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Thus the surface facing us periodically approaches and
recedes from us, thereby causing the spectral lines to shift periodically to the blue and to the red.
When we plot a Cepheid's variations in velocity against time we
obtain a velocity curve for the star.
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The light curve (top) and the velocity curve (bottom)
of Delta Cephei.
The velocity curve shows the radial velocity changes due to pulsation alone, and
since the star itself is moving toward the sun with a velocity of - 10 miles per second,
its radial velocity relative to the sun is also given.
The star is expanding
whenever
its radial velocity is between - 10 and - 20 miles per second; it is contracting
ever its radial velocity is between - 10 miles per second and O.
when-
It is expanding
its maximum rate at point (1) and contracting at its maximum rate at point (3).
at
The
star ceases expanding and begins to contract at point (2), where the Doppler shift
would change from a violet to a red shift if the star were at rest relative to the sun.