science scope

by Chuck Fidler and Sharon Dotger

O

ne of the largest challenges of teaching astronomy is bringing the infinite scale of the universe into the four walls of a classroom. However, concepts of astronomy are often the most interesting to students. This article focuses on an alternative method for learning about stars by exploring visible characteristics of the constellation Orion and applying these observations to an inquir y-based modeling project. By the end of this lesson, middle school students will be able to gain a better understanding of

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• stars by using a hands-on project in order to see the variation of stellar distances; • methods and interpretations of scaled astronomical distances; • problems with Earth-based space exploration; and • the impact of advanced technology on modern space exploration.

Constellation Orion the Hunter

US Naval Observatory Library

FIGURE 1

When we gaze into the cosmos, our perspective from Earth may lead to inaccurate interpretations of celestial objects. Stars appear to be similar points of light in the night sky, however studies of space using high-powered telescopes indicate this is not true. Figure 1 depicts several stars of the constellation Orion, which appear brighter than the stars that make up the dotted stellar background. The seemingly two-dimensional plane, or ceiling effect (Black 2005), in which these points of light in the night sky reside may lead to misconceptions about actual stellar distances. In fact, many common student misconceptions about stars exist: All stars are the same distance from Earth, stars in a constellation are near each other, all stars are the same size, the brightness of a star depends only on its distance from Earth, and the galaxy is ver y crowded (AIP and LSU Department of Physics and Astronomy 1998). The use and manipulation of experts’ discoveries of actual star distances can be implemented in the classroom in an inquir y-based lesson that effectively models scaled distances of the constellation Orion.

Why the constellation Orion?

The choice to use the constellation Orion the Hunter was quite deliberate. Orion is a very well-known constellation observable from anywhere in the world due to its low position on the celestial equator. At the very least, Orion’s belt, which is composed of three bright stars in a seemingly straight line (from Earth’s perspective), is often identifiable by a novice observer. In addition, Orion contains various types of celestial objects. The constellation contains stars of various sizes and magnitudes by classification. For example, Betelgeuse is considered a red giant (approximately 700 solar diameters), and Rigel is a blue giant (approximately 62 solar diameters). The constellation also contains the Orion Nebula, the remnants of an exploded star approximately 30 lightyears across. However, because the nebula resides approximately 1,600 light-years away from Earth, it is observed by the unaided eye as a single point of reddish light. Orion’s belt also offers some interpretation. Although these three stars seem close to each other and are similar in apparent brightness, they actually reside at drastically different distances from Earth. How can they be the same apparent magnitude? I also chose Orion because of its high profile in our culture. Numerous movies, NASA missions, and spacecraft make use of the famous constellation’s name. As we look into the stars at night, quite often you hear people say, “I see Orion’s belt!”

A lesson plan to study the relative distances of stars in the constellation Orion This lesson can be conducted in one and a half block schedules of 80 minutes or two 55-minute periods. Required math skills include basic conversions and scale manipulation, accurate measurement skills, and basic geometr y.

Preassessment (5 minutes) The lesson begins with a preassessment where paired students are asked to respond to the question “How are distances to the stars determined?” and then write their responses in their science journals. Common student responses include the following:

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Astronomical Scale of stellar distances

FIGURE 2

Two sample photos used for student comparison

“The stars are too far to be measured.” “I know you can measure them but I have no idea.” “They are all just really far away.” “Stars [other than our Sun] reside within our galaxy.” “You use star light.” Teachers can refer to Murphy and Bell’s article “How Far Are the Stars?” (2005) for a clear explanation of stellar distance measurement and the concept of a light-year. When students are finished writing, guided questioning begins to demonstrate varying scales of distance. Students may be asked to think about the distance between their school and home, school and the nearest shopping mall, or school and a state landmark. This exercise asks students to look at relative scales among more familiar distances and begins discussion of how common language (other than exact measurements) can be used to describe surrounding distances. For example, students may not know the exact distance (i.e., mileage), but may describe the distance from school to the nearest shopping mall to be twice that of the distance from school to home. The use of common language will be useful when comparing star distances using the three-dimensional model.

