- They obey well-defined period-luminosity relations that have very
low intrinsic scatter (±0.1 mag in the I band Udalski
et al. 1999)
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- Their characteristic "sawtooth"
variability allows very easy, unambiguous detection.
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- Cepheids are young supergiant stars which are very bright and can
be observed over extragalatic distances (~25Mpc).
- They are numerous and common in spiral galaxies. This allows us
to calibrate the P-L relation (in principal) locally and observe
the same objects in galaxies out to ~25Mpc.
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