STELLAR FINGERPRINTS: THE SPECTRA OF STARS
Time: Two class periods
to National Standards
Before beginning this activity, teachers should be aware of the misconceptions that their students might harbor concerning light and color. For some examples of common student misconceptions on this subject and suggestions for probing them, see the Amazing Space Web site. http://amazing-space.stsci.edu/resources/myths/light.php.
The following paragraph outlines the prerequisite for this lesson. First, students should be aware that white light is composed of colors and these colors correspond to certain wavelengths, frequencies and energies. They should be familiar with the quantum theory of matter (see list of terms for definition). Specifically, they should know that each kind of atom or molecule can gain or lose energy only in discrete amounts, and thus can absorb and emit light only at wavelengths corresponding to those amounts. For example, hydrogen atoms will absorb and emit visible light when their electron jumps from or to the second energy level. In addition, students should have an understanding of intensity as the amount, degree, or quantity of energy passing through a point per unit time. For more information on spectral lines and quantum theory, see the following Web site. http://www.colorado.edu/physics/PhysicsInitiative/Physics2000/quantumzone/
The lesson is broken into four activities. Each activity builds on the information developed in the previous section. Each activity however, can stand alone. Depending on your purpose, you may choose to address any or all of the objectives from simply explaining how an element can be identified using emission spectra to explaining how the spectrum of a star can reveal the composition of the star. There are four activity sheets, one for each activity, which can be used as the students engage in the activities. Teachers are encouraged to review the lesson materials to identify and define terms that may be unfamiliar to students.
Introduction to Stellar Spectra:
1. To introduce the stellar spectra activity, show students the stellar spectrum of Vega using Activity Sheet 1: "Introduction to Stellar Spectra" and/or Overhead A: "Stellar Spectrum of Vega." Provide motivation by explaining that spectroscopy is a tool used by astronomers. You may need to define the term spectroscopy and any other terms that may be unfamiliar to your students. Consider having students read the sections titled It's All About Light and The Hubble Space Telescope found in the introduction to this lesson.
2. Explain that the visible light coming from the star is broken into its components and the intensity of light is recorded as a function of the wavelength.
3. Ask students to examine the spectrum and formulate three questions.
4. Collect these questions and rank them according to how frequently the questions were asked, or ask students to rank them according to interest. Many of the questions may focus on the significance of the "dips" in the graph. These questions can be used as a guide to student learning. You have the option of allowing students to explore and research the answers to their questions at this point, or proceed with the activities as written and address their questions again at the end of these activities. If their questions are still unanswered at that point, individual research can be assigned.
Identifying Emission Spectra:
1. Now that you've introduced the concept of stellar spectra, develop the idea of the emission spectra produced by hydrogen and other elements as they relate to quantum theory. Prepare for the emission spectra activity by placing the hydrogen discharge tube into the power supply and positioning it so students can use diffraction gratings to create a hydrogen emission spectrum. Provide motivation by explaining that astronomers need to know how to identify elements using the emission of light. You may need to define the term emission and any other terms that may be unfamiliar to your students. Consider having students read the sections titled It's All About Light, Continuous Spectra, Fraunhofer Lines, Elemental Fingerprints, Quantum Theory Revisited, and The Hubble Space Telescope found in the introduction to this lesson. Note: If you don't have access to the equipment needed for this section of the activity, the following Web site allows you to choose an element and view its emission spectrum. http://webmineral.com/help/FlameTest.shtml
2. Pass out diffraction gratings and instruct students to hold them so they get a rainbow of colors when they look through the grating to one side of the classroom lights. (If the rainbow is above and below the diffraction grating, the students need to rotate the grating 90 degrees.)
3. Dim the classroom lights and turn on the power supply containing the hydrogen discharge tube. Safety caution: Gas discharge tubes use high voltage power supplies. Caution students that they should not touch the power supply.
4. Have students view this light through their diffraction gratings. Instruct students that they must look through the grating off to the side of the light to see the bright bands of color.
5. Have students use colored pencils to sketch the pattern they see on activity sheet 2: "Comparing Emission Spectra." Label the spectrum they draw as hydrogen. Ask students to think about how the bands of light are produced. You can help them draw the right conclusions by reminding them to think about quantum theory. Then discuss how the bands of color relate to the emission of energy as the electron drops to a lower energy level.
6. Use other discharge tubes containing different elements to compare the patterns produced by these elements with that of hydrogen. To do this, insert another gas discharge tube (containing a different element) in the power supply, identify the element they are viewing, and allow students to view this element's spectrum through their diffraction gratings. Again, have students use colored pencils to sketch the patterns they see and to label their spectra with the elements contained in the tubes.
7. After students sketch two or three elements other than hydrogen, place one of the previously viewed gas discharge tubes in the power supply and ask students to identify the element. Ask them to explain how they identified the unknown element.
8. Ask students to think about why astronomers must know how to identify elements in this manner, Tell them to write their explanations on their activity sheets. Be sure to discuss their ideas so that students begin to see the "real-world" application.
Comparing Emission and Absorption Spectra
1. Now that students understand how emission spectra are produced and that each element has a unique pattern, use Activity Sheet 3: "Comparing Emission and Absorption Spectra" and/or Overhead B: "The Emission and Absorption Spectra for Hydrogen" to show students the emission and absorption spectra for hydrogen. Provide motivation by explaining that astronomers also need to know how to identify elements using the absorption of light. You may need to define the term absorption and any other terms that may be unfamiliar to your students. Consider having students read the sections titled It's All About Light, Continuous Spectra, Fraunhofer Lines, Elemental Fingerprints, Absorption Spectra, Quantum Theory Revisited, and The Hubble Space Telescope found in the introduction to this lesson.
