Evolution of a Planetary System

Transcription

1 Sample Lesson for Grades 5-6 from Evolution of a Planetary System The Life in the Universe Series was created by children, teachers, and scientists at the SETI Institute for grades 3-9, with funding from the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA). Edna DeVore Director Education and Public Outreach Pamela Harman Manager Education and Outreach SETI Institute 515 North Whisman Road Mountain View, CA phone fax ed_dept@seti.org

2 Life in the Universe Series - Book 2 2 Mission 4 Investigating Types of Stars Is Our Sun Unique? Overview In Mission 3, the students did an experiment in which they simulated the formation of our Solar System. In Mission 4, they will learn about properties of different types of stars. Is the bright, yellow Sun the only kind of star that might have planets orbiting it? How would planetary systems be different for other colors or types of stars? In this activity, your students will experiment to find out that a star's color is determined by its temperature. White stars are hotter than yellow stars, and red stars are the coolest. White stars burn up the fastest, yellow stars have a longer lifetime, and red stars live the longest. If two stars are the same size, the hotter one will radiate more energy than the cooler one. Concepts There are many different colors, sizes, and temperatures of stars. The largest, hottest stars are white (and blue; still other large stars are cool and red.) Medium sized stars are cooler, and are yellow or orange. The smallest, coolest stars are red. All stars are hot. Star types are catalogued by the letters O B A F G K M which can be remembered by the sentence, "Oh Be A Fine Girl (or Guy) Kiss Me!" These star types span the range of star temperatures. Spectral type is determined strictly by temperature: O-type stars are the hottest stars and M-type stars are the coolest. (We study types A, G, and M today.) A star "burns" by converting hydrogen to helium at its center. The larger, hotter stars, such as O, B, and A-type stars, use up their hydrogen fuel and burn out faster. Smaller, cooler stars burn slower and live longer. A- type stars probably do not live long enough for life to evolve on any nearby planet as it has on the Earth.

3 Life in the Universe Series - Book 2 3 Skills Measure with a radiometer. Time an experiment. Take averages. Compare models and simulations to real objects. What You Need PART ONE It is ideal if a Star Center station can be set up for every group of 5-6 students. However, if your materials budget allows you to set up only a single station, arrange other projects to engage the students while groups wait their turn. For Each Station: 1 Clear 200 watt light bulb 1 Ceramic light bulb socket 1 Rotary-dialed dimmer switch (that can be installed into lamp wire) Electrical tape 3 to 6 Feet of lamp wire 1 Electrical Plug 1 Wire cutters 1 Wire strippers 1 Blade screwdriver 1 Phillips screwdriver 1 Fine point permanent marker 1 30 cm Metric ruler or Meter stick 1 "New" Radiometer older radiometers lose their vacuum seal) 1 Stopwatch Optional: Clear plastic box to protect radiometer For the Class: Butcher paper Marking pens Optional: "Life Story of the Earth" Image Transparencies Optional Overhead projector Optional: Calculator For Each Student: Student Logbook and pencil

4 Life in the Universe Series - Book 2 4 Getting Ready PART ONE One or More Days Before Class: 1. Build your Star Center(s). Refer to Figure 4.1, Star Center Assembly. First consider where you will want to locate your Star Center(s). Use enough lamp wire to span the distance from the Star Center to a wall socket easily. Attach the ceramic light bulb socket to one end of the lamp wire, following the directions on the box. Then use electrical tape to cover any exposed wire and/or metal on the bottom of the socket. Splice in the dimmer switch about a foot from the socket, using the directions on the package. Attach the plug at the end of the lamp wire. Use electrical tape to cover any exposed wire and/or metal. Figure 4.1. Star Center Assembly. light bulb socket dimmer plug 2. Calibrate your dimmer switch. Screw your clear 200 watt bulb into the socket, and plug in the Star Center. Set out the 30 cm ruler and place your radiometer at the end of it as shown in Fig. 4.2, Star Center Set-Up (or mark 30 cm on a meter stick, and place the radiometer at 30 cm away from the bulb). Figure 4.2. Star Center Set-Up. dimmer plug bulb and socket 30 cm ruler radiometer STAR CENTER DIRECTIONS

