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It seems very logical, to call the star, closest to earth, "Proxima" ("Proximus" is Latin for "close" and the word for star, "stella", is feminin, hence "Proxima"), but there's one thing I don't understand: in order to call a star ±"The closest one", you must be sure that in future, no closer star will be found.
This is not so simple: lots of very close stars, like Barnard's star, Kapteyn's, ..., are very weak and even not visible to the naked eye.
So, how can we be that sure that Proxima is indeed the nearest one? Have astronomers indeed examined the complete sky in order to prove this?

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    $\begingroup$ "Atom" means indivisible. But after atoms were discovered and named, they were found to be divisible. Science is full of premature naming. $\endgroup$ – Anders Sandberg Nov 22 '18 at 11:37
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So, how can we be that sure that Proxima is indeed the nearest one? Have astronomers indeed examined the complete sky in order to prove this?

The minimum absolute magnitude for a star is, according to this well-known diagram about +16. Proxima Centuari is at the very low end of this scale with an absolute magnitude of +15.60. Still, it has an apparent magnitude of +11.13. That means that if a star is closer to us, it needs to be +11.5 or brighter. Every object of that magnitude or brighter has been categorized, and a hypothetical close star would've stood out due to its large parallax. So it's safe to say there are no closer stars ... at this moment.

you must be sure that in future, no closer star will be found.

Stars move (they rotate around the galaxy center), so in the (far) future, there might be a star closer to use than Proxima Centauri, just like sometimes Venus is the planet closest to Earth, and sometimes Mars. So it's possible that Proxima might not be the nearest star in the far future. This is not something we have to worry in our lifetimes, though :)

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  • $\begingroup$ Thanks for your quick answer. My concern was not about the far future, with possible other stars moving by, but about the current situation. Your analysis seems correct, in the following sense: in case a star is closer but so weak that the absolute magnitude is larger than +16, this would reveal other questions about stars' clarity. In case the star is obfuscated by something (like a dust cloud), then we might expect such a cloud being detected already by other means. $\endgroup$ – Dominique Nov 22 '18 at 13:02
  • $\begingroup$ Yeah, I thought it was interesting to mention but it might've come off as a little snarky. I hope it's better this way. Yes, it's theoretically possible to have a dark matter dust cloud to cover a closer star; but even then, we might be able to measure its gravitational influence. $\endgroup$ – Glorfindel Nov 22 '18 at 13:11
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    $\begingroup$ @Glorfindel a “dark matter dust cloud”?? Given our current understanding of dark matter, it’s exceedingly unlikely there’s a “cloud” of it in our immediate stellar neighbourhood, and even if there were, dark matter doesn’t interact with light so it couldn’t cloak a star. $\endgroup$ – Chappo Hasn't Forgotten Monica Nov 23 '18 at 10:14
  • $\begingroup$ Agreed, my "theoretically possible" implied the same as your "exceedingly unlikely". I mean, there are still so many things to be discovered in the universe but things of that scale won't be found at our doorstep. $\endgroup$ – Glorfindel Nov 23 '18 at 10:20
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A 1928 paper mentions this very point:

Iɴɴᴇs has suggested the name Proxima Centauri for this star. Since it is the nearest of the three components of the system, the name is most appropriate and may well be retained even though some star nearer the Sun be subsequently discovered in the same constellation.

There are a couple of points to note here. "Proxima Centauri" can be taken to mean "close one of the Centaur". So a more close star in a different constellation would not invalidate the name.

Innes, the discover of the star, was hopeful that his discovery was important. He attempted to make a distance measurement by parallax (seeing how far the star appears to move as the Earth orbits the sun.) The value for the distance he published was slightly smaller than the accepted distance of Alpha Centauri, but the error bars in he measurement meant that in truth he did not know if it was slightly nearer, or slightly further from the Sun. Nevertheless he suggested:

If this small star had a name it would be convenient—it is therefore suggested that it should be referred to as Proxima Centaurus. (quoted)

This is basically grandstanding. Innes hoped that his star might turn out to be the very closest star and suggested that name. In 1928, it was established to be the closest star in the Alpha centauri system, the name was set and seen to be appropriate, even if some other star was later discovered to be closer.

In the future other stars will come closer. A star called Ross 248 will briefly be the closest star in about 30000 years time.

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  • $\begingroup$ and indeed, by the time other stars come closer, the stars in the Centauri constellation will have move to the point where it may not even be the same constellation any more. $\endgroup$ – Michael May 18 '20 at 22:49

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