Freaky freaky Cepheid variables
In act III, scene I of Shakespeare’s play Julius Caesar, Caesar says, “I am constant as the northern star.” Well, Caesar is wrong: it turns out that the northern star isn’t as constant as he thinks it is. It belongs to the family of Cepheid variables—the type of stars that pulsate in size and luminosity. This is strange; usually, stars don’t expand and shrink back multiple times. Size change signifies evolutionary change, and stellar evolution is very much a one-way road. Cyclicity and periodicity in stellar size variation is very rare, so scientists can’t help but wonder: why do Cepheid variables pulsate?
The secret lies in their insides: deep beneath the surface of Cepheid variables, there is a special region in which partially ionized helium ionizes a second time (i.e. loses another electron). The energy from the stripping and combining of electrons is actually the driving force behind Cepheid variables’ pulsation. Temperature fluctuates quite frequently in the stellar interior, and occasionally it becomes so hot that the entire star expands. Strangely, the Cepheid variable doesn’t cool down with expansion—thanks to the doubly ionized helium carrying on electron-ion recombinations and releasing heat. Eventually, the electron-ion recombination will be completed. One source of heat will be lost, and the Cepheid variable will contract. Here, the second ionization energy of the helium uses up part of the gravitational potential energy, stopping the contraction heat from seeping away. When the ionization completes, the electron-ion recombination starts, and the cycle continues.
If you plot all the observable stars in our universe onto a chart ranked by their luminosity and temperature, you will discover that the Cepheid variables clump together into a thin, long, and vertical region near the top of your diagram. Scientists label that region “the instability strip.” The geometry of the strip reveals many fundamental properties about Cepheid variables: its verticality implies that these stars span a wide range of luminosity, while its narrowness shows that they stretch only a tiny range of temperature (from 5500 K to 7500 K). The specificity of the temperature range is vital for pulsations to happen: if the star’s temperature exceeds 7500 K, the ionization zones will be located very close to the surface, where there is not enough mass to drive the pulsations. Otherwise, if a star’s surface temperature is lower than 5500 K, the ionization zones will be located in a convection zone, where efficient heat transfer would give all the energy needed for pulsation away.
Polaris is one of the most well-studied, well-used, and well-loved stars in human history; sailors, explorers, and navigators have traveled the globe for centuries with the help of its gentle gaze. However, very few have ever had the privilege of bearing witness to its tender blinks. Polaris shimmers with such subtlety that its pulsation remains hidden and secretive to most of its observers, including Julius Caesar. Now that you’ve learnt Polaris’s secret, think about Cepheid variables and the physics behind them the next time you gaze upon the northern sky!