Prep: 15 mins | Activity: 20-30 mins
Use this activity as a student-based or teacher-led phenomenon for a unit on stellar evolution. Using an inexpensive spectroscope with wavelength units, students determine the visible spectrum for white light, hydrogen, helium, and a hydrogen/helium mixture. Spectral analysis guides students in determining the spectral pattern of medium-sized stars like our sun.
Take a CD and rotate it around under a light. What do you notice?
White light should be broken up into the spectrum.
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What do you think is causing the spectrum to appear?
Students may say that the CD is acting like a prism or some type of lens. A CD is constructed of 3 layers—2 plastic and 1 aluminum. One plastic layer is burned, so there are bumps called pits and flat areas called land. A laser reads the pits and land as 1s and 0s, digitizing the medium. The burned plastic layer is coated with aluminum, making the shiny side we see. Another layer of plastic encases the aluminum. The shiny pits and land surface act similarly to a diffraction grating, which is a thin piece of plastic with many parallel slits. In both cases, the pits and slits diffract light, breaking it up into its component wavelengths.
How does the composition of a star like our sun change over time?
HS-ESS1-3. Communicate scientific ideas about the way stars, over their life cycle, produce elements.
Obtaining, Evaluating, and Communicating Information
ESS1.A: The Universe and Its Stars
Energy and Matter
Gather student spectroscopes and ensure that the diffraction grating is in place over the slit. Inspect the spectral tubes for any signs or cracks.
Hydrogen Spectrum
Helium Spectrum
Hydrogen and Helium Mix
Students may not record the exact same wavelengths for the colors they see. It’s important to notice the additional colors in helium and that the mixture of the two elements produces a combined spectrum.
Explain how the 3 spectra compare.
Hydrogen produces lines on the purple and blue end of the spectrum with one red band. Helium adds a yellow, green, and orange. The mix of elements produces a combined spectrum.
2. If a star in the same class as our sun produced a spectrum similar to helium, what you would conclude about its age?
You would expect the star to be in the old stage. Helium is the product of hydrogen fusion. The hydrogen fuel would have been used up producing the helium.
3. Explain what kind of spectrum you would expect our sun to produce in middle age.
You would expect both hydrogen and helium to appear in the spectrum. Some hydrogen would have already fused to make helium, but there would still be a supply of hydrogen. The spectrum would show both hydrogen and helium.
4. Using the spectra, illustrate the life cycle of our sun.
Young: hydrogen spectrum. Middle age: combined spectrum. Old:helium spectrum.
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