Student Classifications of Stars
Students should use the Get Spectra tool to find spectra for each of the 14 stars in the table. Many of the lines are labeled, but a few are not. If a student cannot read a label because the spectrum obscures the label, they can look at another star’s spectrum and know that the lines are in the same order on both spectra. Just a note: the four lines from the Balmer series are always labeled as Ha, Hb, Hg, and Hd.
Exercise 1 has no right or wrong answers. Hopefully, students will put some thought into their classification systems, and will be able to justify them to to others. You may have the students exchange their guidelines and see if someone else can reproduce their classifications. Students will not derive the same classification scheme as astronomers because they are working with such small samples. Question 3 tries to get students to revise and improve their ideas, as professional scientists do in their research.
Energy Levels of Electrons
You can choose how much you want to cover the energy levels. Many chemistry classes talk about energy levels, but don’t go over the mathematics, even of the simple hydrogen molecule. This information may be new to some students.
Absorption and Emission Lines
The wavelength values of the absorption lines in the table are approximate. From these values, students should be able to see the lines on the spectra. Exact numbers are not necessary, because the resolution of the .gif images prevents students from reading down below about the nearest 40 to 50 angstroms.
Questions 5 and 6 should be treated as practice. They are designed simply to show students the type of reasoning they will need to employ to properly classify stars as they go through the lesson. Be sure students realize that it is OK if they do not get the star’s spectral type correct the first time. Classifying spectra is not easy.
On all the spectra, the positions of the absorption lines are marked whether or not there is a line present. The table gives positions of some lines in addition to the ones marked automatically on the SDSS spectra. The titanium oxide lines are quite numerous, but occur in distinct bunches at the listed wavelengths.
Exercises 5, 6, and 7 give the students an opportunity to practice classifying stars by doing an informal survey of the prevalences of different spectral types. You might wish to assign each group of students a different plate to work from to avoid duplication of data. Or, you might assign two groups to each plate to provide a check on each other’s data. Either way, you should have students discuss whether they feel they truly did a random sample of the stars in the sky, or whether their sample is somehow biased toward a certain part of the sky.
Exercises 8, 9, and 10 show different types of stars that do not fall into the OBAFGKM sequence. Try to foster discussion of the fact that these are not the only types of stars, and there are several rare classes not represented in the OBAFGKM sequence.