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It is often said that the Sun is a medium size, or even dim star. Is this true?

According to this list of stars within 21 light years, there are, out of the closest 121 stars, only six brighter than the Sun. This means that the Sun is in the top 6%. If you also count brown dwarfs, the Sun ranks even higher.

Doesn't this indicate that the Sun is actually a relatively large star?

7/121

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  • $\begingroup$ I think your comment is fundamentally correct. It used to be said that the sun was a medium sized/small star based on preliminary observations. Better telescopes reversed that observation as many many many more smaller stars became observable. It's worth pointing out that very few of the stars we can actually see in the sky are smaller than the sun. None of the visible stars are red dwarfs or smaller. A similar correction was made on binary stars being the majority. $\endgroup$
    – userLTK
    Jan 10, 2016 at 3:44
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    $\begingroup$ Define your terms. By "medium" do you mean mean or median? By "size" do you mean mass or radius? $\endgroup$
    – Mike Scott
    Jan 10, 2016 at 6:41
  • $\begingroup$ @MikeScott Just give me any size metric that actually makes the Sun a medium sized star. As for "medium", I meant median. $\endgroup$
    – SE - stop firing the good guys
    Jan 10, 2016 at 13:25
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    $\begingroup$ The Sun s about 3 times more massive than a median field star and is more luminous than about 90% of stars. It is only typical in the sense of being on the main sequence, where most stars are. $\endgroup$
    – ProfRob
    Jun 2, 2016 at 11:31
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    $\begingroup$ See also: http://physics.stackexchange.com/a/262732/59023 and http://physics.stackexchange.com/a/262909/59023. Rob's answer is much more informative, as should be expected (as far as I am concerned, he is the star whisperer on these SE sites). $\endgroup$
    – honeste_vivere
    Aug 18, 2016 at 12:27

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It is true that a surprisingly large number of stars are smaller (and thus less massive) than the Sun. However, the stars that are bigger than the Sun are often much bigger.

Look at this chart:


Image courtesy of Wikipedia user Jcpag2012 under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Notice how small the Sun is compared to some of the other stars. It's tiny! It is indeed a small star - in technical terms a main sequence dwarf.

However, despite its size, it is clear that there are many more stars less massive than the Sun that there are stars more massive than the Sun. Why? There are two reasons:

  1. Lower-mass stars live longer.
  2. More low-mass stars can form in a given region than high-mass stars.

Encyclopedia of Astronomy and Astrophysics

The distribution of masses can be quantified in an initial mass function, typically given in the form $$\xi(m)=km^{-\alpha}$$

When you integrate this over a range of masses, you can find how many stars are within that range. Not surprisingly, this number gets lower and lower as you slide the endpoints to more massive stars. You can see this decrease from the fact that $\xi'(m)<0$, so long as $k>0$ and $\alpha>0$ - which is assumed by the model, according to empirical data.

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    $\begingroup$ See also what-if.xkcd.com/83 : "Our Sun isn't a grain of sand on a soft galactic beach; instead, the Milky Way is a field of boulders with some sand in between." $\endgroup$
    – j-g-faustus
    Jan 9, 2016 at 23:00
  • $\begingroup$ Sorry, but I do not really like this answer because it focuses on spectacular and very rare objects too much. I tend to agree what Sun is medium sized among main-sequence stars. $\endgroup$
    – Free Consulting
    Jul 28, 2016 at 21:05
  • $\begingroup$ @FreeConsulting What do you mean by "spectacular and very rare", and why do you object to it? $\endgroup$
    – HDE 226868 ♦
    Jul 28, 2016 at 21:11
  • $\begingroup$ Because Sun is rather large compared to most abundant stars. $\endgroup$
    – Free Consulting
    Jul 28, 2016 at 21:29
  • $\begingroup$ @FreeConsulting I'm aware of that, and stated as much: However, despite its size, it is clear that there are many more stars less massive than the Sun that there are stars more massive than the Sun. $\endgroup$
    – HDE 226868 ♦
    Jul 28, 2016 at 21:34

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