The Universe in the Infrared
What can we learn from infrared light and how do we see it?
Funded by NASA’s Spitzer Science Center
Images courtesy NASA/JPL - Caltech
Pilachowski / August 2005
The Universe in the Infrared
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Outline
The electromagnetic spectrum Atmospheric windows The colors of infrared light Sources of infrared light Detecting IR light Infrared telescopes Distances
Pilachowski / August 2005
The Universe in the Infrared
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Understanding the electromagnetic spectrum
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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 m (microns)
– A micron is one-millionth of a meter, and is abbreviated as µm
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What does “electromagnetic” mean?
Properties of waves speed (distance per
second) wavelength
(length) frequency (cycles
per second)
speed of light = wavelength x frequency
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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 = x 109 /sec
)(3.0sec)(/10
sec)/(103)(
9
8
metersmx
metersW
speed = wavelength x frequency
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Terminology
decimeter 10-1 meterscentimeter 10-2 metersmillimeter 10-3 metersmicrometer 10-6 metersnanometer 10-9 meters
decameter 101 metershectometer 102 meterskilometer 103 meters
Visible light has wavelengths between 400 and 700 nanometers
Microns and nanometers…
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The Colors of Infrared Light
Astronomers refer to different types of infrared light
The precise wavelength ranges are somewhat arbitrary
Near IR: 1-5 m Mid IR: 5-30 m Far IR: 30-200 m
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Atmospheric Windows
Some near-IR light reaches mountain-top 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.
Earth from GOES-8 @6.7 m
At different wavelengths of light, the Earth’s atmosphere can be either
transparent or opaque
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Sources of IR Light
Stars
Gas
Dust
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All matter glowswith light
Cool matter glows primarily with radio
or infrared light
Warmer matter glowswith higher energy
light
Matter at about10,000 degrees centigrade
glows white hotEven hotter matter
glows blue hot
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The glow of matter because of its
temperatureBlackbodies 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
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Stars as Black Bodies
A very hot star will peak in the ultraviolet, but we will see it as a blue star
A very cool star will peak in the infrared, but we will
see it as a red star
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• “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
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Wien’s Law
max
000,000,3
T
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
We can determine the surface temperature from the wavelength of the
peak brightness for any star
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• The energy emitted is directly proportional to
T4
• As stars get hotter, their energy output increases quickly!
• A star 10 times hotter than Sun has 10,000 times more energy output
Temperature
Matters!To be bright in the infrared, cool sources must be BIG
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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
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Temperature and peak brightness
Radio < 0.03K
Microwave 0.03-30KInterstellar Space
Infrared 30-4100KHumans
Visible 4100-7300KSun
UV 7300-3 x 106KHottest Stars
X-ray 3x106-3x108KNeutron stars
Gamma Ray > 3x108KBlack holes
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Scattering and Extinction
• Dust also scatters starlight
Dust clouds block visible light but are transparent
to infrared light T.A.Rector (NOAO/AURA/NSF) and Hubble Heritage Team (STScI/AURA/NASA)
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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.
24 mVisible
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Why infrared?
• Near-infrared (1-5 mm)– stars– warm gas– dust is transparent
• Mid-infrared (3-30 mm)– dust warmed by
starlight– protoplanetary disks
• Far-infrared (30-200 mm)– cold gas & dust
Dust is more transparent to infrared light. We can see what’s hidden in the dust.
Cold gas and dust is invisible in visible light, but glows in infrared light.
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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
InSb array detectors by Raytheon (SBRC).
Courtesy Univ. of Rochester Astronomy
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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
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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
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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
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More false color
• Astronomers also “colorize” black and white images to highlight certain aspects.
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Inverse Square Law
If we know a star’s apparent AND absolute brightness, we can calculate its distance
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
brightness = 1/distance2
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Parsec: the distance to an object with a stellar parallax of one arc second
The parallax of Alpha Centauri = 0.76 arcseconds
A parallax of ~0.001 arc secondsis the smallest we can measure
What is a Parsec???
1 parsec = 3.26 light years
A star at a distance of 1 parsec showsa parallax of 1 arc second
How big is onearc second?
The size of adime at adistance of2.3 miles!
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Wrapping Up
The electromagnetic spectrum Atmospheric windows The colors of infrared light Sources of infrared light Detecting IR light Infrared telescopes Distances