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Physics - Formulas - Cepheid Rulers

Cepheids are a type of variable star that pulsates regularly. The properties of variable stars will be discussed later, but because these stars have a particular brightness to variation ratio, they server as good rulers for determining distances.

By using the Distance Modulus:


we can determine distance by a re-write:

If we record a Cepheid variable that pulsates at a particular rate in some distant star cluster or galaxy, we know the absolute magnitude (M) based on the Period-Luminosity relationship. All we have to do is measure the apparent brightness (m) of the recorded Cepheid and plug in the rest. An example, a Cepheid that pulsates once every three days has an absolute (M) magnitude of -2. Every Cepheid that pulsates at this rate will have this magnitude.

RR Lyrae variables can also be used as rulers, but they pulsate faster and have about a 1 magnitude variation in brightness. The benefit is they seem to reside mostly in globular clusters.

As far as data, the AAVSO has a Light Curve Generator that can help determine the exact data required to accurately solve this equation.

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