In order to calibrate the equipment and determine the transparency of the atmosphere, the signals of the photomultiplier tube from the bright stars Arcturus and Capella were measured. And although these stars have close magnitudes and were located at the same height above the horizon, the signals were very different. From which of these stars could the signal be greater and why?
The condition does not say in which spectral region the measurements were carried out. But it is known that the brightness of the radiation of cold (red) stars decreases sharply in blue rays (as indicated by Planck’s formula). Therefore, if we measure the brightness of Arcturus and Capella with a blue or violet light filter, then, despite the close apparent magnitudes, the signal from the orange Arcturus will be significantly less than from the yellow Capella.
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