2002, astronauts on space shuttle mission STS-109 saved NICMOS. Further repairs in the early twenty-first century allowed HST to continue front-line infrared observations before the 2021 launch of the James E. Webb Space Telescope.

The last member of NASA’s Great Observatories, the Space Infrared Telescope Facility (SIRTF), was launched into a solar orbit on an expendable Delta II booster on August 25, 2003 (thirteen years to the day after Hubble was placed in Earth’s orbit), rather than being deployed from a space shuttle by astronauts. After launch, the observatory was renamed the Spitzer Space Telescope (SST) in honor of the astronomer and longtime proponent of placing large telescopes in orbit, Lyman Spitzer. The Spitzer telescope was designed to observe infrared radiation from 3 to 180 microns for at least five years, being outfitted with an infrared camera (operating from 3 to 180 microns), an infrared spectrometer (operating between five to forty microns), and far-infrared detector arrays. The observatory was cooled by liquid helium to 5.5 kelvins to achieve these goals, aided dramatically by passive cooling with a sunshield. Spitzer was going strong after five years, with much of its liquid helium remaining.

The James E. Webb Space Telescope (JWST) was designed to be set up in an operation position at the L2 Lagrangian point, a spot 1.5 million kilometers from Earth where the gravitational influence on the observatory from Earth is balanced by that from the Sun. This observatory, fully devoted to infrared astronomy, detects emissions from 0.6 to 28 microns. JWST was designed to be half as massive as Hubble but with a folded optical system of eighteen individual hexagonal segments with a light collecting area six times greater than HST. JWST is designed to rely primarily on sun shields to achieve its intended mission and remain cooler than forty kelvins. It was launched on December 25, 2021, by the European Space Agency atop an Ariane V booster. It is a NASA observatory with ESA instruments launched by ESA in exchange for time using the telescope. JWST has four primary objectives: (1) search for light from the earliest stars and collective structures formed in the first hundreds of millions of years after the Big Bang; (2) produce images and data that will assist in expanding our understanding of the galactic formation and subsequent evolution; (3) produce images and data that will assist in expanding our understanding of the planetary formation and subsequent evolution; and (4) search for the existence of organic material essential for the development of life in the universe.

Applications

Infrared radiation can give astronomers valuable information about the formation of stars. Stars are believed to be formed from large clouds of rotating dust and gas that condense under their gravity. The energy released in the collapse causes the forming star, or protostar, to increase temperature until nuclear reactions begin. It is not until the star turns on in this way that it emits radiation in visible wavelengths. As a result, the process of star formation is difficult to study optically. As the star begins to shine, newly created energy warms the surrounding dust, which radiates the energy away as infrared waves. The process is not understood completely, and astronomers have learned much from studying infrared and radio wavelengths.

Many infrared sources are clouds of dust heated by a nearby star. Infrared stars are generally very young or old and associated with dust clouds. One of the early infrared discoveries was of a giant cloud of gas and dust in the constellation of Orion: the Kleinmann-Low Nebula, named for its discoverers. It was found to have a mass greater than two hundred times that of the Sun, yet it is invisible at optical wavelengths. In the infrared, it outshines the Sun more than 100,000 times. It was determined to be a relatively close area of active star formation (within 1,600 light-years). Detailed studies of the Kleinmann-Low Nebula in the infrared and radio bands indicate that it contains several young stars and clouds of dust and gas that may be in the process of collapsing to form new stars. By studying this nebula, astronomers are learning more about the process of star formation. ISO detected such “stellar nurseries” in the Milky Way and other galaxies, where, its data suggest, star formation occurs at a higher rate than astronomers expected.

One of the most exciting discoveries made by IRAS was a disk of dust grains around the star Vega. Scientists believe this disk of material may be remnants of the dust cloud from which the star formed. The theory in 1990 of planet formation suggests that a similar but smaller disk of material around the Sun provided the raw material from which Earth and other planets were formed. If the disk of material around Vega follows the same pattern, it could eventually form asteroid or planet-sized bodies. The findings from IRAS suggest that such material is common around other stars as well.

A year after the disk of material was found around Vega, a small companion object was found orbiting a faint star. The object was between thirty and eighty times the mass of Jupiter. It was too small to sustain nuclear reactions, as a star would, and some astronomers suggested initially that this object heralded the discovery of the first planet outside our solar system. It was theorized that the object was a brown dwarf, an object between a star and a planet. It was an important discovery, as astronomers were finding there may be many more brown dwarf-type objects than expected. It was thought that if these objects outnumbered visible stars, astronomers’ theories regarding the amount of matter in the universe (and its eventual fate) would be in need of revision. In due course, however, it was realized that brown dwarfs could not account for all the missing mass in the universe, even as more brown dwarfs were being found by the Spitzer Space Telescope.

