The Wikipedia article on the Milky Way states that it has an estimated 100 to 400 billion stars. The one on the Andromeda states that it has about a trillion stars. With this, we can conclude that the Andromeda has between 2 and 10 times the number of stars than the Milky way.

But then there are articles like this that state that its about the same mass as the Milky Way.

The only way for both of these facts to be correct is that the stars in Andromeda are somehow lighter than the ones in the Milky way. But there is no good reason to expect this I can think of, especially over the kinds of massive numbers of stars we're talking about. If the process of star formation is equivalent, it should produce a similar distribution of stars in the two places and also the same average mass.

So, is this simply because the estimates in the Wikipedia articles are outdated? If so, what are the latest numbers?

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    $\begingroup$ This article (found after posting): forbes.com/sites/startswithabang/2019/03/14/… sheds some light on this. $\endgroup$ Sep 5, 2022 at 22:03
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    $\begingroup$ Most of the mass of galaxies isn't in stars. $\endgroup$
    – notovny
    Sep 5, 2022 at 23:50
  • $\begingroup$ So the Milky Way has much more diffuse gas and dark matter than the Andromeda? $\endgroup$ Sep 6, 2022 at 0:20

1 Answer 1


TL;DR - things are complicated and we're confused.

The article linked in the comments of the question summarizes this. It seems it's very hard to estimate the number of stars in the Milky Way since we're inside it. Hence, the wide estimate of between 100 and 500 billion stars. The Andromeda has about a trillion stars. However, those stars seem to be older, lighter and the general rate of new star formation there is lower.

For the Andromeda, measuring its mass seems to be much more difficult than the Milky Way. Quoting the relevant text:

Although there are large uncertainties, the total mass estimates of Andromeda range from about 800 billion solar masses up to 1.5 trillion solar masses. These estimates are so different because they're arrived at by using different techniques, which poses an interesting puzzle at present.

So somehow, the Milky Way and Andromeda seem to be either fundamentally different species of galaxies or at different stages of their evolution.

Summing it all up, it looks like the Milky Way may be larger in extent and more massive in nature than we realized, while Andromeda may be more diffuse, spread-out, and less massive than we previously suspected.

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    $\begingroup$ Please edit your answer to make it clear whether you are talking about stellar mass or total mass (which is dominated by dark matter) . $\endgroup$
    – ProfRob
    Aug 9, 2023 at 18:48

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