Doppler ShiftDoppler Shift

October 19, 2009

Taking Care of Business Taking Care of Business (TCB)(TCB)

Read textbook Units 25 and 26Read textbook Units 25 and 26 No Homework next week!No Homework next week! Moon Observations – Monday, Moon Observations – Monday,

October 26October 26 Need total of 9 observations, all Need total of 9 observations, all

columns filled.columns filled. Test #2 – Friday, October 16 to Test #2 – Friday, October 16 to

Monday October 19Monday October 19 Reserve a test time!!!!Reserve a test time!!!!

What can we learn by analyzing starlight?

• A star’s temperature

• A star’s chemical composition

- peak wavelength of the spectral curve

- dips in the spectral curve or the lines in the absorption spectrum

• A star’s motion•Moving towards us/ moving away•How fast the star is going

The Doppler Effect

• Definition: “The change in wavelength of radiation (light) due to the relative motion between the source and the observer along the line of sight.”

Astronomers use the Doppler Effect to learn about the radial (along the line of sight) motions

of stars, and other astronomical objects.

On a boat, in the front you can see shorter waves

Boat, in the back the waves are longer

Real Life Examples of Doppler Effect (Sound or Light)

• Sound, NOT the volume of the sound, it’s a build of sound waves (pitch change)

• Trains, airplanes, moving vehicles, police sirens, bats

• Car headlights, radar gun

Doppler Effect• When something which is giving off light moves

towards or away from you, the wavelength of the emitted light is changed or shifted

V=0

Doppler Effect• When something which is giving off light moves

towards or away from you, the wavelength of the emitted light is changed or shifted

V=0

longer

shorter

Doppler Effect

• When the source of light is moving away from the observer the wavelength of the emitted light will appear to increase. We call this a “redshift”.

Red because the wave length is the longest.

Doppler Effect

• When the source of light is moving towards the observer the wavelength of the emitted light will appear to decrease. We call this a “blueshift”.

Blue, shifted towards the blue

Astronomy Application

V=0

Doppler Shifts

c

v

0

(700-400)/700 = V/3x10^8 m/s

V= 34% C

The Doppler Effect causes light from a source moving away to:

1. be shifted to shorter wavelengths.

2. be shifted to longer wavelengths.

3. change in velocity. (In space, light will travel in a constant speed.)

4. Both a and c above

5. Both b and c above

You observe two spectra (shown below) that are redshifted relative to that of a stationary source of light. Which of the following statements best describes how the sources of light that produced the two spectra were moving?

BLUE RED

Spectrum A

Spectrum B

1. Source A is moving faster than source B.

2. Source B is moving faster than source A., we are told it is redshifted!

3. Both sources are moving with the same speed.

4. It is impossible to tell from looking at these spectra.

A bright star is moving toward Earth. If you were to look at the spectrum of this star, what

would it look like?1. an absorption spectrum that is redshifted relative to an

unmoving star

2. an emission spectrum that is redshifted relative to an unmoving star

3. a continuous spectrum that is blueshifted relative to an unmoving star

4. an absorption spectrum that is blueshifted relative to an unmoving star, blueshifted (moving towards us), absorption

5. a continuous spectrum that is redshifted relative to an unmoving star

Links to In-Class Links to In-Class ProblemsProblems

Go to astro.unl.eduGo to astro.unl.edu Click on “Class Action” box on right.Click on “Class Action” box on right. Light: #11, #12, #13Light: #11, #12, #13