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I am thinking about the Bright Star Catalogue and how it is not a good representation of a typical population of stars. The Bright Star Catalogue includes all the visible stars from Earth when viewing with the naked eye. Therefore, it only includes higher mass stars at various stages of evolution since these are more easily seen; however, most stars in any stellar population will be of lower mass. Lower mass stars are much more plentiful in comparison (see initial mass function/IMF for details). The Bright Star catalogue is limited to stars ~6.5 apparent magnitudes and brighter (so lower numbers since magnitudes run in the "opposite sense". ie. brighter = a lower number). The question is, which star is at the seeing eye limit (~6.5 - 7 apparent magnitudes), yet has the lowest absolute magnitude? Another way of phrasing this question is, what is the lowest mass star we can see from our Earth with the naked eye? I am not looking to receive an answer about how the answer depends on absolute magnitude and distance (ie. the distance modulus). I understand that already. I am curious what the star actually is (ie. name etc).

Edit to add for clarity:

Luminosity does not increase monotonically with mass. For the sake of this question, let's stick with the faintest absolute magnitude main-sequence star.

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  • $\begingroup$ Not sure that there is a definitive answer - as the limiting magnitude depends on both, you geographic location, the weather conditions and especially the quality of your eye sight. So if you define the magnitude you can still see, it's a simple star catalogue query. $\endgroup$
    – planetmaker
    Commented Aug 2, 2021 at 16:47
  • $\begingroup$ I'm looking for an answer that does not depend on fine details such as seeing and geography. Assuming typical conditions, what is the lowest mass star anyone in any hemisphere would likely see from Earth? Let's say 7 magnitudes and above. $\endgroup$
    – Astroturf
    Commented Aug 2, 2021 at 16:49
  • $\begingroup$ Surprisingly more difficult as sinbad does not have stellar masses. Assuming main sequence and the later the spectral type, the lower the mass, this query will give you a list of K-type stars with a magnitude brighter than 6. I'd focus on the two K5V stars and see which is lowest mass. $\endgroup$
    – planetmaker
    Commented Aug 2, 2021 at 17:24
  • $\begingroup$ simbad.u-strasbg.fr/simbad/… $\endgroup$
    – planetmaker
    Commented Aug 2, 2021 at 17:24
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    $\begingroup$ 61 Cygni must be in the running... $\endgroup$
    – James K
    Commented Aug 2, 2021 at 19:44

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WP says:

The brightest-known M-class main-sequence star is M0V Lacaille 8760, with magnitude 6.7 (the limiting magnitude for typical naked-eye visibility under good conditions is typically quoted as 6.5), and it is extremely unlikely that any brighter examples will be found.

I think what this really tells us is that there is no perfectly well-defined answer to this question. Looking at planetmaker's linked search, it seems like as you go from K0 to M0, it gets more and more doubtful/fuzzy whether the human eye can see any star at this magnitude.

It sounds like it would be a super fun project -- but not an easy one -- to try to observe M0V Lacaille 8760. You'd probably want to recruit a young person, create a very detailed finder chart, and go to a remote dark-sky location at high elevation. You'd probably need to create some pretty elaborate protocol to make sure that the person really did see it. Maybe you'd need to do it as a blind or double-blind experiment, with controls where the person was told to look for a star where there was none.

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    $\begingroup$ +1 Note that the linked search explicitly limits to luminosity class V, thus main sequence, thus lower mass. Non - main sequence stars, thus higher luminosity class, all must be heavier, as the main sequence lifetime of K and M dwarfs is longer than the universe's current age. $\endgroup$
    – planetmaker
    Commented Aug 2, 2021 at 22:45
  • $\begingroup$ A significant issue here is that stars lose mass through their lifetime. The sun will have a mass of 0.5 to 0.55 stellar masses by the time it’s a white dwarf, which is lighter than Lacaille. An initial 0.9 solar mass star would become a little lighter than that, this leaves the possibility of a giant star being lighter $\endgroup$
    – blademan9999
    Commented Apr 4, 2023 at 3:09

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