• Simon Fraser University

• PHYS 102

Astronomy 1102/3311 Spectroscopy Part II - 1 Saved: 1/8/2026

Purpose: To study real stellar spectra and understand how astronomers define spectral classes of stars

Preliminary questions: Use the following HR diagram to answer the following two questions. Notice the correspondence between the spectral class of a star (OBAFGKM) and the surface temperature.

a) If two stars are the same size, which is

brighter, a red star or a blue star?

b) Estimate which color dominates the

visible light of the following stars:

• Sirius B (25000 K)

• Sun (5800K)

• Betelgeuse (3000K)

Question 1: Guessing the surface temperature. Here are 4 actual spectra from four different stars. Label the stars, Star A, Star B, Star C and Star D from the top down. Without calculations, list the stars from highest surface temperature to lowest surface temperature and explain why you chose that ordering.

Name: __________________________

Partner(s): _______________________

1102 or 3311: ______

Desk # ____________

Date: _____________

Spectroscopy Part II

Astronomy 1102/3311 Spectroscopy Part II - 2 Saved: 1/8/2026

Question 2: Calculating the surface temperature of a star. The wavelength of the peak of the blackbody curve λpeak, can be calculated using Wien’s Law. The peak wavelength is in nanometers and the surface temperature of the star is in Kelvin.

or

a) Fit by hand a blackbody curve to the spectra shown above of the stars A, B and C, like the

curve on star D. Star D has the curve already drawn for you. The shape of the curve for this

star was difficult to trace because of the deep absorption lines around 520 nm, probably due to

molecules in the star. The deep absorption around 380 - 400 nm is due to singly ionized

calcium.

b) Find the peak wavelength of each curve and calculate the corresponding temperatures.

Star A: Star C:

Star B: Star D:

Do your calculations agree with the predictions you made in Question 1?

c) How does the light that astronomers see from distant stars and galaxies tell them what they are

made of? Why are spectral lines often referred to as "atomic fingerprints"?

d) Remember what you did in the previous spectroscopy lab. Distinguish among emission

spectra, absorption spectra, and continuous spectra in how they look. (It’s alright to use the

word rainbow.)

Astronomy 1102/3311 Spectroscopy Part II - 3 Saved: 1/8/2026

e) (ASTR 1102). Distinguish among emission spectra and absorption spectra in how the spectra

are formed physically—that is, what is going on in the atom. You can draw a diagram. If you

do not know, have a look at Question 4.

Question 3: Identifying elements. Below are the spectra of three stars, Abba, Babba, and Cabba. They are made primarily of hydrogen and helium (just like all of the other stars in the sky). However, in this imaginary case, one star has a high percentage of carbon in its atmosphere, another has a high percentage of nitrogen, and the third has a high percentage of oxygen. Below these are 3 spectra of a number of different elements.

Star Abba

Star Babba

Star Cabba

Carbon (laboratory emission spectrum)

Nitrogen (laboratory emission spectrum)

Oxygen (laboratory emission spectrum)

Xenon (laboratory emission spectrum)

Hydrogen (laboratory emission spectrum)

Helium (laboratory emission spectrum)

Astronomy 1102/3311 Spectroscopy Part II - 4 Saved: 1/8/2026

a) Which star has a high abundance of nitrogen? Explain.

b) Which star has a high abundance of oxygen?

c) Which has a high abundance of carbon?

d) Do any of these three stars show evidence of having xenon in their atmospheres? Explain.

Question 4: The hydrogen atom. For many years, it was assumed that matter was made of small, indivisible, particles called atoms. Near the end of the nineteenth century, it was found that atoms were not indivisible, but contained both positive and negative particles. Nowadays, thanks to quantum mechanics, we do not describe electrons as particles any more. However, for the purpose of this lab, the particle model is sufficient.

We are mainly interested in the hydrogen atom. The symbol for hydrogen is H. Hydrogen is the simplest and lightest element in the universe. It consists of a proton and one electron in orbit around the proton. An electron can only occupy certain levels of energy.

The electron transitions that correspond to the first level (from n = 1 to n = 2, 3, 4, 5, 6 …) is the Lyman series.