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The Hubble century

26 January 2000

The name of Hubble will be eternally linked with 20th-century astronomy. At the beginning of the 20th century astronomers knew of planets and stars, and anything else was called a
nebula (derived from the German word for fog). US astronomer Edwin Hubble was to change all that.

In his 1936 book The Realm of the Nebulae, Hubble said, “They are scattered
throughout space as far as telescopes can penetrate. We see a few that are large and bright. These are the nearer nebulae. Then we find them smaller and fainter, in constantly increasing
numbers, and we know we are reaching out into space… until, with the faintest nebulae that can be detected with the largest telescope, we arrive at the frontiers of the known universe. This
last horizon…is a vast sphere, perhaps a thousand million light-years in diameter.”

Until Hubble applied his eye to the new 100-inch telescope at Mount Wilson in 1919, we thought
that the universe consisted only of the Milky Way – our galaxy. Although a few bold astronomers talked of remote “island universes”, the nebulae were merely clouds on the astronomical
horizon. By harnessing the power of the world’s largest telescope, Hubble saw a star in the Andromeda nebula that was about a million light-years away, more than ten times the diameter
of the Milky Way. A curtain had been lifted on a much larger universe. Nebulae – now known as galaxies – are clouds of distant stars beyond the Milky Way.

Having drawn back
the curtain, Hubble’s work went on to show that this larger universe is continually expanding. In what we now call the “Hubble flow”, distant galaxies appear to be rushing away from us.
Hubble’s law states that the further away the galaxy, the faster it appears to recede. The constant of proportionality – the Hubble constant – fixes the age of the universe. Back in Hubble’s
day astronomers did not receive Nobel prizes.

The thick blanket of the Earth’s atmosphere totally blocks the light from the faintest and furthest stars. In 1990, the first major
astronomical satellite to be placed in orbit above this star-blocking layer was aptly named the Hubble Space Telescope. Initially hampered by faulty optics, this new eye on the universe has
gone on to reveal images of unparalleled beauty and significance and it has revolutionized optical astronomy in a way that is worthy of its namesake. The Hubble Space Telescope has
shown that the universe is about 12 thousand million light-years across. This is not that much bigger than Hubble had described in 1936, however, is far more spectacular than he could
have imagined from the images obtained from the Mount Wilson telescope.

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