I know Edwin Hubble used Cepheid variables to prove the Andromeda Galaxy is not part of the Milky Way. But how did scientists at that time decide the boundary of the Milky Way?
And how did they find out both Andromeda and Milky Way are just the local group galaxies and the whole universe is much bigger than that?
In the early 1920s, mathematicians used Albert Einstein’s equations for general relativity to predict that the universe should be expanding. However, at the time, astronomers didn’t have the observations to settle the Great Debate about the size of the universe; some argued that the universe could not extend beyond the Milky Way.
The Great Debate
A fascinating milestone on determining the size of the universe occurred on just over a century ago on April 26, 1920. During The Great Debate, also known as the The Spapley-Curtis Debate, Heber Curtis and Harlow Shapley theorized on the true nature of the cosmos. At the heart of the issue was the nature spiral nebulae like Andromeda: were they a part of the Milky Way, as Shapley asserted, or separate island universes as Curtis claimed?
Ironically, it was Shapley who believed the Milky Way was vastly larger than accepted estimates. To him, objects such as Andromeda could not be outside our galaxy because of its size. Curtis accepted a more conservative, smaller Milky Way, and though he conceded that the Milky Way might be as large as Shapley claimed, nonetheless Curtis speculated that spiral nebulae and Andromeda were outside the Milky Way.
It was later in the same decade that Edwin Hubble measured Cepheid variable stars in Andromeda to establish Andromeda as a distant galaxy, separate from the Milky Way.
Discovery of Cepheid Variables
Henrietta Swan Leavitt worked as a human "computer" in the photometry department Harvard College Observatory. Leavitt was given the tedious task of cataloguing “variable” stars. In 1908, she published her results and noted that the brightness of a number of stars showed a pattern: brighter ones had longer periods. In 1912, Leavitt confirmed that the Cepheid variable stars with greater intrinsic luminosity also have longer periods, and that the relationship is close and predictable.
Leavitt’s discoveries gave astronomers a better idea as to the vastness of the cosmos.
History of the Local Group of Galaxies
In a paper published in 2003, Sidney van den Bergh outlined a History of the Local Group. He mentions the contributions of Edwin Hubble and others leading to the discovery of objects beyond the bounds of the Milky Way.
During the 1920s and '30s, Hubble had access to the Mount Wilson Observatory in California and the 100-inch Hooker Telescope. Hubble used the telescope to observe faint, cloud-like patches of light called nebulae at the time. In 1929, he published a paper, A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae in which he describes his use of distance and radial velocity techniques. Hubble remarked:
The apparent luminosities of the brightest stars in such nebulae are thus criteria which, although rough and to be applied with caution, furnish reasonable estimates of the distances of all extra-galactic systems in which even a few stars can be detected.
It was Hubble who introduced the term "Local Group" in his book The Realm of the Nebulae published in 1936. He described it as "a typical small group of nebulae which is isolated in the general field" and he assigned several nebulae to the group. Today, we know them as the Andromeda Galaxy, the Milky Way, the Triangulum Galaxy, Large Magellanic Cloud, Small Magellanic Cloud, M32, NGC 205, NGC 6822, NGC 185, IC 1613 and NGC 147.
Subsequent generations of astronomers have improved upon Hubble’s original methods and developed new ones. Although astronomers now make remarkably precise measurements of many more galaxies and stars, different measurement methods still produce dissimilar results.