FIGURE 3

Sample slide with star names, distances, and guiding questions How did you do?

Betelguese 427ly Bellatrix 243ly A C

Alnilam 1350ly

E

Mintaka 916ly F

D Alnitak 815ly

Correct order: C, A, H, I, D, F, B, E, G Let’s compare some stars…look at Orion’s belt stars. How can they look very similar yet be different distances away? Why does Betelguese look reddish in color?

Orion Nebula 1600ly G What else looks reddish in color? B Hatsya 1300ly I Distances retrieved from Sloan Rigel 773ly H Digital Sky Survey at the UniverSaiph 720ly sity of Chicago supported by NSF, NASA, and U.S. Dept. of Energy

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Optical illusion (10 minutes) The direct obser vation of the stars from the Ear th’s sur face is confounded by the atmosphere. In order to illustrate this point, we turn to the nearest celestial neighbor for clarification, as the Moon’s proximity to Ear th makes it much easier to obser ve and discuss. Two images depicting various Moon diameters (Figure 2) are shown in order to facilitate discussion about apparent sizes and distances of objects in the sky. Questions to facilitate discussion are “Why do objects in the sky look the way they do?” “Do objects in the sky always look the same? Why or why not?” “Can you compare dif fer-

Astronomical scale of stellar distances

ent objects in the sky? How?” “What are some similarities and dif ferences between objects you see in the sky?” Common student responses include the following:

FIGURE 4

Constellation Orion the Hunter cutout

“Planets look like stars in the sky.” “Clouds may block our view of stars.” “It [celestial object view] depends on the where the Sun is.” “The Sun is the brightest object in the sky.” “Some stars are brighter than others.” This activity offers students an opportunity to understand why celestial objects are sometimes not as they may appear from our Earth-based vantage point.

The challenge activity now applies the difficulties of sky obser vation to the stars, a task that is a bit harder to conceptualize due to the apparent smaller stellar diameters and larger stellar distances. In pairs, students are shown a high-quality image of the constellation Orion via computer/ overhead projection as shown in Figure 3. The nine major stars (including the Orion Nebula) are labeled with the letters A through I. Students rank the stars in order from the closest to the farthest from Earth, establish criteria for their selections, and generate a class list. Students should • rank the stars of Orion (with peers) in order from closest to farthest; • discuss and write down their rationale for the ordering of star distances in their science notebooks; • identify some common criteria they developed to support their decision making; and • be prepared to share with the class their common criteria for ranking the star distances.

photos courtesy of NASA

Challenge activity (10 minutes)

Three-dimensional model (30 minutes) Materials (per team of two to three students) • safety goggles (to be worn at all times) • 9 wooden shish-kebab skewers sharpened at one end (about $5 for 100). Plan ahead to replace skewers, as students may make mistakes and need more skewers. (To reduce time in class and for safety, we recommend that these be sharpened by the teacher ahead of time.) Students will determine the length of the skewers based on peer-developed scales. • 10.2 cm × 12.7 cm (4” × 5”) precut piece of 1/2inch foam board. Regular cardboard can be used, as long as it is thick enough to support the skewers safely and securely.

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• sharp scissors • metric rulers • constellation Orion cutouts (see Figure 4) • glue sticks/tape • calculator (optional) • 1 bottle of correction fluid per group Safety note: The cutting of sharpened wooden skewers may be difficult for some students and should be done with safety in mind. The teacher should super vise students while the skewers are being sharpened, and provide help to anyone needing assistance. Protective goggles must be worn at all times to shield students’ eyes from any wooden splinters that may be ejected during the act of cutting.

