The Universe in the Infrared
What can we learn from infrared
light and how do we see it?
Images courtesy NASA/JPL - Caltech
Funded by NASA’s Spitzer Science Center
Outline
 The electromagnetic spectrum
 Atmospheric windows





The colors of infrared light
Sources of infrared light
Detecting IR light
Infrared telescopes
Distances
The Universe in the Infrared
Pilachowski / August 2005 Slide 2
Understanding the
electromagnetic
spectrum
The Universe in the Infrared
Pilachowski / August 2005 Slide 3
The Electromagnetic Spectrum
• Infrared (IR) is a form of light (or electromagnetic
radiation)
• IR light is found between visible light and radio waves
• Wavelengths extending from 1 to 200 mm (microns)
– A micron is one-millionth of a meter, and is abbreviated as µm
The Universe in the Infrared
Pilachowski / August 2005 Slide 4
What does “electromagnetic” mean?
 Properties of waves
 speed (distance per
second)
 wavelength (length)
 frequency (cycles
per second)
speed of light
= wavelength
x frequency
The Universe in the Infrared
Pilachowski / August 2005 Slide 5
speed = wavelength x frequency
 speed – 300,000 km per second (3 x 108
meters per second)
 frequency – say, one billion cycles per second
(109 cycles per second)
 What is the wavelength?
 What kind of light is this?
3 x 108 m/sec = l x 109 /sec
3x108 (m / sec)
W (meters) 
 0.3(meters)
9
10 (/ sec)
The Universe in the Infrared
Pilachowski / August 2005 Slide 6
Terminology
Microns and nanometers…
decameter 101 meters
hectometer 102 meters
kilometer
103 meters
decimeter
centimeter
millimeter
micrometer
nanometer
10-1 meters
10-2 meters
10-3 meters
10-6 meters
10-9 meters
The Universe in the Infrared
Visible light has
wavelengths
between 400 and
700 nanometers
Pilachowski / August 2005 Slide 7
The Colors of Infrared Light
Near IR: 1-5 mm
Mid IR: 5-30 mm
Far IR: 30-200 mm
 Astronomers
refer to
different types
of infrared
light
 The precise
wavelength
ranges are
somewhat
arbitrary
The Universe in the Infrared
Pilachowski / August 2005 Slide 8
Atmospheric Windows
Earth from GOES-8 @6.7 mm




