|
|
|
|
|
Why Observe in the Far IR and Submillimeter?
Beyond the Rainbow
Remember the first time you saw "The Wizard of Oz," and the drab
black-and-white of Kansas changed to the dazzling full color of
Munchkinland? Imagine if landing in Oz also enabled you to see
colors that no human eye had ever been able to detect - and these
new colors revealed fantastic objects that had previously been
invisible!
That is what it was like when astronomers began observing in the
infrared.
In fact, some of the most interesting things in the Universe are
visible only in light beyond the rainbow of colors that people can
see.
Radio waves, microwaves, x-rays, gamma rays, and the spectrum of
visible colors are all really the same thing - electromagnetic
energy. The differences are their wavelengths. Radio waves are long,
measuring as much as hundreds of meters between peaks. Gamma ray
wavelengths are extremely short, as little as trillionths of a
meter. A photon of shorter-wavelength light packs more energy than a
photon of longer-wavelength light.
Within the limited range our eyes can perceive, we interpret
different wavelengths as colors, from violet (about 400 nanometers)
to red (about 700 nanometers). Light with wavelengths just shorter
than violet is known as "ultraviolet" (UV). We can't see it (though
some insects can), but our skin responds to it. That's the portion
of sunlight we're trying to block when we wear sunscreen.
The part of the spectrum with wavelengths just longer than red is
called "infrared" (IR). Like UV light, IR light is invisible to the
naked eye. But we can feel it as heat, and we can see it with the
aid of technology.
"Night vision" devices enable people to see by the light of the
near-infrared, the wavelengths just beyond visible light.
Instruments aboard the Herschel Space Observatory will enable us to
see the far-infrared and submillimeter portion of the spectrum, from
about 60 to 670 microns in wavelength.
Why is infrared radiation important to astronomy?
Most of the light in the Universe is in infrared and longer
wavelengths. There are three basic reasons for this:
-
It's cold out there.
|
|
Cold objects like this comet radiate most strongly in the infrared
|
Many of the things scientists want to observe in space are far
too cold to radiate at optical or shorter wavelengths. However, even
at temperatures far below the coldest spots on Earth, they do
radiate at far-infrared and submillimeter wavelengths.
To understand how stars form and evolve, we need to understand
their raw materials - the cold atoms and molecules that populate
interstellar space. They radiate most strongly in Herschel's range.
Analyzing their spectra will enable scientists to determine the
temperature, density, luminosity, composition, magnetic fields, and
dynamics of the chemicals and their environments.
In our own solar system, cold objects such as comets, asteroids,
and the planets themselves reveal most of their characteristics by
infrared light.
Brown dwarfs, protostars, dusty disks around young stars, and
planets in other star systems are all too cold to radiate in the
optical range, but shine at infrared wavelengths. (The stars will
outshine their planets in the IR, but the relative glare is much
less in the infrared.)
-
It's dusty out there.
A number of things of great interest to astronomy are hidden
within or behind vast clouds of gas and dust. In the early stages of
forming, stars and planets are concealed by the stuff. So are the
enormously powerful cores of active galaxies, the center of our own
Milky Way, and most of the early Universe.
|
Stars in the main band of our galaxy, the Milky Way, are obscured by
dust
in optical light (left), but shine through in infrared light (right).
|
Our view is blocked in visible light because the dust grains are
about the same size as optical wavelengths - about one micron or
less - and so are very effective at scattering or absorbing that
light. But longer infrared wavelengths undulate around the dust. And
the longer the wavelength, the thicker the layer of dust it can
penetrate. So far-IR and submillimeter radiation can move freely
through the Universe, unobstructed by dust.
The visible and ultraviolet light that the dust absorbs warms the
grains just enough for them to reradiate the light at infrared
wavelengths. So in addition to allowing infrared light to pass
through, the dust itself radiates at that range of wavelengths. And
by analyzing the dust's radiation, scientists can deduce information
about the optical sources it is hiding.
-
The Universe is expanding.
Galaxies outside our own group are traveling away from us with
the expansion of the universe, and the more distant they are, the
faster they're receding. As they speed away, their light is
"redshifted" to longer wavelengths. Light that starts out at optical
wavelengths may be stretched into the infrared.
What Infrared Astronomy has shown us so far?
Optical astronomy has been around since the first humans looked
up and started to chart the motions of the heavens. We've boosted
our powers of observation with instruments since 1609, when Galileo
pointed a telescope at the moon. But it is only within the last
half-century that we have begun to explore the Universe in the
infrared. And the results have been astonishing.
The first infrared survey of the sky, published in 1965, revealed
ten objects that optical telescopes couldn't see. By 1969, thousands
of new objects had been discovered in the infrared.
More recently, infrared astronomy made the surprising discovery
that Jupiter, Saturn, and Neptune have internal sources of heat. It
found a hundred thousand red giant stars in the central bulge of the
Milky Way, and ices of water, methane, carbon dioxide, formaldehyde,
and carbon monoxide in interstellar space.
|
|
|
Visible-light image of Andromeda galaxy.
|
Andromeda galaxy in infrared.
|
Old, familiar objects have revealed new features by the light of
the infrared. By optical light (left), the Andromeda galaxy appears
as a spiral that grows more tenuous at the outer regions. But in the
infrared (right), we can see that a giant ring of dust at the
outskirts of the galaxy is a hotbed of star formation.
Infrared observations of galaxies 10 billion light years away
found star-formation at a rate three to four times greater than
optical surveys had indicated, dramatically changing our
understanding of the early Universe.
Deeper into the Infrared and Submillimeter
Herschel's observations of far-IR and submillimeter light promise
to reveal even more wonders.
At these wavelengths, Herschel will be able to probe deeper into
dusty interstellar nurseries to see details of star formation that
have so far remained hidden. It will have a much clearer view of our
galaxy's core, as well as the centers of neighboring galaxies. And
it will peer through massive dusty toroids for an unprecedented look
at the cosmic powerhouses known as Active Galactic Nuclei.
This is also the best part of the electromagnetic spectrum for
observing key chemicals in space. About 130 kinds of chemicals have
been detected so far in the interstellar medium, and most have
rotational spectra - the photon emissions induced by the rotation of
the molecules - with wavelengths that peak in the submillimeter
range. These include the many forms of water and the organic
molecules thought to be necessary for life.
Herschel will uncover new information about how these chemicals
come to be, their role in the life cycles of stars and galaxies, and
how they work in the atmospheres of our neighboring planets and
comets.
And finally, this is the optimal range for observing the
earliest, most distant galaxies whose light has been redshifted into
wavelengths too long even for Herschel's predecessor, the Infrared
Space Observatory, to detect. But Herschel should be able to see
light from this era, when the Universe was just coming out of its
"Dark Age," and the first galaxies were beginning to form.
To Learn More
NASA's Infrared Processing and Analysis Center (IPAC) offers
tutorials to help you learn more about
Infrared Astronomy
and
Infrared Spectroscopy
.
|
|
|
|
|
|
