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Learning to Think Spatially: GIS as a Support System in the K-12 Curriculum http://www.nap.edu/catalog/11019.html SPATIAL THINKING IN EVERYDAY LIFE, AT WORK, AND IN SCIENCE 65 FIGURE 3.8 Map of the solar neighborhood. This diagram projects the position of younger clusters of stars onto the galactic plane. The Sun is denoted by S in the diagram. From observations of external galaxies we know that the spiral arms are apparent largely because of the presence of bright stars in relatively young clusters of stars. In showing the position of these stars near the Sun we can see a similar pattern. Parts of three arms of our galaxy are visible from the diagram. SOURCE: Morgan et al., 1953, p. 318. Reproduced by permission from the American Astronomical Society. parsecs). So even in our description of the far reaches of the universe, the distance scale is tied to the distance from Earth to the Sun. This distance scale is an appropriate homage to the process of spatial thinking that led from measuring the diameter of Earth to an understanding of some of the most fundamental properties of the universe. Copyright © National Academy of Sciences. All rights reserved.
Learning to Think Spatially: GIS as a Support System in the K-12 Curriculum http://www.nap.edu/catalog/11019.html 66 LEARNING TO THINK SPATIALLY FIGURE 3.9 The period-luminosity diagram for the Cepheids. This diagram shows Henrietta Leavitt’s graph of data for the Small Magellanic Cloud. It illustrates the relationship between the period of a Cepheid variable and its average luminosity. Left: m refers to the average apparent magnitude of the variable as observed; right: M refers to absolute magnitude of the variable stars. SOURCE: http://www.astro.livjm.ac.uk/courses/one/ NOTES/Garry%20Pilkington/cepinp1.htm. FIGURE 3.10 The characteristic light curve of a Cepheid variable. This star has a period of approximately 5.5 days (log 0.74), which implies that it has an absolute magnitude of approximately –1.5. Recalling from Figure 3.9 that the magnitude scale is a logarithmic scale of luminosity and that stars whose absolute magnitude differs by 5 differ in intrinsic luminosity by a factor of 100, we can calculate the distance to the star. The average apparent magnitude (visual magnitude in the figure) is 3.75, making the difference between absolute and apparent magnitude slightly more than 5. Therefore, the star appears approximately 100 times less luminous than it would if it were at the standard distance of 10 parsecs. Applying the inverse square law (luminosity decreases in proportion to 1/r 2 ), one can infer that this star would be slightly more than 100 parsecs away. SOURCE: Abell, 1969, p. 480. Copyright © National Academy of Sciences. All rights reserved.
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