SIRTF science objectives
Posted: April 14, 2003

An artist's concept of SIRTF. Credit: NASA/JPL/Caltech
Imagine listening to the sweeping movements of a symphony, but the only notes your ears pick up are just a few pitches around middle C. With such limited perception you would miss much or most of the music, its structure and beauty. Historically, astronomers have found themselves in a similar situation. Though their trade is not in music appreciation, they have attempted to capture what they thought was the full grandeur of the cosmos in only visible light.

Seeing the invisible
Our eyes detect only a sliver of the enormous bombardment of energy the universe washes over us at any instant. The remainder of this energy comes in the form of radio waves, microwaves, heat, X-rays, gamma rays and other types of what scientists call electromagnetic radiation. The various types of radiation differ in frequency, wavelength and the amount of energy. The shorter the wavelength, the higher the energy and frequency, and vice-versa. At one end of the scale are radio waves, which have low frequencies, low energy and long wavelengths. At the other end of the scale are gamma rays, with high frequencies, high energy and extremely short wavelengths.

Midway between radio waves and gamma rays is the spectrum of visible light. Here, what the eye perceives as color reflects the light's wavelength, energy and frequency. At one end of the scale is red light, which has a low frequency, low energy and long wavelength. At the other end of the scale is violet light, which has a relatively higher frequency, higher energy and shorter wavelength. Wavelengths of light in the visible spectrum ranges from about 0.7 micron for red light to 0.4 micron for violet (a micron is one-millionth of one meter, or about 1/50th the width of a human hair).

Infrared light lies just beyond the red portion of the visible spectrum. It has lower energy and a lower frequency than red light, and is therefore called "below red," or "infrared." The infrared region stretches from a wavelength of about 1 micron (the "near infrared") to 200 microns (the "far infrared").

The importance of temperature
Any object in the universe with a temperature above absolute zero (minus 273.15 C or minus 459.4 F) will emit electromagnetic radiation. The type of radiation depends on the object's temperature. For an object to radiate gamma rays, for example, its temperature must exceed 1 billion C (1.8 billion F). Since gamma rays are thought to be the most energetic form of light, gamma-ray telescopes capture some of the most extreme, cataclysmic events in the universe such as supernova explosions and colliding neutron stars.

More familiar celestial objects like stars have temperatures near 10,000 C (18,000 F), and radiate energy mostly as visible light. Following this pattern, objects with cooler temperatures - perhaps a few hundred degrees or cooler, around the temperature of the human body, for example - emit most of their light in the infrared range. Infrared telescopes therefore provide an excellent tool to probe the cool, otherwise invisible universe, including such objects as vast clouds of dust floating between stars, to planets orbiting nearby stars that are too distant and dim to detect in visible light.

Lifting the cosmic veil
Some of the most dramatic objects in the universe are completely obscured from us in visible light. For example, vast clouds of interstellar dust conceal what is thought to be a super-massive black hole lurking in the center of our Milky Way galaxy. However, this dust becomes transparent in near- and mid-infrared light, thereby allowing astronomers to peer at such fascinating phenomena.

Looking back in time
Nearly all galaxies in space appear to move away from each other, the result of cataclysmic event dubbed the Big Bang. As a result, just as the pitch of a siren seems to lower as a fire truck passes by, light from distant, receding objects in the universe also appears to shift to longer wavelengths. This is called red shift. The farther away an object is, the faster it appears to be moving away from us and the greater its light shifts toward the red. Light takes a finite time to reach us, so when we look out at very distant objects we actually see them as they appeared when the universe was much younger. As a result of the expansion of the universe, most of the optical and ultraviolet radiation emitted from very distant stars, galaxies and quasars since the beginning of time now lies in the infrared. As a result, infrared observations will help answer how and when the first objects in the universe formed.

Colors and chemicals
Astronomers often determine the chemical makeup of an object in space by using an instrument based on the simple prism that separates a beam of light scrutinizing the spectrum of colors in light given off by that object. For example, the atoms in a particular object, say a star, absorb light at certain frequencies and emit light at other frequencies. This results in very finely etched lines in the color spectrum from the star, what scientists refer to as "absorption lines" and "emission lines." Each pattern of these lines serves as a distinctive fingerprint of the particular chemicals that make up the star. This technique of analyzing the light from stars is called spectroscopy. If the star or other object lies in the far reaches of the universe where it speeds away from us quickly, the red shift causes its spectral lines to move into the infrared. In fact, scientific detective work to determine exactly how far a given line has shifted into the infrared is one way to gauge how quickly an object races away from us. From this information astronomers can estimate the object's age and distance.

