THE SPECTRUM OF A STAR

Transcription

1 THE SPECTRUM OF A STAR Overview: Stars can be classified by using the general shape and specific lines of their spectra. Objectives: The student will: perform a simple analysis of spectral lines to classify 15 spectral models into four classes; and compare three plots of unknown stars to the Jacoby-Hunter-Christian Atlas to find the closest classification match. Targeted Alaska Grade Level Expectations: Science [10] SA1.1 The student demonstrates and understanding of the processes of science by asking questions, measuring, classifying, making generalizations, analyzing data, developing models, inferring, and communicating. [10] SA1.2 The student demonstrates and understanding of the processes of science by reviewing pertinent literature, hypothesizing, making qualitative and quantitative observations, controlling experimental variables, analyzing data statistically (i.e., mean, median, mode), and using this information to draw conclusions, compare results to others, suggest further experimentation, and apply their conclusions to other problems. (L) [10] SD4.1 The student demonstrates an understanding of the theories regarding the origin and evolution of the universe by recognizing phenomena in the universe (i.e., black holes, nebula). Vocabulary: absorption spectrum the pattern of dark lines and colors made when light passes through an absorbing medium, such as a gas or liquid; the dark lines represent the colors that are absorbed; because each type of atom absorbs a unique range of colors, the absorption spectrum can be used to identify the composition of distant substances, such as the gaseous outer layers of stars continuum spectrum produced by white light and is composed of all wavelengths, or all colors, in the visible spectrum emission spectrum the radiation, such as light, given off by a substance whose atoms have been excited by heat or other radiation; the atoms of different elements give off radiation at specific frequencies as they return to their normal energy level; the radiation can then be passed through a prism, forming a pattern of colored bars (one bar for each frequency); by analyzing these bars, scientists can determine of what kinds of atoms the substance is made spectra (plural of spectrum) values that vary over a continuum spectrograph a spectroscope able to photograph or otherwise record spectra; the image produced is called a spectrogram spectroscope an instrument designed to separate light waves into a spectrum; light enters a narrow slit, is focused into a thin beam of parallel rays by a lens, and passes through a prism that separates it into a spectrum spectrum a range of possible wave frequencies; a distribution of the frequencies and intensities of a group of waves, usually arranged by frequency wavelength the distance between the peak or crest of a wave and the next peak or crest Whole Picture: Most of the elements with an atomic number greater than four occur naturally on Earth, but it is theorized that some elements were created by primary synthesis during the early formation of the universe and in the later

