- Cepheids are variable stars. Stars that are more than 5 times
the mass of our Sun go through an "instability strip" in the H-R diagram. This instability strip notes where
stars go through a radial pulsation mechanism.
The mechanism is basically a battle between opacity, the temperature
of the atmosphere and gravity (stars are in hydrostatic equilibrium).
When the outer layers of the star are sufficiently opaque (dense), the
temperature increases and the atmosphere expands. This expansion
happens until the opacity (density) gets low enough for the atmosphere
to cool and then there isn't sufficient pressure support to keep these
layers from falling downwards. The star is brightest when the
temperature of it's outer layers is highest (which corresponds to a
maximum outward velocity of these layers). It is dimmest when the
temperature is smallest and opacity is largest (which corresponds to
maximum inward velocity).
The mechanism is basically a battle between opacity, the temperature of the atmosphere and gravity (stars are in hydrostatic equilibrium). When the outer layers of the star are sufficiently opaque (dense), the temperature increases and the atmosphere expands. This expansion happens until the opacity (density) gets low enough for the atmosphere to cool and then there isn't sufficient pressure support to keep these layers from falling downwards. The star is brightest when the temperature of it's outer layers is highest (which corresponds to a maximum outward velocity of these layers). It is dimmest when the temperature is smallest and opacity is largest (which corresponds to maximum inward velocity).
This produces a characteristic "sawtooth" pattern. The absolute
magnitudes of Cepheids vary from -2 to -6 (visible) and their periods
range from several to several hundred days.
- In 1912, Henrietta Leavitt (a human calculator working for
Charles Pickering at Harvard) noticed that some 2,400 stars in the SMC
went through variability in brightness and that the more luminous ones
had longer variability periods. With the little free time she had, she
plotted the magnitudes of these stars against their periods. Here's a
link to the March
1912 report of this discovery by her employer, Edward
C. Pickering. It starts, "The following statement regarding the
periods of 25 variable stars in the Small Magellanic Cloud has been
prepared by Miss Leavitt."
