1. Variable Stars & Distance The “Standard Candle”

3. Cepheid Variables Stars that exhibit pulsation periods on the order of a few days to months, are 4–20 times more massive than the Sun, and up to 100,000 times more luminous. Cepheids are supergiants of spectral class F6 – K2 and their radii change by several million km (30%) in the process.spectral class There exists a well-defined relationship between a Cepheid variable's luminosity and pulsation period, securing Cepheids as viable standard candles for establishing the Extragalactic Distance Scale.standard candlesExtragalactic Distance Scale Over 700 classical Cepheids are known in the Milky Way Galaxy, and several thousand extragalactic Cepheids have been discovered.Milky Way The Hubble Space Telescope has identified classical Cepheids in NGC 4603, which is 100 million light years distant.Hubble Space Telescope NGC 4603light years http://www.phys.ufl.edu/~buchler/ceph/anim.html

4. Discovery of Cepheids On September 10, 1784 Edward Pigott detected the variability of Eta Aquilae, the first known representative of the class of Cepheid variables.Edward PigottEta Aquilae The namesake for classical Cepheids is the star Delta Cephei, discovered to be variable by John Goodricke a few months later.Delta CepheiJohn Goodricke The period-luminosity relation of Cepheids was discovered in 1908 by Henrietta Swan Leavitt in an investigation of thousands of variable stars in the Magellanic Clouds. [9] She published it in 1912 with further evidence.Henrietta Swan Leavitt Magellanic Clouds [9]

6. RR Lyrae variables RR Lyrae variables are periodic variable stars, commonly found in globular clusters, and often used as standard candles to measure galactic distances.variable starsglobular clustersstandard candles This type of variable is named after the prototype, the variable star RR Lyrae in the constellation Lyra.RR LyraeLyra RR Lyraes are pulsating horizontal branch stars of spectral class A (and rarely F), with a mass of around half the Sun's. They are thought to have previously shed mass and consequently, they were once stars with similar or slightly less mass than the Sun, around 0.8 solar masses.horizontal branchspectral classSun RR Lyrae stars pulse in a manner similar to Cepheid variables, so the mechanism for the pulsation is thought to be similar, but the nature and histories of these stars is thought to be rather different. (The average absolute magnitude of an RR Lyrae is 0.75, only 40 or 50 times brighter than our Sun. [citation needed] ) Their period is shorter, typically less than one day, sometimes ranging down to seven hours.Cepheid variablesabsolute magnitudeSuncitation needed The relationship between pulsation period and absolute magnitude of RR Lyraes makes them good standard candles for relatively near objects, especially within the Milky Way. They are extensively used in globular cluster studies, and also used to study chemical properties of older stars.standard candlesglobular cluster

7. The RR Lyrae stars are divided into 3 main types [1], following a classification of S.I. Bailey based on the shape of the stars' brighness curve: [1]S.I. Bailey RRab — the majority, with steep rise in brightness (about 91%) RRc — having shorter periods, more sinusoidal variation (about 9%). RRd — rare double-mode pulsators.

8. In contrast to Cepheids, RR Lyraes are old, relatively low mass, metal-poor "Population II" stars.Population II RRs are much more common than Cepheids, but also much less luminous. RR Lyrae stars were formerly called "cluster variables" because of their strong (but not exclusive) association with globular clusters; conversely, about 90% of all variables known in globular clusters are RR Lyraes.globular clusters RR Lyrae stars are found at all galactic latitudes, as opposed to classical Cepheid variables, which are strongly associated with the galactic plane.Cepheid variables Several times as many RR Lyraes are known as all Cepheids combined; in the 1980s, about 1900 were known in globular clusters. Some estimates have about 85000 in the Milky Way [1]. [1] RR Lyrae vs Cepheid Variables

9. He+ is ionized to He++ within the partial-ionization layer near the surface of the star whenever T > 40,000K.

10. The outer layers contracts and reheat to 40,000K, and the ionization cycle begins again. This sort of stuff happens all the time within stars – including the Sun – wherever the internal temperature crosses the ionization threshold of important elements. Along the instability strip, however, the period for He++ ionization cycling is approximately equal to the overall acoustic oscillation period within the star, and so these fluctuations become greatly amplified and readily detectable. When this happens, the opacity of the layer goes up due to the increased number of free electrons. - - - -

11. The greater opacity drives up the temperature and pressure gradients across the partial-ionization layer, which physically expands outward and cools.

12. As the layer cools, He++ recombines with free electrons to form He+, driving the opacity back down and decreasing the temperature and pressure gradient.

13. The outer layers contracts and reheat to 40,000K, and the ionization cycle begins again.