At different wavelengths of light, the
Earth’s atmosphere can be either
transparent or opaque
Some near-IR light
reaches mountaintop observatories.
Clear IR windows
are centered at
1.25, 1.65, 2.2, 3.5,
4.8 microns.
High-flying
airplanes and
balloons get above
most of the
atmosphere
Only space-borne
infrared
telescopes provide
an unimpeded view
of the infrared
universe.
The Universe in the Infrared
Pilachowski / August 2005 Slide 9
Sources of IR Light
Stars
Gas
Dust
The Universe in the Infrared
Pilachowski / August 2005 Slide 10
Cool matter glows
primarily with radio
or infrared light
All matter glows
with light
Warmer matter glows
with higher energy
light
Even hotter matter
glows blue hot
The Universe in the Infrared
Matter at about
10,000 degrees centigrade
glows white hot
Pilachowski / August 2005 Slide 11
The glow of matter because
of its temperature
Blackbodies emit light at all wavelengths
Cooler object peak at longer wavelengths (redder)
Hotter objects peak at shorter wavelengths (bluer)
The higher the temperature, the
shorter the peak wavelength
Very cool objects peat at radio wavelengths and very hot
objects peak at ultraviolet, x-ray, or gamma-ray wavelengths
The Universe in the Infrared
Pilachowski / August 2005 Slide 12
Stars as Black Bodies
A very cool star will peak in
the infrared, but we will see
it as a red star
A very hot star will peak
in the ultraviolet, but we
will see it as a blue star
The Universe in the Infrared
Pilachowski / August 2005 Slide 13
• “Black bodies” glow at ALL wavelengths
• The wavelength at which the black body
is brightest tells us the temperature
(hotter = shorter wavelength)
• As the temperature increases, the
blackbody radiation also gets BRIGHTER
Black Body Radiation Applet
The Universe in the Infrared
Pilachowski / August 2005 Slide 14
Wien’s Law
We can determine the surface temperature
from the wavelength of the peak brightness
for any star
3,000,000
T
lmax
The sun is brightest at
a wavelength of 520
nanometers. What is
the temperature at the
surface of the Sun?
3,000,000 / 520 = 5770 K
The Universe in the Infrared
Pilachowski / August 2005 Slide 15
Temperature Matters!
•
The energy
emitted is directly
proportional to
4
T
•
•
To be bright in
the infrared,
cool sources
must be BIG
As stars get
hotter, their
energy output
increases quickly!
A star 10 times
hotter than Sun
has 10,000 times
more energy
output
The Universe in the Infrared
Pilachowski / August 2005 Slide 16
Temperature – The Kelvin Scale
• Named after Lord (William Thompson)
Kelvin
– 19th century Scottish physicist
– a one degree difference on the Kelvin (K)
scale is the same as for the Celsius (or
centigrade) scale
• The zero-point is defined to be
absolute zero
– the coldest possible temperature
– atomic and molecular motion ceases
– no negative temperatures
• Note: no degree symbol (°) with the
Kelvin scale
The Universe in the Infrared
Pilachowski / August 2005 Slide 17
Temperature and peak brightness
Visible 4100-7300K
Radio < 0.03K
Microwave 0.03-30K
Interstellar Space
Infrared 30-4100K
Humans
Sun
UV 7300-3 x 106K
Hottest Stars
X-ray 3x106-3x108K
Neutron stars
Gamma Ray > 3x108K
Black holes
The Universe in the Infrared
Pilachowski / August 2005 Slide 18
Scattering and Extinction
•
Dust also
scatters
starlight
Dust clouds block visible
light but are transparent to
infrared light
The Universe in the Infrared
T.A.Rector (NOAO/AURA/NSF) and Hubble Heritage
Team (STScI/AURA/NASA)
Pilachowski / August 2005 Slide 19
The Pleiades – Optical & IR
A dust cloud passing near the Pleiades scatters blue starlight in
this visible light image. The dust radiates in the infrared.
Visible
The Universe in the Infrared
24 mm
Pilachowski / August 2005 Slide 20
Why infrared?
Dust is more
transparent to
infrared light. We
can see what’s hidden
in the dust.
•
Near-infrared (1-5 mm)
•
Mid-infrared (3-30 mm)
•
Far-infrared (30-200 mm)
– stars
– warm gas
– dust is transparent
– dust warmed by starlight
– protoplanetary disks
– cold gas & dust
Cold gas and dust is invisible in visible
light, but glows in infrared light.
The Universe in the Infrared
Pilachowski / August 2005 Slide 21
Detecting Infrared Light
• Single-pixel bolometers, 1960’s
• first semi-conductor arrays, 32x32 pixels, in early 1980’s
Top left: 58 X 62 pixels, 1987
Middle left: 256 X 256 pixels, 1991
(SIRTF, IRAC)
Lower left: 1024 X 1024 pixels (1 Mega
Pixel), 1996
Right: 2048 X 2048 pixels (4 Mega Pixel)
2001
Courtesy Univ. of Rochester Astronomy
The Universe in the Infrared
InSb array detectors by Raytheon
(SBRC).
Pilachowski / August 2005 Slide 22
Observing at Nonvisible Wavelengths
• Astronomical objects radiate in wavelengths
other than visible (blackbody radiation)
– Stars
– Hot, warm and cold gas
– Dust
• Telescopes for each wavelength region
–
–
–
–
Require their own unique design
All collect and focus radiation and resolve details
False-color pictures to show images
Some wavelengths must be observed from space
The Universe in the Infrared
Pilachowski / August 2005 Slide 23
Infrared Telescopes
• Space-Based Advantages
– No atmospheric blurring
– No atmospheric absorption
– No atmospheric emission
• Ground-Based Advantages
– Larger collecting area
– Better spatial resolution
– Equipment easily updated
• Ground-Based Considerations
– Weather, humidity, and haze
– Atmospheric transparency
The Universe in the Infrared
Pilachowski / August 2005 Slide 24
False Color
• Astronomical images begin as black & white (grayscale) digital data
from a single spectral region, often using wavelengths outside of
the range of human vision
• A "true" color image or photograph recreates what our eyes would
see in visible light under natural conditions
• To create a color image from data at other wavelengths,
astronomers represent it in "false" colors
• Three of grayscale images from different wavelengths may be
mapped to red, green, and blue and overlaid to form a color image
The Universe in the Infrared
Pilachowski / August 2005 Slide 25
More false color
• Astronomers also “colorize” black and white
images to highlight certain aspects.
The Universe in the Infrared
Pilachowski / August 2005 Slide 26
Inverse Square Law
If we know a star’s apparent AND absolute
brightness, we can calculate its distance
brightness = 1/distance2
The inverse square law describes how
the brightness of a source light (a star!)
diminishes with distance
For nearby stars, stellar parallaxes
provide a way to measure distance
The Universe in the Infrared
Pilachowski / August 2005 Slide 27
What is a Parsec???
Parsec: the distance to an object
with a stellar parallax of one arc second
A star at a distance of 1 parsec shows
a parallax of 1 arc second
1 parsec = 3.26 light years
A parallax of ~0.001 arc seconds
is the smallest we can measure
How big is one
arc second?
The size of a
dime at a
distance of
2.3 miles!
The parallax of Alpha Centauri = 0.76 arcseconds
The Universe in the Infrared
Pilachowski / August 2005 Slide 28
Wrapping Up
 The electromagnetic spectrum
 Atmospheric windows





The colors of infrared light
Sources of infrared light
Detecting IR light
Infrared telescopes
Distances
The Universe in the Infrared
Pilachowski / August 2005 Slide 29