Hubble's original estimate of the constant now bearing his name, based on observations of Cepheid variable stars as " standard candles " to measure distance, was 500 (km/s)/Mpc (much larger than the value astronomers currently calculate). Later observations by astronomer Walter Baade led him to realize that there were distinct " populations " for stars (Population I and Population II) in a galaxy. The same observations led him to discover that there are two types of Cepheid variable stars with different luminosities. Using this discovery, he recalculated Hubble constant and the size of the known universe, doubling the previous calculation made by Hubble in 1929. He announced this finding to considerable astonishment at the 1952 meeting of the International Astronomical Union in Rome.