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; fâz→ phase. | 