IRAS examined many peculiar galaxies, one of which is a galaxy known as Arp 220. IRAS found that the galaxy was emitting eighty times more energy in the infrared than in all other wavelengths. Although the object is not excessively bright at optical wavelengths, its infrared brightness would make it about as energetic as some quasars. (Quasars are extremely powerful, bright sources of energy located in a very small area at the center of a galaxy that outshines the entire galaxy around them.) Arp 220 is actually two galaxies that are colliding. While the individual stars of the galaxies are not likely to collide, huge clouds of dust and gas would collide, generating shock waves and heat by compression. This energy would be radiated in the infrared.

Researchers using IRAS employed a rigorous observational screening process to weed out any stray infrared detections caused by charged particles. They screened out all but the sources that remained stationary over time and were repeatable. This method of observation lent itself to the discovery of some fast-moving objects that were eliminated because they moved too quickly from one observation to the next. In studying the rejected observations, scientists discovered a comet in 1983—the IRAS-Araki-Alcock—named for the satellite and for G. Iraki and E. Alcock, the independent discoverers. The comet passed closer to the Sun than any other comet in the last two hundred years, and IRAS was able to study it in detail, along with other ground-based observations. In total, six comets were discovered by the satellite, and five other known comets were studied.

Among the most important discoveries made by scientists using ISO was the signature of water around planets within the solar system and in regions of stellar formation. With regard to the latter, such a signature had been hidden from detection by the presence of dust within such forming star systems. With regard to the former, ISO determined that as much as ten kilograms per second of water “rain” down in the upper atmosphere of Jupiter, Saturn, and Uranus. ISO’s data did not answer the question as to the origin of that water. Water was also found in the thick atmosphere of Titan, one of Saturn’s satellites—a tantalizing result that Cassini-Huygens scientists eagerly hoped to verify and investigate further when that probe entered the Saturn system beginning in July 2004. Cassini found evidence of water clouds in Saturn’s lowest cloud deck at a distance of about 130 kilometers under the tropopause. At that atmospheric level, the local temperature is near the freezing point of water. As for the water in the gas giant planets’ upper atmosphere, one leading hypothesis for the source was the influx of small cometary nuclei.

ISO was also used for extragalactic studies. For the first time, evidence of dust was confirmed for the otherwise rather empty space between galaxies. One particularly outstanding extragalactic finding from ISO was that intergalactic dust in a large group of more than five hundred galaxies clustered together within the constellation Coma Berenices was heavily concentrated toward the cluster’s center. Determining intergalactic space is laden with very low-density dust concentrations held implications for cosmological models.

NICMOS provided a means for HST to observe celestial objects in infrared, which could then be contrasted with images of the same sources taken in visible light. One major comparison involved deep-sky observations of distant, dim galaxies. The visible light images and infrared images of the same tiny areas of the sky were combined to indicate the differences between different classes of galaxies. The false-color scale used in making these survey images indicated galaxies with strong infrared emissions as reddish, while galaxies glowing more strongly in visible light appeared bluish. These sorts of survey images included blue dwarf galaxies, red elliptical galaxies, and spiral disk galaxies. Examination of those images provided insights into the populations of dust-obscured galaxies at the earliest times in galaxy formation, shortly after the Big Bang.

Closer to home, NICMOS was used to examine Uranus’s cloud features. In visible light, Uranus reveals little of its atmospheric structure. Using infrared, however, HST’s NICMOS instrument found as many as twenty clouds near a bright band in the planet’s atmosphere. Wind speeds in the region were determined to be between 300 and 500 kilometers per hour.

Naturally, infrared astronomers expected great performance from the Spitzer Space Telescope, but they were pleased beyond those preliminary expectations when initial images taken by Spitzer of the dust disk surrounding a forming star, the glow of a stellar nursery, and the swirling dust in a large galaxy revealed tremendous detail. The spectroscopic capability of Spitzer revealed the signature of organic material. SST was the first observatory to detect light directly from extrasolar hot Jupiters (specifically HD 209458b and TrES-1). Views of our neighboring galaxy, the Andromeda galaxy (or M31), taken by Spitzer, clearly show the spiral arms by noting the dust lanes in them. SST studies were the first to examine the core of the Milky Way in such a way as to determine that the galaxy's core has a barred structure.

