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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?

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    $\begingroup$ How much history of astronomy have you read? If you are interested in astronomy this is the sort of basic fundamental reading better answered using Wikipedia or similar. $\endgroup$
    – Rory Alsop
    Sep 8, 2023 at 15:29
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    $\begingroup$ This one astronomy.stackexchange.com/questions/5994/… didn't answer my question but they are related. $\endgroup$
    – Qiulang
    Sep 9, 2023 at 8:36
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    $\begingroup$ @RoryAlsop after reading the first answer to this question - I'm not convinced that "basic fundamental reading better answered using Wikipedia or similar" is actually true here. Which Wikipedia article or articles actually answers this as well as the current, excellent SE answer here? $\endgroup$
    – uhoh
    Sep 10, 2023 at 2:08
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    $\begingroup$ @uhoh - having a better answer here (and it is an excellent answer) doesn't invalidate my initial clarification question. I managed to find out fairly quickly - basics like this are well documented. $\endgroup$
    – Rory Alsop
    Sep 10, 2023 at 17:30
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    $\begingroup$ @RoryAlsop well, it wouldn't be Stack Exchange if we could only ask questions who's answers could only draw from sources that were poorly document. But you are right in that questions should in general make an effort to include prior research as our friend the tooltip advises, (note I'm the lone up-voter (I don't always take my cues from M. Tooltip) but interestingly when I hover over the down arrow it looks far more highlighted than the up) Where did you find fairly quickly how scientists decided the boundary of the Milky Way at Edwin Hubble times? $\endgroup$
    – uhoh
    Sep 10, 2023 at 19:58

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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.

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    $\begingroup$ Nice answer. It would be worth adding that H. S. Leavitt developed a method for determining the distance to the cepheids, known as a "standard candle," which became an important tool in modern astronomy. This method was used by Hubble to resolve the Great Debate. Her contributions to astronomy are immense as they enabled us to completely revolutionize our understanding of the Universe. See here and references therein en.wikipedia.org/wiki/Henrietta_Swan_Leavitt#Scientific_impact $\endgroup$ Sep 9, 2023 at 22:22
  • $\begingroup$ Yes it is a good answer just that it did not really answer my question. But thanks. $\endgroup$
    – Qiulang
    Sep 10, 2023 at 2:08
  • $\begingroup$ @Qiulang I understood your interest in astronomy at the time of Hubble's use of Cepheid stars as the first step in determination of scale of the universe. The Shapley-Curtis Debate demonstrated the limited consensus concerning the size of the Milky Way, the true nature of spiral nebulae (we call then galaxies), and the validity of Leavitt's standard candle (now known as intrinsic luminosity). The answers to Where does the Milky Way end? reflect current consensus, but are they contemporary to Hubble's Andromeda measurements? $\endgroup$
    – creidhne
    Sep 10, 2023 at 3:28
  • $\begingroup$ Yes I alway also added this Q&A in my comments. But I still don't know the answer to my second question, how did sicentists find out both Andromeda and Milky Way were just the local group galaxies and the whole universe is much bigger than that ? $\endgroup$
    – Qiulang
    Sep 10, 2023 at 4:25
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    $\begingroup$ @Qiulang I added Hubble's introduction of the term "Local Group" and some references to how astronomers measured distances to the observed nebulae. Determination of the size of the universe was and is a complex evolution of discoveries and observations. I hope this helps answer your question. $\endgroup$
    – creidhne
    Sep 10, 2023 at 17:33

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