Selecting a scale model of stellar distances (15 minutes)

FIGURE 5

Sample student-generated data for star distances and conversions Converted cm length of skewer

Plus 1 cm for foam board

427

2.1

3.1

1,300

6.5

7.5

Bellatrix

243

1.2

2.2

Alnitak

815

4.1

5.1

Alnilam

1,350

6.8

7.8

Mintaka

916

4.6

5.6

1,600

8.0

9.0

Saiph

720

3.6

4.6

Rigel

773

3.9

4.9

Star Betelgeuse Hatsya

Orion Nebula

Distance from Earth in light-years

Students cannot directly measure stellar Source: Sloan Digital Sky Survey, University of Chicago. distances, therefore star distances (lightyears) should be provided by a credible found in Figure 5.) Note: Measuring the wooden source such as the Sloan Digital Sky Sur vey (www. skewer should begin just above the tapered section sdss.org). A brief review of the light-year can be of the skewer to allow for the skewer to be inserted found in Robertson’s article “Science 101: Why Is into the foam board. The scaling theme used in a Light-Year a Unit of Distance Rather Than a Unit this example would then convey relative stellar disof Time?” (2006). Students should begin by reviewtances without asking students to think in terms of ing the data table of star distances provided (see extremely large numbers, such as those associated Figure 5). Students work with their peers to deterwith light-years. mine an appropriate scale in which to convert the light-year distance into centimeters. (This process Putting the pieces together is analogous to a map scale where 1 cm may equal (20 minutes) 10 km.) Students enter the calculated scaled cenOnce students have set a defined scale, the timeter length in the column titled “converted cm wooden skewers should be cut to scaled, mealength of skewer.” Next, students fill in the column sured lengths for each respective star in order titled “plus 1 cm for foam board” to determine the to assemble the three-dimensional model of the total cut skewer length. The following paragraph constellation Orion. Students should glue down provides an example of how this process may look a printout of the constellation Orion (Figure 4) in the classroom. onto a piece of appropriately sized foam board. A student can choose to represent 1 cm to apNext, students pair each scaled piece of wooden proximately 200 light-years (ly). This ratio can be skewer with the appropriate star by inser ting the applied to the Orion Nebula by cutting a wooden tapered end of the skewer into the foam board, alskewer to measure 8 cm (1 cm/200 ly = X cm/1,600 lowing the skewer to protrude ver tically into the ly = 8 cm) because the approximate distance to the surrounding air. This process should be repeated Orion Nebula is 1,600 light-years. Using this ratio, for all nine celestial objects. Once all skewers the distance from Earth to the enormous red giant are in place, students use white paint to paint the Betelgeuse would measure approximately 2.1 cm. tops of the skewers for added contrast. The result (A listing of sample scaled stellar distances can be

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Astronomical scale of stellar distances

Orion 3-D model as seen from directly overhead

FIGURE 7

Orion 3-D model from side

photos courtesy of the author

FIGURE 6

will be a three-dimensional scaled model of the relative distances to most of the stars in the constellation Orion.

Using the three-dimensional model (10 minutes) The model can now be used to represent an Earthbased perspective of the relative stellar distance of the constellation Orion. In order to do so, the model should be held at eye level, horizontal to the floor, where students can obser ve the relative scaled skewer lengths of star distances with respect to each other. Careful obser vations can be directed toward the relative distances between the three stars that make up Orion’s famous belt. In this example, the longer the skewer length, the farther away from Earth’s surface, which may be quite a contrast to the seemingly equidistant positions of the three stars on Orion’s belt. Another perspective allows students to mimic a real-life obser vation by inverting the model to an

overhead position. The model should be oriented so that the vertical skewers are pointing down directly at the obser ver’s eyes. (Students should be wearing protective goggles.) From this vantage point, the three-dimensional skewers appear to be depthless, similar to students’ familiar Earth-based perspective (Figure 6). The model can be rotated along three axes to offer various perspectives of relative star positions. Note: The model is not intended to represent stellar distances from the eyeball to the cut end of the skewer. This may create an inaccurate representation that longer skewers represent closer positions to Earth. See Figures 6 and 7 for student work samples. Teachers should direct students to take special notice to how the stars, which appear to look two dimensional above at night, are actually var ying distances away. Teachers should demonstrate that the model should turn on the vertical axis so that skewers point up into the air when held below eye level, and toward the floor when overhead.

