Hubble, EdwinPowell

Hubble, EdwinPowell (1889–1953)Americanastronomer. He first studied nebulae, concluding in 1917 that the spiral-shaped ones (which we now know as galaxies) were different in nature from diffuse nebulae, which he found to be gas clouds illuminated by stars. From 1923, using the 100-inch (2.5-m) telescope at Mount Wilson Observatory, he resolved the outer regions of the spiral nebulae M31 and M33 into stars, identifying over 30 Cepheid variables in them. This proved that such ‘nebulae’ were truly independent star systems like our own—other galaxies. In 1925 he devised the so-called tuning-fork diagram of galaxies, dividing them into ellipticals, spirals, and barred spirals, which he wrongly believed to indicate an evolutionary sequence. By 1929 Hubble had good distance measurements for over twenty galaxies, including members of the Virgo Cluster. By comparing distances with their velocities, as revealed by the redshifts in their spectra, he concluded that galaxies were receding with speeds that increased with their distance, a relationship known as the Hubble law. This was powerful evidence that the Universe is expanding.