| While the luminosity of the sun is relatively constant, some stars undergo substantial oscillations in brightness. |
| These are called variable stars. |
| One type, Cephid variables, exhibit a relationship between the period of their oscillations and their absolute brightness. |
| At their brightest, Cephids can be seen in nearby galaxies. |
| So the distance scale to these galaxies was set by measuring the periods of Cephid variables, deducing their actual brightness, and comparing with their apparent brightness. |
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