Spitzer was used to determine the atmospheric temperature of the extrasolar planet HD 189733b, the first time such a measurement was made. Astronomers used Spitzer to perform surveys with long-time exposures. At the American Astronomical Society meeting in St. Louis, infrared astronomers presented an infrared image of the Milky Way that consisted of a collection of 800,000 individual images. This composite image showed the distribution of dust within the galaxy in greater detail than had previously been possible.

In early 2008, the Spitzer Space Telescope’s infrared spectrometer recorded the first evidence of the existence of water in protoplanetary disks. Observations were made of DR Tau and AS 205A, which are 457 and 391 light-years distant from Earth, respectively. Water is an essential ingredient in an evolving solar system for the possibility of an Earth-like planet forming, one that might permit the development of life. Water is also, however, important in a protoplanetary system for the formation of icy satellites around large planets in the outer fringes of the system. Water closer to the young star at the center of the protoplanetary disk could be in gaseous or perhaps even liquid form. Both of the aforementioned protoplanetary systems produced large numbers of water emission lines.

Spitzer provided insight into numerous astrophysical phenomena, one of the most bizarre being magnetars. These are stars with magnetic fields approximately fourteen orders of magnitude more intense than that of typical stars on the main sequence. Magnetars are the highly compressed remains of massive stars that went supernova, but they are hardly dead stars. In addition to their intense magnetic fields, magnetars pulsate in the X-ray portion of the electromagnetic spectrum. In late May 2008, the Spitzer Science Center, run by the California Institute of Technology, reported on observations of the magnetar SGR 1900+14. In addition to having the usual attributes of a magnetar, this object was surrounded by a ring of material seven light-years across that was energized by the energetic X-ray pulsations. Heated dust in the ring resulted in the ring glowing strongly in the infrared.

Although results from IRAS, HST, ISO, and Spitzer have been spectacular, ground-based telescopes are still useful for observing many infrared phenomena. Infrared observations from NASA’s Infrared Telescope Facility on Mauna Kea have revealed volcanic eruptions on Jupiter’s moon Io. A volcano that had been erupting at the time of the Voyager flybys in 1979 was still erupting, and a new volcano was detected. Observations like these help to gather valuable information over time that can elaborate on the findings of other missions.

Context

Infrared astronomy is part of the revolution that has been called the new astronomy. Instruments of modern astronomers give them access to information from the entire range of the electromagnetic spectrum. This revolution has occurred mostly since the early 1960s, when it became possible to place remote detectors above Earth’s atmosphere. Before then, astronomers relied, for the most part, on the optical range of wavelengths for their information about the universe.

It was not until 1800 that the first sign of another way to look at the universe was discovered. While analyzing sunlight by separating the white light into a spectrum, English astronomer Sir William Herschel noticed that a thermometer placed in the dark area just outside the red limit of the spectrum registered an increase in temperature. In 1881, American astronomer Samuel Pierpont Langley developed the bolometer, an electrical detector that measures heat over a broad range of wavelengths. In measuring the Sun’s energy from a high altitude, Langley found that the radiant energy of the Sun extended far past the visible portion of the spectrum and far past the region that Herschel had discovered previously. Herschel had discovered the near-infrared, whereas Langley was detecting the longer-wavelength middle-infrared band.

Infrared radiation from the Sun was fairly simple to detect, but more sensitive instruments had to be developed before detecting the infrared from far distant sources. In 1856, near-infrared radiation was detected from the Moon, but it was not until the 1920s that it began to be detected from the other planets and bright stars. Available instruments were still unable to see into the far infrared. While working on superconductivity experiments in the late 1950s, physicist Frank Low began the development of more sensitive instruments. By the early 1970s, Low was among the first to attempt observations of the far infrared by leading observations aboard high-flying jets.

An infrared satellite was first proposed in the mid-1970s. NASA was facing troubled times with budget cuts, inflation, and cost overruns in other projects. It might have scrapped the project entirely except for the interest of the Dutch space agency. The Dutch had completed several successful satellite programs and were interested in collaborating on an infrared satellite. England then joined the project, which came to be known as the Infrared Astronomical Satellite program. The project was a difficult one, but the diplomatic aspects of an international collaboration helped to give the program stability, and the satellite was launched successfully in 1983. A succession of increasingly sophisticated space-based observatories expanded upon the groundwork laid by IRAS. Hubble, ISO, Spitzer, and eventually the James E. Webb Space Telescope turned infrared astronomy into an integral component of astrophysical investigations, leading to insights into the interstellar medium, thermal processes on planets in the solar system, protoplanetary disks, nebulae, galaxy formation, brown dwarfs, and many other phenomena.

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