1. Infrared Astronomy
The image above is an aitoff projection of the sky centered on the center of the Milky Way Galaxy (in the constellation Sagittarius). The right and left endpoints of lines of latitude in this image are the same location in the sky. This image is a composite of previous infrared surveys of the sky done by the 2MASS and IRAS projects.

2. Infrared Light In 1800 William Herschel discovered “invisible light”
It’s energy with all the same characteristics as visible light, but is not sensed by the human eye
The light Herschel discovered was just beyond the red part of the spectrum. So it was named “infrared”
The image is a painting depicting William Herschel conducting his famous experiment where he spread sunlight into a spectrum and placed thermometers in the different colors to measure temperature differences. He found that when he placed the thermometers beyond the red portion of the visible part of the spectrum the thermometers in line with the visible spectrum still read a higher temperature than control thermometers. He realized that there must be energy in the sunlight that behaved just like light, but was invisible.
Notice the telescopes pictured in the background. Herschel was a musician by profession, but as an amateur astronomer he not only discovered infrared light, but also the planet Uranus.

3. The Spectrum of Light
Visible light is a tiny fraction of the Electromagnetic Spectrum
Gamma rays--billions of waves per inch
Radio waves--up to miles-long wavelengths
Low Energy Waves
High Energy Waves

4. The Physics of Light
All objects in the Universe emit light depending on their temperature.
Cool objects emit primarily long wavelength light
Hot objects emit primarily short wavelength waves
The graph is a plot of the intensity of light coming from an object as a function of the wavelength of that light. Opaque objects of different temperatures all have curves with the same shape. But the total amount of energy coming from the object (the area under the curve) and the peak of the curve (where most energy comes out) all change with temperature. These curves are often called black body curves or the Planck function.
The visible color of luminous objects depends on their temperature. Hot objects put out more blue light than red and appear bluish. Cooler objects put out more red light than blue and appear redish. Objects glowing white are putting out roughly equal amounts of all colors, like the Sun.

5. The Physics of Light
Objects emit light depending on their temperature.
Cool objects emit primarily long wavelength light
Hot objects emit primarily short wavelength light.
Human
Electric Stove
Spot Light
Lightning Bolt
The visible color of luminous objects depends on their temperature. Hot objects put out more blue light than red and appear bluish. Cooler objects put out more red light than blue and appear redish. Objects glowing white are putting out roughly equal amounts of all colors, like the Sun.
Infrared
33° C (306 Kelvin)
Red Light
4,130° C (4,400 Kelvin)
White Light
5,230° C (5,500 Kelvin)
Ultraviolet Light
30,000° C (30,273 Kelvin)

6. The Range of Infrared Light
Infrared light lies just beyond the red portion of the visible spectrum ("below red“). Infrared wavelengths are about 0.7 to 350 microns.
(a micron is one-millionth of one meter, or about 1/50th the width of a human hair).
In the image on the left, Yerkes observatory is seen in visible and near infrared light. All of the light in these two photos is reflected sunlight. Notice how the trees are very reflective in near infrared, but absorb much of the visible light that falls on them.
In the image on the right, a person is shown in visible light and mid infrared light. The visible light image is reflected light, but the infrared image is emitted light from the person. Where the infrared light is brighter the temperature of the person is warmer. Notice that infrared light passes through the black plastic bag, while visible light does not. But also notice that infrared light does not pass through the glass in the person’s glasses where visible light does.
Near Infrared
Mid Infrared

7. The Range of Infrared Light
SPECTRAL REGION
WAVELENGTH RANGE (microns)
TEMPERATURE RANGE (degrees Kelvin)
WHAT WE SEE
Near-Infrared
0.7 – 5
740 – 5,200
Cooler red stars
Red giants
Dust is transparent
Mid-Infrared
5 – 40
93 – 740
Planets, comets and asteroids
Dust warmed by starlight
Protoplanetary disks
Far-Infrared
40 – 350
11 – 93
Emission from cold dust
Central regions of galaxies
Very cold molecular clouds
The different parts of the infrared spectrum are not exact divisions. Some astronomers would use slightly different numbers. What they would use depends on many things including the physical processes producing the infrared light and the kind of detector being used to observe it.