5 Life in the Universe Series - Book 2 5 Use the permanent marker to draw an arrow on the body of the dimmer, as shown in Figure 4.3, Calibrating the Dimmer Switch. The arrow will show the students where to set the dial for each of their measurements (A, G, and M). Figure 4.3. Calibrating the Dimmer Switch. Calibration for the A type Star: Turn the switch up to its highest setting. Mark the dial with a small line that lines up with the arrow, and write "A" by the mark. Calibration for the M type Star: While watching the bulb, slowly spin the dimmer down to a point where the filament glows orange/red. When it is at a very low setting it will flicker. Turn it up until the flicker is less noticeable. Test your radiometer to see if it's vanes turn at this setting. If not, turn the dimmer up until the radiometer does turn, or replace your radiometer (it may be too old). Make a mark for the M star at the lowest setting that will turn the radiometer's vanes. Calibration for the G type Star: Turn the dimmer all the way up to its highest point again, and then turn it down until the bulb glows yellow. This should be about midway between your A and M marks. Make a mark for the G star. Teacher's Note: Starting from the lowest setting and turning towards the highest, gives very different readings. Encourage your students to use a consistent method to collect this data, i.e., starting from highest to lowest. 4. Optional: Radiometers are made from very thin glass, and therefore are breakable. Some teachers have found it useful to glue their radiometer into a small clear plastic box. Just Before the Lesson: 1. Set up one or more Star Centers as shown in Fig. 4.2, Star Center Set-Up. Put the Star Centers in places that do not have another source of heat, such as a sunny window, an incandescent light, or a heater. 2. Before each use of a completed Star Center, try your radiometer(s) with each dimmer setting to be sure it will still turn at the 30 cm distance. If you have an older radiometer, it may tend to turn more slowly because air has leaked in. If necessary, you can shorten the distance at which students make their measurements, as long as the measurements for all bulbs are made at the same distance. 3. Draw a chart like Table 4.1 on the butcher paper or blackboard for students to record their data. Hang the chart at the front of the room. Table 4.1. Radiometer Turns in 10 Seconds.

6 Life in the Universe Series - Book 2 6 Star Types A (White) G (Yellow) M (Red) Group 1 Group 2 Group 3 Group 4 Group 5 Average Classroom Action PART ONE 1. Review. Remind the students that in the last Mission, they learned how planets may form around a star. But are all stars the same? Now we will study how stars can vary in their color and temperature. Star types were first invented to identify stars with different colors. For example, our Sun is a G type star. What color is it? Yellow. Star types are catalogued by the letters O B A F G K M which can be remembered by the sentence, "Oh Be A Fine Girl (or Guy), Kiss Me!" These star types span the range of star temperatures. Spectral type is determined strictly by temperature: O-type stars are the hottest stars and N-type stars are the coolest. We will study three types of stars: white (A), yellow (G), and red (M). 2. Mission Briefing. Have the students refer to the Mission Briefing in their Student Logbook, as one student reads it aloud. In order to decide which types of stars to search first for planets that could have life, we would like you to conduct some experiments to determine some of the ways that stars differ. 3. "What Do You Think?" Activity/Discussion. Let the students write their opinions on the pre-activity questions ("What Do You Think?"). Then invite them to share their answers with the class in a discussion. 4. Demonstration. Show the students the Star Center you set up at the front of the classroom. Explain that a single bulb can represent three different star types by using a dimmer switch at three different settings. The three bulb settings represent stars that are white (A), yellow (G), and red (M). Tell students that the light bulb they see in the Star Center represents a star a burning ball of gases very far away. We are going to observe how the color changes as the temperature of the bulb is changed. Turn the lights off in the classroom and turn the bulb on to its highest, brightest setting. Ask the students what color the bulb is. White. Begin dimming. The color will become noticeably more yellow. Continue slowly dimming the bulb, stopping each time there is a change in color, to ask students what color they see. At the end, ask the students: "What colors did you see?" White, yellow, red and colors in-between. Point out that white, yellow and red are three types of stars: A, G and M respectively. Their Student Logbook has a chart that gives facts about each of the basic types of stars. You may want to refer to this chart.

7 Life in the Universe Series - Book 2 7 Tell the students that their job is to measure the heat (infrared radiation) produced by the model A, G and M type stars. A radiometer is used to measure heat (infrared radiation). The faster the radiometer spins, the more heat (infrared radiation) is being radiated by the bulb. (For an explanation of how a radiometer works, see the Appendix.) Go over the directions in the Student Logbook, demonstrating each step. Emphasize that they cannot begin counting until the radiometer reaches full speed, which takes about thirty seconds. It is important not to bump the radiometer, since it is very fragile. In order to stop the radiometer vanes between measurements, tilt the radiometer slightly for a few seconds and then gently let it down again. Mention safety considerations: No more than one group at the Star Center at one time. Do not touch the bulbs. Handle the radiometers very carefully. Do not remove the radiometers from the Star Center. 5. Activity. Give the students time to complete their observations and datataking. Provide other activities for early finishers. When groups finish, they should record their data on the class data chart that you have drawn on the butcher paper or on the black board. Closure 1. Data. Have a student or teams of students use a calculator to average the results from the various groups. Ask the students for the conclusions that they can draw from this experiment. Go over the questions on the data sheet. The students will find that the A-type star is hottest (gives off the most infrared radiation) and the M-type star is coolest (gives off the least infrared radiation). 2. Lecture. You may also wish to emphasize that all stars are really hot. Even the coolest M star, at 3,300 C is at least 13 times hotter than the hottest kitchen ovens can get. Also, the temperatures listed are surface temperatures of stars. Temperatures at the center or cores of stars are much hotter. 3. Discussion. Ask the students look at Question 3 in the "What Do You Think Now" section of their Student Logbook. Invite discussion. Some students may realize that if our Sun were an A-type star and the Earth were the same distance from it, it would be too hot to live on Earth. Others may point out that the Earth could orbit at a comfortable distance from an A-type star, much further than its current position, but that sunlight would be white, not yellow. 4. Optional Transparencies. Using transparencies, quickly review the Video Image show with emphasis on the Timeline will provide the students with the information that it took over three billion years for complex life to appear on Earth. Use information about the lifetimes of stars of different types from Table 4.3, "Types of Stars Information Table". Direct the students' attention to the Timeline that they created during their last Mission. Ask the students: "How long did it take for complex life to evolve on the Earth?" Over three billion years! Have the students notice the "lifetime" column in table 4.3, and then ask them what would happen if the Earth orbited an A-type star. Complex life would probably have never evolved on Earth, as an A -type star would "burn out" long

