Hubble- the Greatest Astronomer of all Time? Part 1

Having written about Charles Messier, it led me to wonder who the greatest astronomer of all time is. Edwin Hubble (1889-1953), after whom the Hubble Space Telescope was named, must be one of them.

In the Constellation of Andromeda there is a large patch of light, which used to be called a nebula, meaning a cloud of gas in the Milky Way. Historically it was thought that our galaxy, the Milky Way, was all that existed: it was the Universe. Gradually there was speculation that there might be galaxies beyond our own and that the Andromeda Nebula might be one of them. Astronomers had worked out the approximate size and shape of our galaxy; diameter about 100,000 light years. If it could be proven that anything was much further away, it had to be outside the Milky Way.

There is a class of stars which vary in brightness and are called Cepheid Variables (CVs). Whilst most stars remain at a constant luminosity, a CV varies in brightness over a period of time from 4 days to over 50. The period of a CV is the time between the occasions when it is at its most luminous, with a dip in brightness in between. In 1910 an American astronomer, Henrietta Swan Leavitt, discovered that there is a direct relationship between the luminosity of a CV and its period: the longer the period the more luminous the star. The Apparent Magnitude of a star is its brightness when viewed from Earth: its Absolute Magnitude is its brightness as seen by an observer near the star. With a CV, if we know its period we know how bright it actually is (its Absolute Magnitude). If that is compared to its Apparent Magnitude, we can tell how far away it is by using well established physics.

In the 1920s Edwin Hubble observed the Andromeda Nebula through the massive Hooker Telescope on Mount Wilson, near Pasadena. He observed CVs in the Nebula, and worked out their Absolute Magnitude, which indicated that they were about 900,000 light years distant, well outside the Milky Way. By 1929, it was accepted that Hubble had proven that the nebula was a separate galaxy, as was the Triangulum Galaxy, which he had also observed. His calculation of distance to the Andromeda Galaxy was an underestimate, as there are two classes of CV depending on the age of the star. The galaxy is in fact 25,000,000 light years away. He had established the principle of galaxies outside our own. But that was not the end of his work. (to be continued in Part 2)

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