Near versus far infrared
Infrared radiation spans a vastly wider range of wavelengths than visible light. This large stretch means that quite different astronomical objects dominate distinct regions within the infrared spectrum. In the near infrared region, cooler stars such as red dwarfs and red giants glow brightly. Astronomers are exploiting this fact in attempts to map the true distribution of stars in galaxies, a task that is nearly impossible in visible light because the brighter blue stars in a galaxy's spiral arms often drown out the older, dimmer stars in the main plane of the galaxy.

Interstellar dust becomes transparent in the near- and mid-infrared, allowing for a glimpse inside star-forming regions and a probe into the hearts of galaxies.

Moving farther out into the infrared range reveals cooler objects such as comets, asteroids and objects in the far outer solar system, all of which usually escape detection in visible light. In the far infrared, stars all but vanish, and the cold dust - often only a few tens of degrees above absolute zero - that usually goes unseen between these bright beacons takes on an eerie glow.

The chemistry of life
Virtually all molecules - from simple chemicals to complex biomolecules - have emission and absorption lines in the infrared spectrum. Around nearby stars, infrared studies will allow us to compare the inner and outer regions of young planetary discs, and to scan for sites where planets may be forming. Infrared spectral lines will help us identify and map the distribution of organic chemicals such as methane in young planetary systems, as well as life-sustaining water. Comparing these fledgling planetary systems with our solar system will help us understand how the chemical composition of these planet-forming discs may set the stage for life.

Key science goals
In the planning stages of the Space Infrared Telescope Facility, researchers identified several key science goals to help shape the design of the telescope and its instruments. For each of these astronomical objectives, the mission has capabilities beyond those of visible-light telescopes to answer fundamental questions and provide deeper understanding of the cosmos.

  • Brown Dwarfs: Mysterious objects often characterized as "failed stars," brown dwarfs form in the same birth process as stars but never achieve the mass needed to ignite sustained nuclear fusion reactions in their core, and thus never "turn on" and shine like a true star. The exact mass range for brown dwarfs is still somewhat illdefined, but most are thought to range between twice the mass of Jupiter to 1/10th the mass of our Sun. (The Sun is about 1,000 times more massive than Jupiter.) Brown dwarfs were only a theoretical concept until they were discovered in 1995, and astronomers now think there may be as many brown dwarfs as there are stars. The Space Infrared Telescope Facility will add significantly to our understanding of these curious objects, from their number and distribution to their temperatures, sizes and chemical composition.

  • Circumstellar Discs: A substantial amount of the mission's observing time will be spent examining circumstellar discs - faint discs of material surrounding young stars. Astronomers now think these discs are common when stars and then planets form, and have divided them into two main types: protoplanetary discs and planetary debris discs.

    Protoplanetary discs are composed of gas and dust, thought to provide the raw materi- als for future planets. By contrast, planetary debris discs would form at a later stage of evolution, when most of the gas has dissipated. These later-stage discs are composed mostly of small dust grains presumably formed from collisions between small meteorsized rocks and/or larger asteroids. By observing circumstellar discs of various ages, the mission will trace how they evolve into a mature system of planets.

    Attempting to see faint circumstellar discs at visible wavelengths is extremely difficult because of the glare from parent stars. However, this relative difference in brightness is reduced in the infrared. The mission will study hundreds of nearby stars to determine the prevalence of such discs. It will also use images and spectral studies to understand the structure and composition of these discs. This information will be invaluable in deciphering the number and nature of planetary systems beyond our solar system. This will help advance the ultimate search for Earthlike planets, some of which may harbor life.

