Download Spectral Analysis and Doppler Effect

Spectral Analysis

How do we learn about objects in the solar system and beyond?

We can go there and take sample data & images that can be sent back to
Earth. Various probes have landed on Mars and there are numerous
satellites that are orbiting various objects in the solar system and taking
photos of these objects.

We can also study the light spectra emitted from distant objects.

We know that white light passed through a prism consists of all the colours
of the rainbow (ROYGBIV).

A spectroscope is a device that allows light to enter through a fine slit
before passing through a prism. Spectroscopes produce colour spectra
with even finer detail.

We also know that when white light is passed through a gas (e.g.
hydrogen gas), the gas absorbs light from specific sections of the colour
spectrum leaving a series of black lines called spectral lines where this
light has been absorbed.

Every gas (e.g. H, He, Ne) has a characteristic pattern of black absorption
lines, and so, by analyzing the absorption spectra from a given star or
galaxy, we can tell what gases are present in the atmospheres of these
objects.
The Doppler Effect

Have you ever noticed the change in pitch in the sound of car as it passes
by you?

Why does this happen? The Doppler Effect, that’s how!

Sounds travel as waves. The distance between the crests of two waves is
called wavelength, while the number of crests that pass fixed point in a set
amount of time is called frequency. When wavelength is short, frequency
is high. When wavelength is long, the frequency is low.

Suppose there is a firetruck blaring its siren. When the firetruck is
stationary, the sound waves on all sides of the object have the same
frequency and sound the same to all observers (figure 5.25A).

As the firetruck approaches you, the sound waves from the siren get
compressed and pushed closer together (Figure 5.25B). In other words,
the frequency increases causing the siren to sound high-pitched (highfrequency sound waves have high pitch).

As the fire-truck drives away from you, the sound waves from the siren are
no longer compressed but are spread out. In other words, the frequency
has decreased causing the siren to sound low-pitched (low-frequency
sound waves have low pitch).

When sound waves are compressed or decompressed the effect is a
change in the pitch of the sound. When light waves are compressed or
decompressed the effect is a change in colour (blue-shift or red-shift).

What colour does light become if light-waves are compressed? What
colour does light become if light-waves are spread apart?

Compression causes light waves to get closer together or bluer (blueshift), while decompression causes light waves to spread out or get redder
(red-shift).

So, light from a star moving towards us appears bluish, while light from a
star moving away from us appears reddish.

Black absorption lines also get shifted when an object such as a star
moves towards or away from us. When a star is moving towards us, the
absorption lines get shifted to the blue end of the spectrum. When a star is
moving away from us, the absorption lines get shifted to the red end of the
spectrum.
Online Activities:
We will work through first few slides of the spectral analysis interactive activity on
http://www.learnalberta.ca/content/secs/html/space_exploration/SpectralAnalysis
1/index.html as a class and then you will continue to work through the questions
in pairs. Consider the following as you work through this activity:
1) What patterns are the easiest to interpret? Why?
2) What patterns are more difficult and why?
3) What would the spectral pattern look like for the Sun?
Another site to look at for practice with spectral analysis:
http://www.explorelearning.com/index.cfm?method=cExtAccessSecure.dspReso
urce&ResourceID=558
Video about the Doppler Effect: http://www.youtube.com/watch?v=Kg9F5pN5tlI
Using spectral analysis and the Doppler Effect to determine movement of stars:
http://www.learnalberta.ca/content/secs/html/space_exploration/UnderstandingTh
eMovementOfStars/index.html
1) How can you tell if a star is moving towards us?
2) How can you tell if a star is moving away from us?
Another site to look at for more information about the Doppler Effect:
http://www.teachersdomain.org/resource/phy03.sci.phys.energy.doppler/