Mechanism of Variable Luminosity
Astronomers believe that the variable luminosity of a cepheid is caused by the opacity of different types of helium. The more helium is heated, the more ionized it becomes. More ionized helium is more opaque. At the dimmest part of a cepheid's cycle, helium in the outer parts of the star are at their most ionized, and therefore block out more light. Because the helium absorbs more light, it begins to heat up and expand, causing it to cool off and become less opaque. The cycle then repeats as the star's gravity reverses the expansion.
Relationship Between Absolute Luminosity and Period
The brighter a cepheid star, the longer its period. Cepheid luminosity periods typically range from one day to 100 days. A cepheid with a period of around three days most likely has an absolute luminosity of around 1000 times that of the sun. A cepheid whose period is 100 days long has an absolute luminosity closer to 30,000 times that of the sun.
Why Non-Cepheids Cannot Be Used to Calculate Distance
When astronomers look at a normal star, they typically don't have any way to know how bright the star is. As a result, it's impossible to determine the star's distance. It's nearly impossible to tell the difference between a dim, nearby star and a bright, distant star.
Cepheids as "Standard Candles"
Because the period of a cepheid can be measured, astronomers can estimate its absolute luminosity. It can then be used as a "standard candle," a sort of benchmark. Light from a point source becomes weaker with distance, according to a simple mathematical law. By knowing how bright a cepheid truly is, and measuring how bright it seems from Earth's point of view, astronomers can calculate the distance to the star.