It has been argued1 that the extremely faint central stars of some planetary nebulae must be very hot, with most of their energy output in the ultraviolet. We now report the detection of the central star of the planetary nebula NGC2440 in a narrow-band continuum image using a CCD (charge-coupled device) camera at the prime focus of the Anglo-Australian telescope. Its visual magnitude has been measured as 18.9±0.2. Its Zanstra temperature2,3 is about 350,000 K, so that it is one of the hottest stars ever observed. Its radius corresponds to that of a nearly degenerate star. Theoretical calculations can explain the observed temperature and luminosity if it has a mass of at least 1.0 M⊙ and is in its cooling stage, but the predicted age of the nebula is considerably less than that required by the calculations. Furthermore, the present estimates of the progenitor mass of a 1-M⊙ white dwarf are brought into question.