2 THE SPECTRUM OF A STAR formation of stars. By examining stars, and the light emitted, scientists can use spectral analysis to determine elements that are present and even determine the origins of the universe. There are three main types of electromagnetic spectra. A continuum spectrum is produced by white light and is composed of all wavelengths, or all colors, in the visible spectrum. An emission line spectrum is produced when the electrons in an element are excited and produce photons. The specific wavelength shows up as a colored line that acts to identify the element. An absorption spectrum is the pattern of dark lines and colors made when light passed through an absorbing medium, such as the gases in the outer layer of stars. The dark lines represent the colors that are absorbed. Because each type of atom absorbs a unique range of colors, the absorption spectrum can be used to identify the composition of the outer layers of a star. It can also be used to determine the temperature of the outer layers. Using a spectrograph, scientists can literally take an image of the composition of a star and get a fingerprint for that particular star, called a spectrum. Scientists have cataloged different spectra and formed classes of stars stars with similar characteristics and make up based on these spectra. These are called star atlases. Like the bar code activity, the atlases help scientists study existing stars and classify new ones identified in outer space. Stars are classified using letter names. In order from the hottest to the coolest, the range is this: O, B, A, F, G, K, M, L, to T. The letters are out of order because early classifications were based solely on the appearance of various absorption lines. Later scientists discovered a correlation between temperature and the classes were rearranged. If needed, for further review, a tutorial on interpreting the spectra of stars is at: phys230/lectures/spec_interp/spec_interp.html Materials: STUDENT WORKSHEET: Bar Code Stellar Spectra VISUAL AID: Elements in the Stars STUDENT WORKSHEET: Indentifying Stars Activity Preparation: 1. Ask students if they are familiar with how bar codes on packaging work. Bar codes contain information based on the location and width of their lines. Scientists have learned to view stars in a similar manner. Each star has a unique fingerprint based on what elements are present. This fingerprint can be viewed using instrumentation designed to separate the light emitted from the star into different spectrums. This instrument is called a spectrograph. Scientists learn a lot about the make-up of stars in this way. 2. Using a multimedia projector review websites with the class that show spectral analysis. (See Activity Preparation) 3. Hand out STUDENT WORKSHEET: Bar Code Stellar Spectra and review the background information. Allow students to complete the proposed classification section. 4. Show VISUAL AID: Elements in the Stars. By examining the wavelengths of light emitted from the stars, astronomers can compare the wavelengths with the wavelengths of known elements and determine the make-up of the star. Stars come in all different sizes and compositions. The chart on page two of the visual aid shows the wavelength of elements commonly found in stars. 5. Explain many astronomers have cataloged different spectra by plotting the wavelengths and formed classes of stars stars with similar composition and temperature based on these spectra. These are called star atlases. Like the bar code activity, the atlases help scientists study existing stars and classify new ones identified in outer space. One of the Atlases is called the Jacoby-Hunter-Christian Atlas. It contains the spectra of 161 stars within a certain range. 6. Divide students into pairs. Hand out STUDENT WORKSHEET: Indentifying Stars. Explain students will work together to decide the closest match for unknown stars AA, BB and CC by comparing them to examples from the Jacoby-Hunter-Christian Atlas. Once pairs have finished, ask them to consult with another pair to compare findings, then discuss student reasoning behind classification decisions as a class.

3 THE SPECTRUM OF A STAR Extension Ideas: 1. For a more advanced exercise, ask students to visit the University of Manchester Jodrell Bank Observatory site. This site allows students to work through data selection to plot observed spectra of stars. Visit the site at: 2. If available, have students use Graphic Analysis software to examine spectral line wavelengths. 3. Ask students to write a mock article for an astronomy club newsletter explaining how to classify stellar spectra using both general spectral shape and specific spectral lines. Answers: STUDENT WORKSHEET: Bar Code Stellar Spectra Star 1 Proposed Classification (A, B, C or D) Star 2 Star 3 Star 4 Star 7 A D B Star 10 D Star 13 B Star 5 Star 8 B C A Star 11 D Star 14 C Star 6 Star 9 A B C Star 12 A Star 15 D STUDENT WORKSHEET: Identifying Stars 1. M Series 2. F Series (may also accept A series) 3. B Series

4 NAME: BAR CODE STELLAR SPECTRA Background Information: You are probably familiar with the bar codes on sale merchandise in stores. The codes contain important information unique to each item. Scanners use these codes to quickly identify and price the items sought for purchase. In some way these bar codes are similar to the spectra of stars. The light emitted by stars can be broken into different spectra. Astronomers use sensitive instrumentation called a spectroscope to separate the light. Stellar spectra look similar to a rainbow except many dark lines also appear. Those dark lines represent different elements in the atmosphere of the star. The atoms of the element absorb the light at that wavelength and produce a dark line. Each element has a specific signature. All stars of a given spectral class have similar spectral shapes and lines. There are some spectra that have not yet been identified. Vocabulary spectra (plural of spectrum) values that vary over a continuum spectrum a set of values within a continuum spectroscope an instrument designed to separate light waves into a spectrum The visible light spectrum can be seen when light is separated using a prism. A rainbow is formed when light passes through water droplets in the atmosphere. Directions: Use the Key: Bar code stellar classes A-D to classify the bar code spectra of stars A correct classification will have all the key lines and thicknesses. There may also be other lines present that cannot be identified. Similarly, in real stellar spectra some lines cannot yet be identified. Record your proposed classification (A, B, C or D) in the table. Hint: Cut out the key for easier comparison of the codes. Key: Bar code stellar classes A-D: Unknown star spectra bar codes 1 15! Proposed Classification (A, B, C or D) Star 1 Star 2 Star 3 Star 4 Star 5 Star 6 Star 7 Star 8 Star 9 Star 10 Star 11 Star 12 Star 13 Star 14 Star 15!

