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| Georges Lemaître |
Let us look at a
little history.
In various places in
the sky, but especially in the constellation of Cepheus, where the first case was
discovered, there are some stars whose light intensity varies regularly, and
therefore are called variable Cepheids. In 1908, the American
astronomer Henrietta Swan Leavitt discovered that the period of these stars is
linked with their real luminosity: the greater the luminosity, the longer the
period. Therefore, by measuring their period, their real luminosity can be
deduced.
In 1913, the
American astronomer Vesto Melvin Slipher obtained the spectrum of what was then
called the Andromeda
nebula (the giant galaxy closest to ours) and discovered a blue
shift that indicated (according to the Doppler effect) that the nebula moves
towards us with a speed of about 300 kilometers per second, much higher than
expected. Slipher then studied the light of other spiral nebulae and made the
unexpected discovery that most of them, unlike Andromeda, show redshifts, that
is, they move away from the solar system with great speed. In fact, he measured
speeds above 1000 kilometers per second.
In 1919, the
American astronomer Edwin Powell Hubble used the Mount Wilson telescope to
photograph several spiral nebulae, including Andromeda, and showed that, actually,
they were not nebulae, as had been believed, but huge clusters of stars. From
then on they were no longer called nebulae, but galaxies, in honor of our Milky
Way, which also belongs to the class of spiral galaxies. Galactos in Greek means milk.
