SIRTF science objectives
FROM NASA PRESS KIT
Posted: April 14, 2003
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.
An artist's concept of SIRTF. Credit: NASA/JPL/Caltech
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
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
- 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
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
- 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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