  • Star Birth and Death: Giant clouds of molecules, composed mostly of hydrogen gas, provide the basic building blocks for stars. These clouds, scattered throughout the spaces between stars in our Milky Way Galaxy, contain enough dense gas and dust to form hundreds of thousands of Sun-like stars. The Space Infrared Telescope Facility will study the temperature and density of these molecular clouds in order to understand the physical conditions and chemical compositions from which emerging stars form. Stars are born within cocoons of dust and dense molecular gas, a process mostly hidden from view at visible wavelengths. Near-infrared light, at wavelengths of a few microns, pierce this dusty veil to provide astronomers with a peek at newborn stars. The observatory's short-wavelength camera will probe the formation and early evolution of young stars in the first million years of life. The mission's observations will also reveal the extent to which new stars are formed in clusters rather than in isolation.

    In addition, this mission will study old stars. Once it has exhausted most of its thermonuclear fuel over billions of years, a star like our Sun enters a rapidly changing stage of life, when its behavior and ultimate fate depend on its birth mass. During the late stages of its life, a star typically ejects gaseous material from its outer layers, either through a gentle, gradual process known as a "nova," or through a violent cataclysmic explosion known as a "supernova."

    The observatory will study this material ejected by stars that have depleted virtually all their hydrogen and can no longer support themselves with nuclear fusion. It will also provide information about the temperature and chemical composition of the ejected material, and how quickly its parent star loses mass. The gas and dust thrown off by dying stars is an important constituent of the interstellar medium, and a thorough analysis of this material is essential for understanding not only how stars die, but also how they nurture the next generation of stars.

  • Active Galaxies: Many galaxies emit more radiation at infrared wavelengths than in all other regions of the electromagnetic spectrum combined. The most dramatic of these ultra-luminous infrared galaxies are a hundred to a thousand times brighter in the infrared than our own Milky Way Galaxy. Studies at both infrared and optical wavelengths reveal that most of these objects are in fact pairs of galaxies colliding or merging. Understanding the nature of these interacting galaxies and how stars form within them is one of the most compelling problems in astrophysics.

    Astronomers believe that many of these bright galaxies are powered by central black holes, while others produce "starbursts" spawned by colliding galaxies. The Space Infrared Telescope Facility will study the properties and evolution of ultra-luminous infrared galaxies out to great distances. By studying the spectra of the infrared light emitted from them, astronomers will better understand physical conditions in the optically obscured interiors of these galaxies, providing insight into the ultimate power source for these bright beacons.

  • The Early Universe: Because light from the most distant objects in space is shifted toward the red end of the spectrum, and because faraway objects are so distant in time, infrared telescopes provide an excellent window into the early universe. Astronomers will exploit this capability to observe dusty galaxies being born in the infant universe. By examining properties of galaxies at different red shifts, or cosmological ages, scientists will trace the history of star formation as a function of galactic environment, and try to explain why the rate of star formation across the entire known universe was much higher about 7 billion years ago than it is now. These observations will also help astronomers understand the distribution of galaxies in space and why they often appear in clusters.

    Some of the most important discoveries about the early universe may come from observations of the cosmic infrared background. This faint infrared glow in the distant, early universe is thought to result from a collection of countless unseen galaxies, too faint to be detected individually. By studying the intensity of the background at different wavelengths, astronomers can interpret the history of star formation, the history of galaxy formation and the presence or absence of dust in the earliest primeval galaxies. The Space Infrared Telescope Facility should be able to characterize the sources of this radiation.

  • Enduring Mysteries of the Solar System: Earth's own neighborhood, the solar system, still holds many secrets. Asteroids, comets, interplanetary dust and some of the member planets and their moons have defied full understanding when studied in visible light. But snooping in the infrared will allow scientists to examine some of the elusive properties of such features and help explain how our solar system developed and continues to evolve over time. These observations may then help astronomers more effectively scout out other solar systems in the nearby universe.

The importance of serendipity
The technical capabilities of the Space Infrared Telescope Facility will almost certainly lead to discoveries that no one can predict before the start of the mission. In some respects the mission will be hundreds, if not thousands of times more sensitive than past infrared missions. Compound that with the fact that the universe has not been studied extensively at many of these target wavelengths, and we have a recipe for serendipity. Much of the observing schedule has been left flexible, open to the astronomical community to delve into unanticipated territories. There will also be room for follow-up observations on unexpected findings as they are made. As history has taught, astronomers can always expect the unexpected.

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