In 1912 at the Lowell Observatory (est.1894) Slipher was analysing the spectra of spiral nebulae. He discovered that Andromeda was blueshifted and was travelling towards Earth at about 300 km/s. He found that many other nebulae were also moving at high speed and by 1925 had found spectral shifts for forty-five nebulae, forty-one of which were redshifted and therefore moving away from us.
Around 1924 Hubble discovered a faint object in Andromeda. The photographic records in the Mount Wilson Observatory archives revealed it to be a cepheid with a relatively lengthy period of about one month. But the longer the period of a Cepheid the greater is its luminosity. For this star to appear so faint it together with its nebula had to be at a great distance from Earth. Hubble calculated this distance to be ≈800,000 light years, thus proving that Andromeda was far outside the Milky Way. This was the first evidence of galaxies outside our own and soon the idea of separate ‘island universes’ began to be generally accepted.
Andromeda appeared to resemble the Galaxy except that the novae in Andromeda seemed to be fainter than those in the Galaxy. Knut Emil Lundmark (1889-1958) noted that these anomalies would disappear if Andromeda was twice as large and far more distant than as it was then thought to be. Ernst Julius Opik (1893-1985) used a ratio method in which he assumed Andromeda to have the same luminosity-to-mass as the solar area and determined the distance to Andromeda to be about 1,400,000 light years.
In 1929 Hubble, building on the work of Slipher and assisted by Milton La Salle Humason (1891-1972), established the relationship between redshift and distance of galaxies, known as Hubble’s Law. This showed that the Universe is expanding, and suggested that it had a definite origin at a certain moment in time. Hubble’s early measurements suggested that the Universe was a few thousand million years old.
Wilhelm Heinrich Walter Baade (1893-1960) worked with Hubble on studies of supernovae and the distances to other galaxies. In 1943 Baade identified two different populations of stars in Andromeda: Population-I are young hot blue stars and lie in the spiral arms, while Population II are older cooler redder stars and lie in the centre.
In 1948 Baade discovered that the Cepheid variables at the centre were four times fainter than those in the rest of the Galaxy. He then discovered that the period-luminosity relationships for each of the two populations were different; Hubble had compared Population II Cepheids in our Galaxy with Population-I Cepheids in Andromeda. In 1952 Baade recalculated the distance to Andromeda as two million light years. As Hubble’s measurement of 800,000 light years had been a key step in his calculation of the distances to the galaxies, all these distances were now doubled.
In 1958 Allan Rex Sandage (1926-2010) found that some of what Hubble thought were ‘bright stars’ in distant galaxies and used them as measuring rods were in fact nebulae lit by many stars. This more than tripled the size of the Universe and increased its estimated age to at least 13 billion (13×109) years.
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