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Wrap-up/extension (15 minutes) The scaled, three-dimensional model concept can be used to represent a variety of astronomical concepts that can be applied to the scaled diameters, magnitudes, and temperatures of numerous celestial objects. For example, students in class extended this activity to compare the diameters of the Sun and the red giant Betelgeuse (the upper-left shoulder of Orion). Betelgeuse is approximately 700 times the size of the Sun, and discussions included using cut skewers to compare the diameters of the two; therefore, a 2.54 cm (1 in.) Sun diameter would equal a 1,778 cm (700 in.) diameter of Betelgeuse (1:700). Using this scale, the Betelgeuse skewer would extend 17.78 m (58.3 ft.) long! For this extension, skewers can be substituted with string. A piece of string can be cut to represent one unit of distance, and then the string can be rolled out in the school hallway or outside. This prompted students to respond using such common language such as “Betelgeuse is absolutely enormous compared to the Sun.”

Assessment In order to assess the use of common terminology, students wrote responses in their science journals to the questions “What does it mean to use ever yday terminology to describe the world around you?” “How can you use ever yday terminology to describe the distances to stars, as well as other objects in the sky? What evidence can you supply?” Strong student responses included statements such as “Ever yday terminology are words used to express comparative properties of objects in the world. Ever yday words, such as larger than, smaller than, twice as far, much closer than, etc., can be used to help us think about the relative properties of stellar objects, since the exact numbers associated with particular star properties are often too big to comfortably think about. The Orion Nebula was the farthest away, while Alnitak is about half that distance. This was evident when the skewer representing the Orion Nebula was about twice as long as the skewer we used to represent Alnitak.” Students revisited Figure 1 in pairs and were asked to individually write in their science journals responses to the following questions: “Thinking back to the beginning of this activity, how can you describe the stars of Orion the Hunter from an Ear th-based obser vation point?” “How can you

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apply the strategies used in this activity to the Ear th-based obser vation of other constellations or stars in the sky?” “Please describe what threedimensionality means in terms of sky obser vations.” Instr uctors reviewed students’ science journals and found that strong responses included statements such as “When I first obser ved the photo of Orion, the stars were presented on a 2-D document, which resembled what it looked like when we see it [Orion] in the sky at night. The stars seemed to be all at the same distance from us [Ear th]. Since stars are far away, it is hard to determine stellar distance using our own eyes. So, we have to make models to help us. The models we made in class help us visualize star distances by using our own developed scales. It is clear that the stars are not the same distance from Ear th, but var y from star to star demonstrated by the cut skewers. This concept can be applied to other constellations, as all stars do not reside in the ‘two-dimension night sky ceiling’ but rather in the three dimensions of space.” n

References

Black, A.A. 2005. Spatial ability and Earth science conceptual understanding. Journal of Geoscience Education 53 (4): 402–14. Murphy E., and R.L. Bell. 2005. How far are the stars? The Science Teacher 72 (2): 38–43. American Institute for Physics (AIP) and Louisiana State University (LSU) Department of Physics and Astronomy. 1998. Children’s misconceptions about science. www.eskimo.com/~billb/miscon/opphys.html. Robertson, B. 2006. Science 101: Why is a light-year a unit of distance rather than a unit of time? Science and Children 44 (2): 17–19.

Resource

Tretter, T.R., and G.M. Jones. 2003. A sense of scale: Studying how scale affects systems and organisms. The Science Teacher 70 (1): 22–25.

Chuck Fidler ([email protected]) is an assistant professor of physical science in the Department of Mathematics and Science at Wheelock College in Boston, Massachusetts. Sharon Dotger (sdotger@syr. edu) is an assistant professor of science education in the Department of Science Teaching at Syracuse University in Syracuse, New York.