Physical variable stars: Cepheids, novae and supernovae
The brightness of physical variable stars changes because of processes in the stars themselves. Cepheids are pulsating supergiants (prototype δ Cephei): their brightness varies periodically with periods of 1–100 days. In 1912 Henrietta Leavitt found that the longer the period the more luminous the star (the period–luminosity relation): measuring the period gives M, and then the distance follows from m – M = 5 lg d – 5; in 1923–1924 Hubble used Cepheids in the Andromeda nebula to show that it lies outside our Galaxy. A nova occurs in a binary system when a white dwarf pulls matter from its companion and the hydrogen on its surface burns explosively: brightness rises by ≈ 10 magnitudes but the star survives. A supernova is a star’s final explosion with a brightness comparable to a whole galaxy: type Ia – when a white dwarf reaches a limit of ≈ 1.4 M☉ (used as a standard candle for distances), type II – the collapse of a massive star’s core. Historic supernovae: 1054 (the Crab Nebula), 1572 (Tycho), 1604 (Kepler), and SN 1987A in the Large Magellanic Cloud in 1987.
Observing a variable star: choose a bright variable (for example Algol or δ Cephei) from the AAVSO or a planetarium program and, on several nights with an adult, estimate its brightness against nearby constant stars and plot it.