I have been reading some books about Great Debate that whether the Milky Way Galaxy was the entire universe and Edwin Hubble settled the debate by identifying some Cepheid variable star (V1) in Andromeda that had 31 days period.

According to the period-luminosity relationship by Leavitt V1 should be much brighter than Hubble observed. So it must be too far way to be in the Milky Way Galaxy. But how did astronomers at Edwin Hubble time measure/decide the size of the Milky Way Galaxy and supported Edwin Hubble claim ? Did they agree on the size of Milky Way back then?

--- update ---

With answers I got so far I learned that Shapley and Curtis disagreed on the size of the Milky Way, which actually made me even more interested in this question because if they disagreed with the size of Milky Way how did Hubble's finding settle the debate ?

https://en.wikipedia.org/wiki/Harlow_Shapley said "Shapley used Cepheid variable stars to estimate the size of the Milky Way Galaxy" without many details. But Mr. Rennie's answer said Shapley used globular clusters to measure the size.

I also read articles like "Where is the edge of the Milky Way? Star discovery could redraw boundaries", which also makes me puzzled that if we just found the new boundary of Milky Way in 2014, how can astronomers in Hubble times agree on the size of Milky Way (with acceptable margin of error)?


3 Answers 3


You are right - there were two key differences between Harlow Shapely and Heber Curtis when they took part in the Great Debate. One was over the distance to the Andromeda nebula; the other was over the size of the Milky Way itself.

There are good accounts of the Great Debate of 1920 in the Wikipedia articles on the Great Debate and Harlow Shapley and this NASA page.

Shapley’s opponent in the debate, Heber Curtis, correctly identified Andromeda as a separate galaxy. But he believed the Sun was close to the centre of the Milky Way whereas Shapley correctly placed the Sun in the outer regions of the Milky Way. Shapley, on the other hand, believed the Milky Way to be much larger than it actually is.

Although Shapley believed at the time that the Andromeda nebula was part of the Milky Way, he watered down his arguments in the debate itself. It is generally believed this was because he thought becoming entangled in a controversy would damage his chances of being appointed as the next Director of the Harvard University Observatory.

Shapley was appointed Director of the Harvard University Observatory in 1921. Ironically, this was where Henrietta Swan Leavitt collected her extensive data on Cepheid variables and established their period-luminosity relationship. It was this relationship that led to Edwin Hubble conclusively proving in 1924 that Andromeda was too far away to be part of the Milky Way.

  • $\begingroup$ Hi thanks for answering my question. I would argue that your answer was actually a detailed version of my question, lol. I want to know how Edwin Hubble proved Andromeda was too far away to be part of the Milky Way, e.g did astronomers at that time agree on the size of Milky Way ? How did they measure the size of Milky Way ? I edited my question a little bit to my question clearer. $\endgroup$ Sep 22, 2021 at 8:37
  • $\begingroup$ @Qiulang In 1920 Shapley and Curtis disagreed on the size of the Milky Way. Swan Leavitt’s discovery of the period-luminosity relationship for Cepheid variables and Hubble’s observations of the populations of Cepheids in Andromeda and in the Milky Way established both the distance to Andromeda and the size of the Milky Way in 1924. $\endgroup$
    – gandalf61
    Sep 22, 2021 at 9:19
  • $\begingroup$ I can understand how Hubble used the period-luminosity relationship to calculate the distance to Andromeda. But how did Hubble (or someone else) use that to calculate the size of the Milky Way ? Can you further elaborate that ? $\endgroup$ Sep 22, 2021 at 9:37

To expand on Charles Tucker's answer, we can observe Cepheid variable stars both inside and outside the Milky Way. If we find the distances to all the Cepheids in the galaxy this gives us the size of the Milky Way.

There are some potential ambiguities. For example we cannot be absolutely certain that a Cepheid we observe in the Milky Way is actually in the Milky Way and not some distant Cepheid that happens to line up with the Milky way and shines through it. To get round this we observe many Cepheids that appear to be in the Milky Way and we find the vast majority are with a hundred thousand light years or so. This implies the Cepheids in this distance range are in the Milky Way and hence gives us the size of the Milky Way.

Also measuring Cepheids gives us a lower limit on the size since in principle there could be non Cepheid regions of the Milky Way that extend to larger distances. But again we observe lots of Cepheids and find they are roughly uniformly distributed throughout the galaxy.

The way this was actually done was that Shapley measured the distance to the globular cluster M13. I think he did this by observing a Cepheid in M13 though surprisingly I cannot find a definitive statement on this. Anyhow, Shapley then assumed:

  1. the globular clusters were all roughly the same size so knowing the distance to M13 the distance to the other globular clusters can be found from their brightness.

  2. the globular clusters like on the edge of the Milky Way so measuring their distance determines the distance to the edge of the Milky Way and hence the size of the Milky Way.

Shapley then determined the distances to all the globular clusters and found they lay at distances that enclosed a disk of diameter about 300,000 light years. He took this to be the diameter of the Milky Way.

  • $\begingroup$ Hi thanks for answering my question and actually that was what I thought: if we can find some Cepheid variable star that has 31 days period and it is much brighter than the one Edwin Hubble observed in Andromeda then we can safely concluded Andromeda is another galaxy. But I would like to know if astronomers at Edwin Hubble time actually did that ? (And hence my question). $\endgroup$ Sep 22, 2021 at 7:43
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    $\begingroup$ @Qiulang I have extended my answer to explain what the experiments done at the observatory were and how they established the distance. $\endgroup$ Sep 22, 2021 at 8:39
  • $\begingroup$ Thanks again. Coincidentally I recently read an article sciencealert.com/…, so if "Astronomers Think They Just Found The Edge of The Milky Way Galaxy" in 2020 how did Shapley determine the edge of Milky Way back then. Sorry I seem to raise another question. $\endgroup$ Sep 22, 2021 at 8:51
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    $\begingroup$ @Qiulang the Milky Way does not have a well defined edge because the density of visible matter like stars tapers off gradually as we move outwards, and the density of dark matter tapers off even more slowly. So the "size" depends on how you define the where the edge is. Shapley defined the edge to be the location of the globular clusters, so measuring the distance to the globular clusters defined the distance to the edge. The article you link discusses how much bigger the Milky Way is if we change the definition of the "edge" to include the dark matter. $\endgroup$ Sep 22, 2021 at 9:01
  • $\begingroup$ Hi I did some further research about globular clusters (I am a layman) I still don't understand why the location of globular clusters can be defined as the edge of Milky Way. I also updated my question. $\endgroup$ Sep 23, 2021 at 11:12

They used period-luminosity-relationship of cepheids, which was found by Leavitt, see https://en.m.wikipedia.org/wiki/Henrietta_Swan_Leavitt.

  • $\begingroup$ Yes I also read Leavitt's story but I have not found how astronomers at that time decided the size of Milky Way. Can you provide more information ? $\endgroup$ Sep 22, 2021 at 7:15
  • $\begingroup$ The link you provided say "As a result of this, it is now known that our own galaxy, the Milky Way, has a diameter of about 100,000 light years." But I want to know how. $\endgroup$ Sep 22, 2021 at 7:17

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