Introduction

The galaxy M33 (= NGC598) is the third-brightest member of the Local Group. It is a late-type spiral of type Sc II–III. The large angular size of the Triangulum galaxy, and its intermediate inclination i = 56° (Zaritsky, Elston & Hill 1989), make it particularly suitable for studies of spiral structure and stellar content (see Figure 5.1). Only M31, the Magellanic Clouds, and the tidally disrupted Sagittarius dwarf have larger angular diameters than the Triangulum galaxy. The spiral nature of this galaxy was first hinted at by visual observations made by the Earl of Rosse (1850).

The modern era of exploration of M33 began with the independent discovery of variable stars in this object by Duncan (1922) and by Wolf (1923). Neither of these papers show any indication that these authors anticipated the revolutionary impact that the discovery of variable stars in nearby “nebulae” would soon have on Man's view of the Universe. In the words of Hubble (1926) “The nature of nebulae and their place in the scheme of the universe have been favorite subjects of controversy since the very dawn of telescopic observations.” Hubble writes that his investigation “followed naturally upon the partial resolution of Messier 33 into swarms of actual stars.” He concluded that “The data lead to a conception of the object as an isolated system of stars and nebulae, lying far outside the limits of the [G]alactic system.” In his paper Hubble was able to show that 35 of the variables in M33 were classical Cepheids, thus demonstrating, beyond reasonable doubt, that galaxies were “island universes, ” and ending the reat debate (Heatherington 1972, Hoskin 1976) on the nature of spiral nebulae.