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Is the object moving away from the observer?

Time: 2 x 45 mins.

Goal of this lesson is to determine the velocity of galaxies moving away from the observer and (indirectly) to determine the age of the Universe. This is to be achieved by utilizing CLEA Hubble Redshift Distance Relation computer program, which simulates the observation process conducted with large optical telescopes equipped with spectrometers.

Lesson plan:

1. Lesson starts with a short summary (or introduction to the concept) of Doppler’s effect. One can utilize graphic materials included in the presentation or use visual materials freely available on the Internet. It is strongly advised to introduce the concept of „wavelenght” and further use it instead of „wave frequency”, as spectral lines (of the emission spectrum) are commonly described in terms of wavelength.

2. Next we explore the question whether one can hear sound of motion - with emphasis on the sound of an object moving away from the observer - in the Outer space. We conclude that sound doesn’t propagate in the Outer space (note that without this inconsistency Star Wars movies would be rather boring to watch;)).

3. One asks the question: „Can one determine whether an object in the Outer space moves towards the observer or away from him?” and we present a slide depicting T-shirt with the phrase „if this shirt looks blue, you’re going too fast”. We conclude the discussion by stating that Doppler’s effect can also be observed in the case of electromagnetic waves (and light, as is the visible part of electromagnetic spectrum commonly known).

4. In this part ot the lesson one recalls the concept of „emission spectrum” and „absorption spectrum” along with the fact that for each chemical element these spectra are unique – each element has its one of a kind spectral „fingerprint”. One can indentify elements consisting a star by conducting spectal analysis of given star (one can recall the discovery of Helium as an example).

5. Next, one presents the slide titled „Stars and laboratory”. One should guide the discussion so the students arrive to the conclusion that despite the objects motion relative to Earth its spectral pattern doesn’t change, however it’s shifted towards the red part of electromagnetic spectrum when the object is moving away from Earth. This presents an occasion to introduce the concept of „redshift”.

6. Each student should be given a worksheet and instructed to play the role of astronomer for the duration of this activity. Students are to conduct spectral measurements of selected galaxies and determine velocity of their movement away from Earth.

7. <time alloted for students to complete the given assignment>

8. Compare and discuss each group’s results, actively support students as they attempt to formulate the Hubble’s Law and calculate the age of the Universe utilizing information presented during the class.