Alexandre Costa
"EAAE Summerschools" Working Group
Escola Secundária de Albufeira, Centro Ciência Viva do Algarve (Portugal)
Abstract
The idea that the Sun was not the first star that was present in our region of the Universe seems at the beginning strange for every student. The way those elements were produced in Big Bang and later on by stellar nuclear reactions will be presented.
Stellar spectra analysis requires the use of spectroscopes that can easily be constructed by teachers in the classroom with their students. The workshop will consist of the construction of a spectroscope and of a device for the explanation of blackbody radiation for the comprehensive teaching of the spectra.
Some proposals for didactical activities
Unlike other scientific areas astronomy cannot have a scientific method based on traditional experimentation. What we call experimentation in astronomy is quite simply observation. We cannot interfere with the experiment, we cannot change the variables, we can only observe.
Yet, observation allows a lot of conclusions if we are using the right instruments. The rest of an astronomers work is joining pieces of evidence just like a detective would do: observation and logic.
Sometimes astronomers conclude that a star has this or that age, that where this star was there was another star before, merely based on logic.
Today we know that the only elements that were created in the Big Bang where hydrogen and helium (and a tiny amount of lithium). Studies about nuclear fusion in nuclear rectors have shown that under the conditions in stellar interior only elements that in the Periodic Table go up to iron can be produced. All other elements can only be produced by supernova [1]. If we now joint to this knowledge, some logical deduction, you obviously conclude that the Sun has not been the first star in this area of the Universe, but of course there has had to be another star that became a supernovae before the solar system's formation (Otherwise, how could there be gold and silver and uranium on earth?).
Astronomers also conclude about the aging of star from their metalicity, i.e., their atmospheric composition in metals, like sodium, calcium, potassium, etc. To analyse the metalicity of a star one only has light. Of course if we used light in the same way it usually gets to are eye stars would only have the colour due their thermal blackbody radiation, which tells you little about their composition, though it can tell you about the temperature of the star [2,3,4].
Blackbody radiation can even explain why the stars are not green. Well, as we pass from low temperature stars to high temperature stars colour goes from red, to orange, to yellow, to white (?), to blue.
In fact the colour of stars is the sum of all visible radiation it emits. A yellow star has its Wien's maximum in the greens but with big emission of red orange and yellow and only a small amount of blue; since red and green radiation produce yellow, which adds to the "real"(monochromatic) yellow wavelengths, then the star is yellow; the small amounts of all wavelengths that do not contribute to the yellow colours produce white with these wavelengths, that is why the stars are not intensely yellow.
When a star should be green, it becomes white because its maximum is in bluewish green wavelengths. In this case the sum of all wavelengths produce white and not green, and that is why green stars with their colour generated by blackbody radiation cannot exist.
This can be shown by showing the difference between light that is transmitted, reflected or emitted.
To understand the difference between different types of radiation students have to understand the difference between atomic spectra emitted by gases at high temperatures, molecular spectra (reflected by opaque body or transmitted by transparent bodies) and continuous blackbody radiation emitted by bodies that are in thermal equilibrium with their surroundings.
The activities in the workshop will consist of the building of a spectroscope to show the differences between these types of radiation and of projectors that allow simulations to comprehensively answer to the question "Why aren't stars green?".
The model
A. Spectroscope
The spectroscope will be build using a Starlab model for spectroscopes.
First one prepares the main tube were the lightpath will occur as presented in Figure 1. After one cuts a copy of the scale presented in Figure 2.
Then one prepares the eyepiece with a card tube a card with a hole and diffraction grating as presented in Figure 3.
Then we prepare the paper body of the spectroscope by cutting in paper and gluing over some card the piece presented in Fig. 4.
We then make the light entrance slit narrower with the use of dark tape, as presented in Figure 5.
Now we can assemble the spectroscope as presented in Figure 6.
The spectroscope is ready to use.
B. Light source models
We can use a simple filament lamp with a variable resistance to illustrate blackbody radiation. Interposing coloured filters between the lamp and the spectroscope students can see what is transmitted by the filters and by comparing with the lamp without filters can understand molecular absorption.
Students can then understand why there are no green blackbodies by playing with a system like schematically presented in Figure 7, that has a blue, a green and a red projector equipped with variable resistances.
Finnaly they can see a simple atomic lamp to see atomic emission and the sun's spectra to see an absorption spectrum.
References
Johnson,P.E., Canterna,R., 1987, Laboratory Experiments For Astronomy, Saunders College Publishing, N.Y.
Zeilik, M., Gregory, S.A., Smith, E.v.P., 1992, Introductory Astronomy and Astrophysics, 3rd Ed., Saunders College Publishing, Orlando, U.S.A.
Rybicki,G.B., Lightman, A.P.,1979, Radiative Processes in Astrophysics,John Wiley & Sons, USA.
Atkins, M., Erickson, J., Friedman, A.J., Gould, A.D., Seltzer, J.M., Planetarium Activities for Student Success, Vol. 8-Colors from Space, jointly published by Lawrence Hall of Science University of California and the New York Hall of Science.
Costa, A., 2001, Experiencias en Astrofísica - ¿Porqué no hay estrellas verdes?, 4ºs Encuentros de Enseñanza de Astronomia, 45-53.