Introduction Astronomy Tools Concepts 1. Electromagnetic Spectrum 2. Atmosphere Limitations 3. Space Observations Equipment 1. Telescopes 2. Radio 3. Space Tools 4. Photography 5. Spectroscopy 6. Computers 7. Advanced Methods 8. Radio Astronomy Basic Mathematics Algebra Statistics Geometry Scientific Notation Log Scales Calculus Physics Concepts - Basic Units of Measure - Mass & Density - Temperature - Velocity & Acceleration - Force, Pressure & Energy - Atoms - Quantum Physics - Nature of Light Formulas - Brightness - Cepheid Rulers - Distance - Doppler Shift - Frequency & Wavelength - Hubble's Law - Inverse Square Law - Kinetic Energy - Luminosity - Magnitudes - Convert Mass to Energy - Kepler & Newton - Orbits - Parallax - Planck's Law - Relativistic Redshift - Relativity - Schwarzschild Radius  - Synodic & Sidereal Periods - Sidereal Time - Small Angle Formula - Stellar Properties  - Stephan-Boltzmann Law - Telescope Related - Temperature - Tidal Forces - Wien's Law Constants Computer Models Additional Resources 1. Advanced Topics 2. Guest Contributions
 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: m-M=5log10(d)-5 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. Back to Top