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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 63 FIGURE 3.7 The Hertzsprung-Russell (H-R) diagram. Developed independently by the two astronomers for whom it is named, the H-R diagram shows absolute magnitude versus spectral type. Since the spectral type is a surrogate for temperature, hot blue stars are found on the left of the diagram and cool red stars on the right. The Sun is a star of intermediate temperature (G spectral type) with an absolute magnitude of approximately 5. The group of stars running from upper left to lower right is called the main sequence and is where most stars are found on the diagram. A simple consideration of the Stefan Boltzman law suggests that a star above the main sequence at the same spectral type (temperature) must be large and one lower is smaller. This led to the designation of the stars in the upper right as red giants and those in the lower left as white dwarfs. Later work showed that the position in the H-R diagram was a product of stellar mass and age. The magnitude scale is a logarithmic scale of luminosity. Stars whose absolute magnitude differs by 5 differ in intrinsic luminosity by a factor of 100. SOURCE: Unsöld, 1969. which is simply the brightness of a star if it were placed at a standard distance of 10 parsecs. Examining the visible spectrum of a star and classifying it according to the observed distribution of features seen in its spectrum determines the spectral type of the star. Second, as ideas of modern quantum mechanics explained the reason for the existence of different spectral features in stellar spectra, it was realized that the star’s spectral type was a surrogate for temperature, so that the horizontal axis of the H-R diagram ran from hot blue stars on the left to cool red stars on the right. When spectral type and absolute magnitude were plotted, most stars were found to fall on a single broad band, called the main sequence, with some hot dim stars found below the main sequence and some cool bright stars found above. In one diagram, therefore, the concept of red giant and white dwarf stars was given meaning. 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 64 LEARNING TO THINK SPATIALLY Over the next several years, there were two approaches to building a picture of the structure of the universe, as revealed by the distribution of the stars. Both approaches involved the concept of a “standard candle,” thus establishing the absolute brightness (magnitude) of a star by nonparallax means. Using the absolute and the apparent brightness, it was possible to infer the distance of the star. Both methods were tied to stars whose parallax had been measured, but they allowed the extension of the distance scale beyond the solar neighborhood and the reach of the measurement of parallax. One approach built on the H-R diagram and developed what became known as “spectroscopic parallaxes.” In this approach, the spectrum of a star was examined and then classified. By the time of the development of this method, it had been shown that spectra gave evidence not only of the temperature but also of the luminosity of a star. For example, it was possible from an examination of a star’s spectrum to distinguish between a main sequence star of spectral K0 (Figure 3.7) and a brighter (giant) star of the same temperature. So, by observing a star’s spectrum, we know about how bright it is if seen from a given distance. By comparing its known absolute brightness with its apparent brightness in the sky, we can infer how far away it is. In fact, the classification approach was sufficiently precise that the magnitude of a star’s absolute brightness could be established within approximately 10–25 percent (a few tenths of a magnitude) and the distance inferred to comparable accuracy. In 1953, Morgan, Whitford, and Code used this technique of spectroscopic parallax to map the spatial distribution of young clusters of stars around the Sun (Figure 3.8). This analysis shows the distribution of these objects forming three distinct arms, and it was a key step in creating a sense of the spatial structure of our galaxy. The second set of standard candles emerged through yet another astrophysical spatialization, called the period-luminosity diagram (Figure 3.9). This approach was made possible by Leavitt’s observation in 1911 that the period of a particular class of pulsating stars is an excellent predictor of its average absolute magnitude. This class of stars, called Cepheids after the prototype delta Delta Cephei, is also easy to identify from its distinctive light curve (the change in brightness over the period of pulsation) (Figure 3.10). Leavitt made her discovery by observing Cepheid variables in a nearby galaxy, the Small Magellanic Cloud. Because the distance of these variable stars relative to each other was small in comparison to their absolute distance from Earth, the scatter in their distance did not hide the relationship. In 1924, Edwin Hubble used this new standard candle and the 100-inch telescope on Mount Wilson to identify Cepheids in the Andromeda Nebula (Figure 3.11), establishing once and for all that it was a galaxy like our own. Hubble went on to extend his system of standard candles and measures to establish the distances to increasingly distant galaxies. In 1929, Hubble made a discovery that opened up the universe even further and thus began the age of modern cosmology. He found that the further a galaxy was from Earth, the faster it was moving away (Figure 3.12). This led to the idea of an expanding universe, which in turn led to the “Big Bang” theory—that the universe was once all together, in a single place. With the discovery from this picture of an expanding universe, astronomy moved on to concepts such as the Big Bang, the discovery of quasars and other objects moving at very high velocities relative to Earth. The Hubble relationship became not only a map of space, but also a map of time. More distant objects, because of the finite velocity of light, are being observed at an earlier time in the history of the universe, opening up a window into the evolution of galaxies and clusters of galaxies and the “three degree” background radiation. However, it is interesting to note that the “Hubble Constant,” the quantity that describes the expansion velocity of the universe, has the units of kilometers per second per megaparsec (a million Copyright © National Academy of Sciences. All rights reserved.
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