PES 106       Spring 2003
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General Astronomy II
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Lecture Notes:

Light: Spectroscopy and Atoms; Doppler Effect

text: Chapter 3, Sections 3.3 - 3.6

Introduction

Sometimes we are interested in determining the composition of a star or the atmosphere of a planet. We can do that by studying the spectrum of light coming to us from these objects.

 

There are 3 types of spectra:

1. Continuous spectrum

continuous spectrum

like a rainbow

source: radiation from heat (like we discussed in the last lecture) from hot solids or hot dense gasses

2. Absorption line spectrum

absorption spectrum

dark lines in a continuous spectrum => light absorbed at certain wavelengths

source: cooler gas with a continuous source behind it

3. Emission line spectrum

emission spectrum

bright lines at certain wavelengths (no continuous spectrum)

source: hot gas

Here is a link to a nice exercise discussing the various types of spectra.

Here is a description of the three types of spectra with good figures: http://csep10.phys.utk.edu/astr162/lect/light/absorption.html

Why line spectra ?

line spectra will lead us to the structure of atoms

electron orbits and rungs of a ladder

the orbits of electrons (and the spacing between them) can be treated like the rungs of a ladder.

Here is a description and animation showing the Bohr atom: http://home.a-city.de/walter.fendt/phe/bohrh.htm

absorption spectrum: If light with just the right amount of energy (wavelength, color) hits the atom, we can get the electron to jump to the next higher level. Only this color of light will be absorbed.

CD: Look under Astronomy Basics / Light and Atoms / Animations. The first choice whose description begins "Energy is released . . " describes atomic shells and light.

[Link to absorption animation]

emission spectrum: If an electron jumps down from a higher level to a lower level, light with a specific energy (wavelength) will be emitted.

Link to animation showing emission spectrum and atomic orbits.

The steps can have different sizes =>

different energies => different wavelengths (colors) absorbed or emitted

Each element has different spacings.

=> use pattern of absorption or emission lines to identify each element

=> a fingerprint

oxygen: Oxygen spectrum

helium: helium spectrum

a more detailed description of this model: http://csep10.phys.utk.edu/astr162/lect/light/bohr.html

Demo with continuous and emission spectra

emission and absorption spectra can also be seen at: http://mo-www.harvard.edu/Java/MiniSpectroscopy.html

The fluorescent lamp is an example of an emission spectrum. The top figure shows this visibly while the graph shows an upward spike for each emission line.

Choose the Sun from the menu in the center top to see absorption lines. Now the graph shows spikes going down for each absorption line.

Doppler Effect

Stationary star emits light which looks the same to a stationary observer in any direction.

  • If two objects are getting closer to one another, the light emitted from one object appears to have a shorter wavelength (more blue) than it would if neither object was moving.
    • Continuous, absorption and emission spectra will ALL be shifted to shorter wavelengths => BLUE SHIFT
  • If two objects are getting farther from one another, the light emitted from one object appears to have a longer wavelength (more red) than it would if neither object was moving.
    • Continuous, absorption and emission spectra will ALL be shifted to longer wavelengths => RED SHIFT

The amount of the shift will get bigger if the objects are moving faster.

Here is a very good animation showing the Doppler effect for a Galaxy moving away from the Earth: http://www.loydnet.com/doppler/

You can not tell from the Doppler effect which object is moving.

Here is a rather complete discussion of the Doppler effect on Earth and in astronomy: http://www.pbs.org/wgbh/nova/universe/moving.html

Examples with sound are car horns or train whistles. Here is an animation showing this: http://home.a-city.de/walter.fendt/phe/dopplereff.htm

Doppler shift will confuse the process of finding temperatures since the peak wavelength gets shifted.

Line spectra are also shifted - but the fingerprint can still be recognized. This can be seen by examining several spectra at: http://mo-www.harvard.edu/Java/MiniSpectroscopy.html. Select Galaxy1 from the menu at center top and notice the bright red emission line around 670 nm. This comes from hydrogen. Select Galaxy 2 and notice that the line has shifted to about 677 nm. Select Galaxy 3 and the line is at about 690 nm. These lines all come from the same physical process and should be at the same wavelength. The differences arise from different speeds that the galaxies are moving away from us.

If a star is spinning, one side of the star may be moving toward you while the other side moves away. You can use the Doppler effect to determine how quickly the star is spinning.

Doppler broadening from spinning star

If a star is orbiting, some times it will be moving toward you and sometimes away. This will cause an oscillating shift in the spectra as demonstrated in the animation at: http://imagine.gsfc.nasa.gov/docs/teachers/lessons/star_size/star_velocity.html

Skip section 3.7

Skip to Chapter 11 to begin our study of STARS !

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