Edwin Powell Hubble was an American astronomer who was born on the 20th November 1889 in Marshfield, Missouri. He died on the 28th September 1953. His father, John Powell Hubble, worked in the insurance industry and his mother was a housewife; they had eight children.
He played a crucial role in establishing extragalactic astronomy and is the leading observational cosmologist of the 20th century. He showed that many objects previously thought to be nebulae (clouds of gas and dust) were actually galaxies. He also developed Henrietta Swan Leavitt’s discovery of the strong relationship between a Cepheid variable’s luminosity and its pulsation period to scale galactic and extragalactic distances.
He is perhaps best known for the Hubble telescope being named after him and for Hubble’s Law, which implies that the Universe is expanding.
In 1906 Hubble won a scholarship to the University of Chicago which he graduated from in 1910 and was selected as a Rhodes Scholar from Illinois. His BA was in Jurisprudence.
Returning to the US in 1913, he taught for a year and then went to the University of Chicago to study astronomy. He managed to finish his dissertation on ‘Photographic Investigations of Faint Nebulae’ in 1917 just before enlisting in the US Army. At the end of the Great War he studied reflection nebulae at Mount Wilson, where the largest telescope in the world was situated, before returning to spiral nebulae, which he had studied for his doctorate.
He found Cepheid variable stars in the Andromeda nebula (the original name for the Andromeda galaxy (M31)) and using the relationship between the Cepheid’s luminosity and fluctuations, calculated its distance away as being 900,000 light-years. As this was further than the size of the Milky Way it clearly had to be a galaxy and not a nebula. Due to improved Cepheid period-luminosity relationship calculations the Andromeda galaxy is currently thought to be 2.48 million light-years away.
Between 1928 and 1936, working at Mount Wilson with the 2.5m Hooker reflector telescope, Hubble linked up with Milton Humason, who measured the spectral shifts of galaxies and Hubble calculated their distances. He knew the distances to nearby galaxies from measurements of the Cepheid variable stars, but had to use other methods for the more distant ones. In particular he used the apparent brightness of galaxies as an indication of their distance. By 1929 Hubble concluded that the recessional velocity of a galaxy is proportional to its distance from us. This became known as Hubble’s Law.
It is quite easy to see how this is evidence of an expanding universe. Imagine you are on the edge of a balloon and that bursts. The pieces are flung out from each other and from the one you are on all other pieces seem to be getting further away and travelling faster.
The greater the distance to a galaxy the greater its recessional velocity. If Hubble’s Law is true and the only physical cause of redshift is recessional velocity then redshift measurements can be used to give distances to distant galaxies.
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