Type of star that's brightness varies over a period of a few days.
The time between the peaks of brightness is called the period.
Correlation between luminosity and its period - the greater the luminosity the longer the period.
Scientists work out the distance to all Cepheid variable stars by:
- measuring the period
-measure the observed brightness
- compare brightness with the luminosity to get distance
Distances are too large to measure using parallax. They find a Cepheid variable then use its period and brightness to find the distance.
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