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My girlfriend and I were watching Cosmos, and something Carl Sagan said got us wondering what the farthest-away visible star is. Obviously "visible to the naked eye" is a fuzzy enough concept that there may be multiple defensible answers, but hopefully not too many. To make the question a little more interesting, let's restrict to individually distinguishable stars; otherwise the answer is pretty clearly some Local Group galaxy, and there aren't many of them to check.

The closest thing to a reasonable answer we came up with was this wikipedia list of stars that are more luminous than any closer star. The farthest-away star on that list with a plausibly visible apparent magnitude is Eta Carinae (7500 ly away, magnitude 4.55). However, there are several reasons why I'm not willing to consider this definitively answered:

  • It's a wikipedia article, and a poorly sourced one at that. So I don't entirely trust it.
  • It sorts stars by bolometric luminosity rather than visual luminosity, so perhaps there's some farther-away star whose spectrum is better-centered in the visible range.
  • The farthest-away visible star isn't actually guaranteed to be on a list of that sort, even assuming the other two points are cleared up. Perhaps the farthest-away visible star is only barely visible, and there's some star both closer and absolutely brighter than it which makes the list.

Given all these points, is it actually the case that Eta Carinae is the farthest-away visible star, or is there some visible star that's farther from us?

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@zephyr: I meant to suggest that with the word "individual", but I guess I should edit to be a little clearer about it. If you don't impose that restriction, the answer is pretty clearly either the Andromeda or Triangulum Galaxy, depending on how stringently you want to define "visible". –  Micah Dec 3 '12 at 3:52
A possible interesting side questions would be how distant is the furthest supernova that was distinguishable to the Mk I eyeball? –  dmckee Dec 3 '12 at 4:00

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up vote 8 down vote accepted

It all depends on the site you are observing from and the atmospheric conditions (and obviously also on you eyesight). A so-called "magnitude 6 sky" is often taken as the standard for a good dark site with no light pollution. This means that the threshold stars you can see have apparent magnitude 6. According to this article :

The farthest star we can see with our naked eye is V762 Cas in Cassiopeia at 16,308 light-years away. Its brightness is magnitude 5.8 or just above the 6th magnitude limit.

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