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• Summary of Chapter 3: Light
Today
•All of Chapter 4: Spectra & AtomsOptional: Ast. Toolbox 4-2Optional: Stephan-Boltzmann Law
•Coming up: The Sun (Chapter 10)
1. Light Gathering Power: The ability to collect light
2. Resolving Power: The ability to see fine details
3. Magnifying Power: The ability to make objects look bigger
Telescopes have Three Powers
Resolving Power: Seeing Details
Resolving Power (or resolution) is a telescope’s ability to see small details. It is proportional to a telescope’s size (diameter D)
Magnification makes things look larger
However it does not improve resolution (seeing fine details).
The Best Location for a TelescopeFar away from civilization – to avoid light pollution
Text
On a mountaintopMauna Kea, Hawai’i (~14,000 feet elevation)
Hubble Space Telescope
Or in Space…
Space Telescopes avoid the blurring effects of Earth’s atmosphere.
Space Telescopes also allow us to see light blocked by Earth’s atmosphere: infrared, ultraviolet, X-rays & gamma rays.
Spitzer (2003)Fermi Gamma Ray Observatory (2008)
Infrared Light
Visible Light
Andromeda Galaxy
A newer IR image from the “WISE” satellite
X-ray Astronomy
X-rays are high energy light with very short wavelength
They are emitted by very hot gas in the universe.
Chandra X-ray Telescope Chandra Image of Supernova Explosion
The Largest Radio Telescope
Since radio waves pass through Earth’s atmosphere,
we can build radio telescopes on the ground.
The 300-m telescope in Arecibo, Puerto Rico is the
largest in the world.
It can hold 13 football fields!
Review of Chapter 3! To understand what’s in space, we must understand light. ! Light is an electro-magnetic wave with three properties:
! Speed: c = 300,000 km/s ! Frequency (f): number of light waves per second. ! Wavelength (λ): distance. from one peak to the next
!We see different wavelengths as colors ! These are related: c = f x λ
! Many forms of light exist, most invisible to humans: ! Infrared radiation and radio waves have longer
wavelengths than visible ! Ultraviolet, X rays, and gamma rays have shorter
wavelengths ! Visible light occupies only a small portion of the
“elctromagnetic spectrum.”
Chapter 3 Review! Visible light waves are small: their wavelengths (λ) are
measured in nanometers (1 nm = 10-9 m) ! λ=400 nm for violet light and λ=700 nm for red ! Light’s energy depends on its frequency
! Telescopes: gather light, reveal details, and magnify images
! The two main types: ! Reflectors produce images using mirrors, ! Refractors use lenses to focus light
! Light gathering ability depends on the telescope mirrors’s area. ! The area of a circle is: A = π r 2
The Power of StarlightChapter 4
By analyzing the light from a star, we learn about its:
1. Temperature 2. Composition 3. Motion
Red Hot Orange Hot Yellow Hot
Using Light to Measure Temperature
Color and Temperature
Orion
Betelgeuse
Rigel
Stars have different colors,
Rigel is blue
Betelgeuse is red
The different colors of stars are due to their different temperatures.
TemperatureAstronomers use Kelvins (K) to measure temperatures.
Example:
The freezing point of water is:
32 F
0 C
273 K
0 K is Absolute Zero
Challenge Question:
Are you sure? What about at sunset?
The Sun produces light that is not yellow.
What Color is the Sun?
Wavelengths of Light
• Light from any source has a variety of colors.• Q: At which color (or wavelength) does the
source emit the most light?
• To answer this question we use a spectrograph.• A spectrograph produces readout of light intensity
vs. wavelength called a spectrum
The SpectrographA spectrograph uses a prism to
split light up into different wavelengths (=colors!)
Chapter 28 Opener
Modern spectra are recorded digitally as plots of intensity vs.
wavelength
A Spectrum
The Sun emits UV, Visible and Infrared light
Brig
htne
ss
(c) C. Baird, ASTM
Spectrum of the Sun
Wavelength
UV Infrared
Visible
Stellar Spectra
• Sun emits more yellow (λ=570 nm) light than any other color.
• We say the “The Sun is yellow” because its spectrum “peaks” in the yellow part.
• We can use this fact to measure the temperature of the Sun, or any other star.