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The false dawn
in a defined area of sky that represented a column that was vertical to the disk. By comparing the distribution of the brightness of those stars, they deduced how the stars were physically distributed within the column of space. Statistically, seeing more bright stars than faint stars indicates that there are more stars close to you than farther away. Alternatively, fewer bright stars and fewer faint stars, but plenty of intermediate brightness stars, suggests a layer of stars at some intermediate distance, and so on. Of course, this ignores some important factors, not the least of which is the fact that not all stars have the same intrinsic brightness. Nor does it account for interstellar dimming by dust along the line of sight.
THE PROBLEM WITH K STARS The vertical motions of the stars were measured using the Doppler effect. In the 1920s and 1930s, astronomers did not have the sophisticated detectors and spectrographs, much less the huge telescopes available today, and so they chose whatever stars were easiest to measure. There is a particular type of star that is very attractive to astronomers when making velocity measurements: K giants.
Figure 2.3. The HertzsprungÂąRussell diagram plots the brightness of stars against their temperature (determined from their spectral type). It was independently devised by the Danish astronomer Ejnar Hertzsprung (1911) and the American Henry Norris Russell (1913). Any large group of stars Âą such as a cluster or galaxy Âą will form well-defined groups depending on the evolutionary stage of the stars.