2. Once students have studied the two spectra, ask them to predict the absorption pattern produced by element X by drawing the absorption lines in the area provided on Activity Sheet 3: "Comparing Emission and Absorption Spectra." In this case, element X is lithium. You may want to encourage students to identify the element either through direct observation of a lithium discharge tube or through the Internet using the emission spectra available at http://webmineral.com/help/FlameTest.shtml.
3. Ask students to use the wavelength scale provided to identify the wavelengths that correspond to the emission bands for hydrogen. Ask them to describe the relationship between the wavelength and color.
4. Ask students to explain how the absorption spectrum was produced. Include a discussion that focuses on the idea that the exact same wavelength of light is absorbed when an electron is excited as when it drops back to its original energy level. Explain that an absorption spectrum is produced when light from an incandescent body, such as a star, passes through a cloud of cooler gas, such as the star's atmosphere. The electrons in the cloud of cooler gas absorb energy when they jump to excited states. The quantity of energy absorbed when jumping to the higher energy level is the same as the quantity of energy emitted when electrons drop to the lower energy state. This absorbed and emitted energy corresponds to a particular wavelength of light. When absorption occurs, these wavelengths of light are removed from the continuous spectrum, leaving a dark area at each position.
5. Again, this section ends with students explaining why knowing the wavelengths at which hydrogen emits and absorbs light energy would help astronomers. Discuss students' thoughts on this topic.
Interpreting Stellar Spectra
1. Now that students know the relationship between absorption and emission spectra, and that each element has a unique pattern of absorption and emission, ask students to look at Overhead Sheet C: "Comparison of Spectra." Discuss how the two spectra are similar and different. Astronomers use as much information as possible to interpret stellar spectra. Therefore, they use spectra similar to the second one that graphs the intensity vs. wavelength. From these graphs they can determine not only the wavelengths but also how much energy was absorbed. Provide motivation by explaining that astronomers use spectroscopy to identify the composition of celestial objects that emit light. You may need to define the term spectroscopy and any other terms that may be unfamiliar to your students. Consider having students read the sections titled It's All About Light, Continuous Spectra, Fraunhofer Lines, Elemental Fingerprints, Absorption Spectra, Quantum Theory Revisited, Types of Stellar Spectra, and The Hubble Space Telescope found in the introduction to this lesson.
2. Once students are familiar with the spectra used by astronomers, ask them to look at the stellar spectrum pictured on Activity Sheet 4: "Interpreting Stellar Spectra" and have them list the wavelengths where absorption occurs for this star. You may need to help students realize that when some of the energy is absorbed, not as much reaches the detector and there is a dip in the spectrum. Is there any significance to the wavelengths they record? Students should realize that most of the wavelengths they have just recorded are the same ones they recorded for the emission/absorption bands for hydrogen.
3. Return to the stellar spectrum of Vega using either Overhead Sheet A: "Stellar Spectrum of Vega" or Activity Sheet 1: "Introduction to Stellar Spectra." This is an opportunity for students to study another spectrum that shows the absorption of hydrogen. Be sure students can link the points where energy is absorbed on the graph to the wavelengths attributed to hydrogen absorption.
4. Now have students return to Activity Sheet 4: "Interpreting Stellar Spectra," and study the spectrum of star BD +75 325. Then ask students to answer the questions about this spectrum. Discuss the idea that there are more absorption lines for this star than for the other ones. Have students explain why this is significant, and discuss how they might decide the identity of the other element(s) responsible for the absorptions.
5. Again, return to the "practical application" idea and discuss why astronomers can use spectroscopy to reveal the identity of the elements that are present in celestial objects.
6. If students wrote questions on Activity Sheet 1: "Introduction to Stellar Spectra," take time to review their questions and address any ones that have not been answered.
Teachers might choose to:
1. Have students investigate the function of filters and colors as they apply to the Hubble Space Telescope using "Behind the Pictures" http://hubble.stsci.edu/sci.d.tech/behind_the_pictures/ and/or the Sky & Telescope article "Creating Hubble's Technicolor Universe." http://hubble.stsci.edu/newscenter/news_media_resources/reference_center/about_hubble/skytel200209028034.pdf
2. Investigate other properties of stars and light using the Amazing Space Online Exploration: "Star Light Star Bright." http://amazing-space.stsci.edu/resources/explorations/. In this activity, students will identify the different properties of waves and the relationship that exists between energy, wavelength, and frequency. Students will correlate images taken by the Hubble Space Telescope and other astronomical instruments to the wavelength, color, and temperature information that can be found in the spectrum.
3. Find out more about Hubble's impact and science discoveries using the News Center's Reference Center section "About Hubble:" http://hubble.stsci.edu/newscenter/news_media_resources/reference_center/about_hubble/
4. Look at additional stellar spectra and determine the elements present based on the absorption spectra for given elements. A good activity is available from The Annenberg/CPB Math and Science Project: The Science of Light, Stellar Spectra. http://www.learner.org/teacherslab/science/light/color/spectra/index.html
Do a flame-test lab to show the characteristic color of elements and how this
can be used to identify the element. Chemistry and physics lab books as well as
the Internet can serve as a source for specifics on how to do the lab. One suggested
procedure can be found at:
for Science Literacy
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