• 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

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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

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(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.