Polaris - Variable Star

Variable Star

Polaris A, the supergiant primary component, is a classic Population I Cepheid variable, although it was once thought to be Population II due to its high galactic latitude. Since Cepheids are an important standard candle for determining distance Polaris, as the closest such star, is heavily studied. The variability of Polaris had been suspected since 1852; this variation was confirmed by Ejnar Hertzsprung in 1911.

Both the amplitude and period of the variations have changed since discovery. Prior to 1963 the amplitude was over 0.1 magnitude and decreasing very gradually. After 1966 it decreased very rapidly until it was less than 0.05 magnitude and since then has varied erratically near that range. It has been reported that the period is now increasing. The period increased fairly steadily by around 4 seconds per year until 1963. It then stayed constant for 3 years, but began to increase again from 1966 onwards. Current measurements show a consistent increase of 3.2 seconds per year in the period. This was originally thought to be due to secular red-ward evolution across the instability strip, but is now considered to be interference between the primary and first overtone pulsation modes. Comparison of the period luminosity relationship and the observed luminosity indicate that the main pulsations are the first overtone.

Research reported in Science suggests that Polaris is 2.5 times brighter today than when Ptolemy observed it, changing from third to its current second magnitude. Astronomer Edward Guinan considers this to be a remarkable rate of change and is on record as saying that "If they are real, these changes are 100 times larger than predicted by current theories of stellar evolution."