The study of evolution of the central stars of PN is motivated by the observed distribution of PN in the H-R diagram. Using Shklovsky distances (Section 16.1.1) and H Zanstra temperatures (Section 7.1.1), O'Dell (1963) was the first to construct a luminosity-temperature diagram for the central stars of PN. Similar results were obtained by Harman and Seaton (1964) and Seaton (1966) using Hell Zanstra temperatures. They found that both low(T* = 30,000 K) and high temperature (105 K) central stars had low luminosities (102 L⊙) and that the intermediate temperature (50,000 K) stars have high luminosities (104 L⊙). Apparently, the distribution of PN in the H-R diagram had the shape of an upside-down horse shoe, which was interpreted as an evolutionary sequence and named the Harman-Seaton sequence.

Harman and Seaton (1964) suggested that PN were formed at the end of the horizontal branch, followed by a rapid increase in temperature and luminosity of the central star. Such rapid evolution in 104 yr (the dynamical age of PN) posed a great challenge to theorists. Early efforts have concentrated on remnant stellar cores without nuclear burning, and undergoing gravitational and thermal adjustments while contracting toward the WD stages. Although artificial models with specific initial conditions could be made to mimic the Harman-Seaton sequence, the results are far from satisfactory (cf. Salpeter, 1968; Shaviv, 1978).