Edwin Hubble and The Expanding Universe

Edwin Powell Hubble (1889-1953)

An Ocean of Galaxies

Henrieta Swan Leavitt (1868-1921), studying Cepheid variables in the Small Magellanic Cloud, became aware of a relation between the Cepheids’ absolute magnitudes and their periods of fluctuation, a notion later developed by the Danish astronomer Ejnar Hertzsprung (1873-1967). The ability to determine the absolute magnitude of a Cepheid variable is important for two reasons: Firstly, if you can determine the absolute magnitude of one Cepheid variable based on its period of fluctuation, then you could infer the absolute magnitudes of all of them. Secondly, if you know a Cepheid’s absolute magnitude you can use the difference between its absolute magnitude and its apparent magnitude to calculate its distance, by taking into account the inverse-square law.

Harlow Shapley (1885-1972), using Cepheid variables in determining distances to globular clusters in our own galaxy, estimated the Milky Way to be approximately 300,000 light-years in diameter. Even though Shapley’s estimate for the size of the Milky Way was considerably off–the actual size being approximately 80,000-100,000 light-years across–the principle of determining distances by using Cepheid variables was, nevertheless, sound.

Cepheid Variable in M31

Image Credit: Mount Wilson Observatory’s History page

Edwin Hubble, during his initial examination of a plate (above) taken of M31, the Andromeda nebula, on the night of October 5-6, 1923, with the 100-inch telescope at Mount Wilson Observatory, located three novae and marked each with an “N”. Later, while examining the plate again and comparing it with other plates taken earlier of the same region, Hubble noted a periodic fluctuation in brightness and realized that one of the novae was actually a variable star, and in tremendous excitement over his discovery, he crossed out the “N” and wrote “VAR!” in its place. He further observed, upon reexamination, that the variable in question conformed to the characteristic pattern of Cepheid variables. Armed with this new knowledge, Hubble was able to build on the work of Shapley and calculate the distance to the Andromeda nebula. Hubble calculated the distance to be 900,000 light-years–the actually distance being around 2.2 million light-years. Even though Hubble’s estimate fell short of the actual distance, it was still well outside the boundary–even as calculated by Shapley–of the Milky Way, and it proved that the Andromeda nebula was a galaxy outside of our own galaxy.

The 100" Hooker Telescope

Image Credit: Mount Wilson Observatory’s History page

The 100-inch Hooker telescope (above) at the Mount Wilson Observatory was used by Edwin Hubble to make his historic discovery of Cepheid variables in the Andromeda galaxy. The telescope was named after John D. Hooker who donated the cost of the mirror. In 1981, the American Society of Mechanical Engineers dedicated the telescope as an International Historical Mechanical Engineering Landmark.

Edwin Hubble at the 100-inch Hooker telescope

Image Credit: Mount Wilson Observatory’s History page

The Expanding Universe

Vesto Slipher of Lowell Observatory, measuring Doppler shifts of distant galaxies, had made the startling discovery that these galaxies were receding from us at incredible velocities. This work was later continued by Edwin Hubble’s assistant Milton Lasalle Humason, and by comparing the distances of the galaxies that he had measured with their radial velocities, as measured by Slipher and Humason, Hubble realized that the further a galaxy was from us the faster it was receding. Hubble also noted that the observed galaxies were moving away from us in all directions. These realizations allowed Hubble to arrive at the idea that the universe was expanding and at a rate which is known as the Hubble constant. The original number that Hubble came up with for the Hubble constant is considered somewhat greater than what it is presently believed to be. The current estimate of the Hubble constant is between 70-74 kilometers per second per megaparsec with a total uncertainty of 10%.

Copyright © 2010 Eric F. Diaz

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