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From XKCD:

Caption: 'The light from those millions of stars you see is probably many thousands of years old' is a rare example of laypeople substantially OVERestimating astronomical numbers.

So, what is the furthest object visible to the naked eye in the night sky from the surface of the earth? Assume a person with good eyes, on a clear, dry night at sea level, away from bright lights.

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up vote 8 down vote accepted

This is partly a question of 'limiting magnitude' - the faintest magnitude that is visible using a particular instrument, or in this case, just the eye.

+6 - maybe +6.5 - magnitude is sometimes used as a baseline expectation of what can be seen in dark skies sites.

The most distant star that can easily be seen is Deneb. It is actually quite bright, at brighter than +2 Magnitude, but is (probably) 1425ly away. So though it is at quite some distance, it's luminosity means that actually, it isn't limiting magnitude that is at play.

There are stars much further away that are at the barely perceivable limit, and seeing them will depend on conditions. The +4 mag mu Cephei is probably fairly achievable, at 5900ly, and V762 in Cassiopeia is about +6 mag and maybe 16000ly, which would probably be the record if you can actually see it, but you'll need special conditions.

When it comes to galaxies, Andromeda is 2500000ly away, and +3.4, very visible and much further away than any individual star. If you have good conditions, the +5.7 mag M33 is slightly further away at 2900000ly, but it would be much harder to see.

While the generally accepted limit is maybe +6.5, some people claim to have seen to +8. If it is possible, M83 is 14700000ly away at +8.2, with naked-eye claims reported.

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I should write to XKCD and complain :D – naught101 Jan 8 '15 at 13:43
Don't bother, these are the few distant ones that can barely be seen. Pick a bright star... pretty much any star. Odds are, it will be fairly near. – Jeremy Jan 8 '15 at 19:55
So, an extension to that question might be, "what's the distribution of distance from earth for the visible objects in the night sky?" I guess it'd be something like log-normal, but I've no idea what the mean or variance would be. A graph would be cool to see - you could even superimpose the distributions of sizes by distance. I'd happily made that graph, if anyone knows a good source of data (a smallish representative subset would be enough, I think). – naught101 Jan 9 '15 at 2:52

In general, the answer given by @Jeremy is completely correct. However, transient phenomena such as supernovae (exploding stars) and the even more energetic gamma-ray bursts can for a brief period be visible to the naked eye, despite distances of billions of lightyears.

In 2008, the gamma-ray burst GRB 080319B reached a magnitude of 5.8, and was visible to human eyes for half a minute. The distance to GRB 080319B is 10 billion lightyears!

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Thanks for digging that one up. It was a very vague memory. – Wayfaring Stranger Jan 8 '15 at 15:54

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