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

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


Slide 2

Outline

The electromagnetic spectrum Atmospheric windows The colors of infrared light Sources of infrared light Detecting IR light Infrared telescopes Distances



Slide 3

Understanding the electromagnetic spectrum

http://www.spitzer.caltech.edu/features/articles/images/20050708.jpg



Slide 4

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



Slide 5

What does “electromagnetic” mean?

Properties of waves speed (distance per

second) wavelength

(length) frequency (cycles

per second)

speed of light = wavelength x frequency



Slide 6

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

http://www.ipac.caltech.edu/2mass/gallery/2mass_msx_gc.html



Slide 7

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…



Slide 8

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



Slide 9

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



Slide 10

Sources of IR Light

Stars

Gas

Dust

http://www.ipac.caltech.edu/2mass/gallery/showcase/allsky_stars/index.html



Slide 11

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



Slide 12

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



Slide 13

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



Slide 14

• “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

http://alamo.bu.edu/applets/Spec/applet.html



Slide 15

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

http://solar.physics.montana.edu/YPOP/Spotlight/Today/infrared.html



Slide 16

• 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



Slide 17

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



Slide 18

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



Slide 19

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)



Slide 20

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

http://www.aao.gov.au/images/captions/uks018.html

http://images.google.com/imgres?imgurl=http://www.arttech.us/images/Sunset.jpg&imgrefurl=http://www.arttech.us/images/&h=263&w=350&sz=22&tbnid=ilFUi6gzuVYJ:&tbnh=87&tbnw=116&hl=en&start=1&prev=/images%3Fq%3Dsunset%26svnum%3D100%26hl%3Den%26lr%3D%26sa%3DG



Slide 21

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.



Slide 22

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



Slide 23

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



Slide 24

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



Slide 25

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



Slide 26

More false color

• Astronomers also “colorize” black and white images to highlight certain aspects.



Slide 27

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



Slide 28

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!



Slide 29

Wrapping Up

The electromagnetic spectrum Atmospheric windows The colors of infrared light Sources of infrared light Detecting IR light Infrared telescopes Distances

Documents

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