5 ELEMENTS IN THE STARS Astronomers place stars into spectral classes based on the light emitted. By viewing the spectra, and corresponding wavelengths, the elements that make up the star can be identified. The wavelengths of all known elements have been cataloged. Unknown Stars AA and BB show the presence of different elements. Unknown Star AA Unknown Star BB

6 ELEMENTS IN THE STARS Unknown Star CC shows yet a different set of wavelength fingerprints. The chart below lists the wavelength of elements in the range shown in Unknown Stars AA, BB and CC. Hydrogen Balmer a 6563 Metals Ca II 3933 b 4861 Metals Ca II 3968 c 4340 Hydrogen Balmer e 3970 d 4101 Helium He e 3970 Metals Mn I 4032 Metals Fe I 4045 Helium He Metals N IV 4058 Helium He Metals Sr II 4077 Helium He Metals Si IV 4089 Helium He Metals N III 4097 Helium He Hydrogen Balmer d 4101 Helium He Metals Fe II 4175 Helium He Molecular Bands CN 4215 Metals Sr II 4215 Metals Ca I 4226 Molecular Bands CH "G band" 4300 Metals Fe II 4233 Molecular Bands CN 4215 Metals Sc II 4246 Molecular Bands C Metals C II 4267 Molecular Bands TiO 4584 Molecular Bands CH "G band" 4300 Molecular Bands TiO 4625 Metals Ti II 4300 Molecular Bands TiO 4670 Metals Fe I 4325 Molecular Bands TiO 4760 Helium He Molecular Bands MgH 4780 Hydrogen Balmer c 4340 Helium He Metals C II 4267 Metals Ti II 4444 Metals C III 4649 Helium He Metals C III 5696 Metals Mg II 4481 Metals C IV 4658 Helium He Metals C IV 5805 Metals Si III 4552 Metals N III 4097 Molecular Bands TiO 4584 Metals N III 4634 Metals N V 4605 Metals N IV 4058 Molecular Bands TiO 4625 Metals N IV 7100 Metals N III 4634 Metals N V 4605 Metals C III 4649 Metals O V 5592 Metals C IV 4658 Metals Na I 5890 Molecular Bands TiO 4670 Metals Mg II 4481 Helium He Metals Si III 4552 Molecular Bands C Metals Si IV 4089 Molecular Bands TiO 4760 Metals Ca I 4226 Molecular Bands MgH 4780 Metals Ca II 3933 Hydrogen Balmer b 4861 Metals Ca II 3968 Metals O V 5592 Metals Sc II 4246 Metals C III 5696 Metals Ti II 4300 Metals C IV 5805 Metals Ti II 4444 Metals Na I 5890 Metals Mn I 4032 Hydrogen Balmer a 6563 Metals Fe I 4045 Helium He Metals Fe I 4325 Metals N IV 7100 Metals Fe II 4175 Metals Fe II 4233 Metals Sr II 4077 Metals Sr II 4215

7 NAME: IDENTIFYING STARS Directions: Compare Unknown Stars AA, BB and CC to the Jacoby-Hunter-Christian Atlas to find the closest match. Pay close attention to scale. Record your findings. Unknown Star AA 1. Unknown Star AA most resembles from the Jacoby-Hunter-Christian Atlas: A. O Series B. B Series C. A Series D. F Series E. M Series Unknown Star BB 2. Unknown Star BB most resembles from the Jacoby-Hunter-Christian Atlas: A. O Series B. B Series C. A Series D. F Series E. M Series Unknown Star CC 3. Unknown Star CC most resembles from the Jacoby-Hunter-Christian Atlas: A. O Series B. B Series C. A Series D. F Series E. M Series

8 NAME: IDENTIFYING STARS O Series B Series

9 NAME: IDENTIFYING STARS A Series F Series

10 NAME: IDENTIFYING STARS M Series