Cepheid Variables

1. Cepheid Variables

2. Hydrostatic equilibrium • When a stars thermo nuclear fusion equals the pull of gravity the star is in equilibrium • Like our sun

3. Pulsating variable stars • Some stars have problems achieving the proper balance between the power welling up from its core and the power radiating from its surface. • Upper layers may be too opaque • Energy and pressure build up beneath the photosphere and the star swells • Then this expansion puffs the upper layers outward and makes them too transparent • So much energy escapes that the underlying pressure drops and the star contracts

4. Pulsating variable star • In a futile quest for a steady equilibrium the atmosphere alternately expands and contracts • Star will rise and fall in luminosity • Any pulsating star has its own particular period between peaks of luminosity • Range from several hours to several years

6. Pulsating variable stars • Many of these stars inhabit a strip on the H-R diagram (instability strip) • Lying in the upper portion of the strip are Cepheid Variables • Very luminous • Fluctuating with periods of a few days to a few months • The longer the period, the more luminous • North star is a Cepheid

9. Cepheid, what is it good for? • Cepheid variables provide our primary means of measuring distances to other galaxies • So we can learn the true scale of the cosmos

10. By discovering the first binary star where a pulsating Cepheid Variable and another star pass in front of one another, an international team of astronomers has solved a decades-old mystery. The rare alignment of the orbits of the two stars in the double star system has allowed a measurement of the Cepheid Mass with unprecedented accuracy. Up to now astronomers had two incompatible theoretical predictions of Cepheid masses. The new result shows that the prediction from stellar pulsation theory is spot on, while the prediction from stellar evolution theory is at odds with the new observations.