<P> In 1917, Heber Curtis observed nova S Andromedae within the "Great Andromeda Nebula" (as the Andromeda Galaxy, Messier object M31, was then known). Searching the photographic record, he found 11 more novae . Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our galaxy . As a result, he was able to come up with a distance estimate of 150,000 parsecs . He became a proponent of the so - called "island universes" hypothesis, which holds that spiral nebulae are actually independent galaxies . </P> <P> In 1920 a debate took place between Harlow Shapley and Heber Curtis (the Great Debate), concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the Universe . To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift . </P> <P> In 1922, the Estonian astronomer Ernst Öpik gave a distance determination that supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object . Using the new 100 inch Mt . Wilson telescope, Edwin Hubble was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way . In 1936 Hubble produced a classification of galactic morphology that is used to this day . </P> <P> In 1944, Hendrik van de Hulst predicted that microwave radiation with wavelength of 21 cm would be detectable from interstellar atomic hydrogen gas; and in 1951 it was observed . This radiation is not affected by dust absorption, and so its Doppler shift can be used to map the motion of the gas in our galaxy . These observations led to the hypothesis of a rotating bar structure in the center of our galaxy . With improved radio telescopes, hydrogen gas could also be traced in other galaxies . In the 1970s, Vera Rubin uncovered a discrepancy between observed galactic rotation speed and that predicted by the visible mass of stars and gas . Today, the galaxy rotation problem is thought to be explained by the presence of large quantities of unseen dark matter . A concept known as the universal rotation curve of spirals, moreover, shows that the problem is ubiquitous in these objects . </P>

Who is the first person to classified galaxies into groups according to their structure