FIGURE 3.6 Astronomical parallax. The measurement of astronomical parallax is a triangulation of the distance to a star, using the orbit of Earth around the Sun as a baseline. A star is said be 1 parsec away if the angle p is 1 second of arc. Tycho’s difficulty with measuring this angle was that it was well below the limit of his ability to detect. The nearest star is Proxima Centauri, 1.3 parsecs away, making the parallax angle less than 1 second. In practice this technique works only for relatively nearby stars, and the parallactic motion is measured against stars much further away that have no detectable parallax. SOURCE: Abell, 1969.

most likely to detect stellar parallaxes. However, the largest observed stellar parallax is less than 1 second of arc and 60 times smaller than the limit of Tycho’s instrumentation of approximately 1 arc minute. The detection of stellar parallax led to the eventual proof that not only was Earth not the center of the universe, but the Sun was just an ordinary star of modest luminosity.

The breakthrough came in 1838 when three astronomers—Bessel, Henderson, and Struve—reported measuring the parallax of three nearby stars, 61 Cygni, Alpha Centuri, and Vega. This measurement served two critical purposes. First, it established the parsec as the basic unit of astronomy. Using the Sun-Earth distance as the baseline, a parsec is defined as the distance at which an object would have a parallax of 1 arc second, a distance equal to 206,265 times the distance of Earth from the Sun. Second, it established the scale of the universe, as then perceived, to be very large. The nearest stars were found to be more than 1 parsec away.

Following the first detection of stellar parallax, astronomers slowly began to build up a picture of the solar neighborhood, the region around the Sun where the parallax of objects could be measured. Edward Hertzsprung in 1911 and, independently, Henry Norris Russell in 1913 gave astronomy its first astrophysical spatialization—the Hertzsprung-Russell (H-R) diagram (Figure 3.7). This diagram plotted absolute magnitude against the newly identified spectral type of nearby stars. Two key concepts were merged in the diagram. The first was that of absolute magnitude,

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