BDanielMayfield wrote:Thanks Stephen. The kappa mechanism was completely new to me. It’s good to have misconceptions corrected.
Well then, next question: When or where in a star’s lifetime does the Cepheid variable phase occur? It would have to be after the main sequence burning of Hydrogen, right?
A cepheid variable is a yellow supergiant, fusing hydrogen in a shell around its core, in an unstable period between the main sequence and the red supergiant phase. The outer layer of the star pulsates, and dims and brightens, because doubly ionized helium in the outer layer absorbs more light than singly ionized helium. Here's what wikipedia has to say:
http://en.wikipedia.org/wiki/Yellow_supergiant wrote:A yellow supergiant (YSG) is a supergiant star of spectral type F or G. These stars have initial masses between about 10 and 40 solar masses, although some yellow supergiants will have lost over half of that. Lower mass stars have lower luminosities and are seen as yellow giants. Higher mass stars do not expand beyond blue supergiants.
Most yellow supergiants are cooling and expanding rapidly towards red supergiants after leaving the main sequence, spending only a few thousand years in that phase, and so are much less common than red supergiants. Yellow supergiants are burning hydrogen in a shell after exhausting the hydrogen in their cores. Core helium ignition occurs smoothly at some point during the development of a red supergiant, but models vary on whether this occurs at the yellow supergiant stage or after the star has become a red supergiant.
Yellow supergiants are in a region of the HR diagram known as the instability strip because their temperatures and luminosities cause them to be dynamically unstable. Most stars observed in the instability strip appear as variables, subgiants as RR Lyrae variables, giants as W Virginis variables (type II Cepheids), and brighter giants and supergiants as Classical Cepheids. ...
 p. 366, The evolution of massive stars with mass loss, Cesare Chiosi and Andre Maeder, Annual review of astronomy and astrophysics 24 (1986), pp. 329–375. Bibcode: 1986ARA&A..24..329C. doi:10.1146/annurev.aa.24.090186.001553.
 Neugent; Philip Massey; Brian Skiff; Georges Meynet (2012). "Yellow and Red Supergiants in the Large Magellanic Cloud". arXiv:1202.4225v1 [astro-ph.SR].
 Bibcode: 2011BSRSL..80..266M
 Georges Meynet; Sylvia Ekström; André Maeder; Patrick Eggenberger; Hideyuki Saio; Vincent Chomienne; Lionel Haemmerlé (2013). "Models of rotating massive stars: Impacts of various prescriptions". arXiv:1301.2487v1 [astro-ph.SR].
http://en.wikipedia.org/wiki/Cepheid_variable#Dynamics_of_the_pulsation wrote:The accepted explanation for the pulsation of Cepheids is called the Eddington valve, or κ-mechanism, where the Greek letter κ (kappa) denotes gas opacity. Helium is the gas thought to be most active in the process. Doubly ionized helium (helium whose atoms are missing two electrons) is more opaque than singly ionized helium. The more helium is heated, the more ionized it becomes. At the dimmest part of a Cepheid's cycle, the ionized gas in the outer layers of the star is opaque, and so is heated by the star's radiation, and due to the increased temperature, begins to expand. As it expands, it cools, and so becomes less ionized and therefore more transparent, allowing the radiation to escape. Then the expansion stops, and reverses due to the star's gravitational attraction. The process then repeats.
The mechanics of the pulsation as a heat-engine was proposed in 1917 by Arthur Stanley Eddington (who wrote at length on the dynamics of Cepheids), but it was not until 1953 that S. A. Zhevakin identified ionized helium as a likely valve for the engine.
 Webb, Stephen, Measuring the Universe: The Cosmological Distance Ladder, Springer, (1999)
 Eddington, A. S. (1917). "The pulsation theory of Cepheid variables". The Observatory 40: 290. Bibcode:1917Obs....40..290E.
 Zhevakin, S. A., "К Теории Цефеид. I", Астрономический журнал, 30 161–179 (1953)
Last edited by Anthony Barreiro on Mon Sep 09, 2013 10:15 pm, edited 1 time in total.
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