8. Getting the WHOLE picture
An object can look radically different depending on the type of light collected from it:
Since shortly after Herschel discovered infrared light astronomers have been observing astronomical objects in Infrared Light to get a more complete picture
The Moon is shown in visible and infrared light. The infrared image was taken while the Moon was being eclipsed in Earth’s shadow. So the light seen is emitted light from the Moon indicating differences in surface temperature. Note that for the most part places that are dark in visible light are giving off more infrared light, indicating that they are warmer. That makes sense because something is dark in visible light if it is absorbing more light than it is reflecting. The notable exceptions are the bright craters. They are not bright in infrared because they are warmer, but rather because the geometry of their bowl shaped surfaces creates a “beaming” effect where more infrared light is directed at the observer than would otherwise be seen in a “flat” surface.
Visible Light Image
Mid-Infrared Light Image

9. Why Study Infrared?
Visible: dark nebula, heavily obscured by interstellar dust (“Horsehead Nebula”)
Near-Infrared: dust is nearly transparent, embedded stars can be observed forming
Mid- and Far-Infrared: glow from cool dust is directly observable
In visible light, the dense cloud in the foreground is seen in silhouette in the shape of a horse’s head. In near infrared light we see right through the gas and dust to view the stars forming within the cloud. In mid and far infrared light we can see the glow of the dust itself.
Visible
Near Infrared
Mid-Infrared

10. Why Study Infrared?
Cool objects--like newly forming stars and solar systems--emit almost exclusively in the Infrared
The image shows the observation in infrared light of a ring of dust surrounding a young star. Probably the remainder of a disk of dust that is being blown away by the young star’s radiation and wind.

11. Why Study Infrared?
Infrared penetrates intervening dust clouds, allowing us to see through or into them
Visible Light
Infrared

12. But there’s a Challenge...
Earth’s atmospheric water vapor absorbs almost all incoming infrared radiation
Even mountain-top observatories get a limited view of the infrared universe
The greenhouse gases in the atmosphere that warm the Earth by absorbing infrared from the ground and oceans also absorb incoming infrared from space. Water is the strongest greenhouse gas in the atmosphere followed by Carbon Dioxide and Methane. Due to the temperature of the atmosphere it then emits infrared light back at the ground. That means if we had eyes that could see infrared light the night sky would be as bright as the daytime sky.
So getting above most of the atmosphere is needed to do most infrared astronomy.
Infrared telescopes need to observe from high altitude or in space

13. NASA’s Infrared Missions
Spitzer Space Telescope
WISE
SOFIA
James Webb Space Telescope
NASA and other nations have had several notable missions in the past that studied the Universe in infrared light. The modern suite of infrared telescopes from NASA include the currently flying Spitzer Space Telescope. In 2009 the WISE mission will launch and survey the entire sky in near and mid infrared wavelengths. Also in 2009 SOFIA begins science flights with a mid and far-infrared telescope that flies high in the stratosphere aboard a Boeing 747 aircraft. In 2013 NASA will replace the Hubble Space Telescope with a 6-meter telescope that views mostly near and mid infrared light called the James Webb Space Telescope.

14. Constellation Orion Visible Light
In visible light we can see the glow from stars and hot gas between the stars.

15. Constellation Orion Mid Infrared Light IRAS
In infrared light we see the glow from cool stars and warm dust and gas. The warmest and dustiest places are also the brightest places in this image. They are locations of new star formation.

16. Trifid Nebula Visible Light NOAO
In this image we see dark dust clouds silhouetted against a cloud glowing red due to being energized by a hot, young stars within it. Scattered, starlight illuminates the edges of the cloud in blue.

17. Trifid Nebula Infrared Light Spitzer
In this image, blue color indicates higher temperatures and red light is cooler temperatures. We see the stars are hotter (blue) where the dust and gas are cooler (red and green).

18. Orion Nebula
Visible
Light

19. Orion Nebula
Infrared
Light
Spitzer

20. Sombrero Galaxy Visible Light HST
In this image of a spiral galaxy seen from the edge we see the dust in the disk of the galaxy silhouetted against the bluish glow of bright, hot, young stars in the disk. While the halo of older stars has a more yellowish tinge.

21. Sombrero Galaxy Infrared Light Spitzer
In this infrared image we see the cool dust in red, and the stars in blue.

22. The Whole Sky Visible Light - Axel Mellinger
This is an image of the entire sky seen in visible light.
Visible Light - Axel Mellinger

23. The Whole Sky Near Infrared Light - 2MASS Survey
This is the starlight of the Milky Way in near infrared light.
Near Infrared Light - 2MASS Survey

24. The Whole Sky Mid/Far Infrared Light - IRAS Survey
Here we see the glow of the disk of dust in the Milky Way (red) and scattered sunlight from the dust inside the plane of the Solar System (blue).
Mid/Far Infrared Light - IRAS Survey