8 Life in the Universe Series - Book 2 8 before complex life could get started. Since an A-type star only lives 100 million years complex life could never have evolved on our planet! 5. "What Do You Think Now?" Have students answer the post activity questions on the Logbook sheet ("What Do You Think Now?") After students have completed their answers, invite students to share their responses. 6. Preview. Tell the students that in the next Mission, we will finding out how a life-bearing planet orbiting other star types, such as A-type and M-type stars, could still be at a comfortable temperature. (Keep the completed "Radiometer Turns in 10 Seconds" Table 4.1 for the next Mission.) Going Further DEMONSTRATION: LIFETIMES OF STARS Set up three Star Centers where they can be seen clearly from all over the room and where it won't be bumped or handled by students. Announce that you are going to demonstrate the comparative lifetimes of the three stars. Turn on the dimmer setting on the first to A-type star, the second to G, the third to M and set a timer for 5 minutes. Explain that the time scale you are using is 5 minutes = 100 million years (0.1 billion years). At this scale the White A-type star would live for 5 minutes. A Yellow G-type star for 8 hours. And a Red M-type star for almost a week (80 hours). Mark your calendar or have a student in charge of turning off the 'stars' at the appropriate times. (If you have only one star center, do the lifetimes in series.) ACTIVITY: MORE RADIOMETER STUFF! Have the students use the radiometer to measure infrared radiation from various things: our sun, other lights, fluorescent lights, heaters, and so forth. What conclusions can they draw? ACTIVITY: COLORFUL STARS AT NIGHT Do some observations of the night sky; try to find stars that are red, white, yellow, or blue and classify them by star type. Use a star chart to get the star's name and look it up in a stellar atlas. ACTIVITY: ASTROPHOTOGRAPHY Take a long exposure (2 seconds) of a familiar constellation from a very dark area. If you move the camera while you take the picture you can streak the stars which will show you what colors they are. From your developed picture determine what types of stars you were looking at.

9 Life in the Universe Series - Book 2 9 Name Date Mission 4 Investigating Types of Stars Dr. Laurance Doyle Stellar Astronomer on the SETI Academy Team. Mission Briefing. In order to decide which types of stars to search first for planets that could have life, we would like you to conduct some experiments to determine some of the ways that stars differ. What do you think? 1. From Earth, stars appear to be different colors: blue, white, yellow, orange, and red. What might cause one star to be a different color than another star? 2. What other ways might stars be different? 3. The Earth orbits a yellow star. What things would be different on Earth and in our Solar System if our star was red?

10 Life in the Universe Series - Book 2 10 Name Date Mission 4: Investigating Types of Stars Star Center Instruction Sheet 1. Turn the dimmer switch all the way off. Place one end of the ruler or measuring stick at the base of the socket. Place the radiometer at the other end. lightbulb ruler or 30 cm measuring stick 2. Stop the radiometer's vanes. Turn the dimmer switch to the A star mark. Wait 30 seconds. Looking from the top follow one blade around with your finger. Have your partner time 10 seconds, saying "Start" and "Stop." Count how many times the vanes go around during the ten seconds. Record the number of turns in 10 seconds below: A Star: turns in 10 seconds Color G Star: turns in 10 seconds Color M Star: turns in 10 seconds Color 3. Turn the dimmer knob down to the G star mark. Stop the radiometer's vanes. Set it back down at the end of the ruler. Wait 30 seconds. Time, count and record the turns of the radiometer vanes just like you did for the A star. 4. Now take the same measurements for the M star. Don't forget to stop the radiometer fully, then wait 30 seconds for it to reach full speed. Record your data. Add all your data to the class chart.

11 Life in the Universe Series - Book 2 11 Name Date Mission 4 Investigating Types of Stars What do you think, now? After completing your investigation, please answer these questions: 1. From Earth, stars appear to be different colors: blue, white, yellow, orange, and red. What causes one star to be a different color than another star? 2. What other ways are stars different? 3. The Earth orbits a yellow star. What things would be different on Earth and in our Solar System if our star was white? (Consider what you learned from the Video Image show about how long it was before complex life appeared on Earth.) Type Color Temperature Lifetime O blue 35,000 C 10 million years B blue-white 21,000 C 40 million years A white 10,000 C 100 million years F yellow-white 7,500 C 5 billion years G yellow 6,000 C 10 billion years K orange 4,700 C 50 billion years M red 3,300 C 100 billion years