An Etymological Dictionary of Astronomy and Astrophysics
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فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory

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Number of Results: 1 Search : phase lag
phase lag
  دگرسانی ِ فاز   
degarsâni-ye fâz

Fr.: différence de phase   

1) General: Same as → phase difference.
2) Cepheids: The observed phase difference between luminosity and velocity in classical (radially pulsating) → Cepheids. On the basis of adiabatic pulsation theory, one would expect the maximum luminosity to occur when the radius of the star is minimal. This means that the maximum outward velocity would be one quarter period out of phase with the maximum velocity. However, in the observations the maximum luminosity and maximum outward velocity are nearly in phase. This effect is due to the → kappa mechanism which is responsible for driving the → pulsations. The pulsations in Cepheids are excited by the helium → partial ionization zone, He+↔ He++, which is located below the He ↔ He+ and H ↔ H+ zones. These latter two regions are too shallow to contribute significantly to the driving of the fundamental modes of Cepheids; so their only effect is to introduce a phase shift.

phase; lag, possibly from a Scandinavian source; cf. Norw. lagga "go slowly."

Degarsâni, → difference; fâzphase.