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From XKCD: http://imgs.xkcd.com/comics/ancient_stars.png

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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2 Answers 2

up vote 5 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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Do you want a single star or a galaxy?

For a single star, I'm thinking Betelgeuse, a red giant 642.5 ly away

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Good point. Maybe one of each? –  naught101 Apr 1 at 5:31
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Betelgeuse may be as near as 500ly and is bright, at least +1.2 and up to +0.2. It's not close to being the furthest star that can be seen with the naked eye. –  Jeremy Apr 1 at 9:37
    
If its a galaxy, Andromeda can be seen with a naked eye and its about 2.5 million light years away (according to google!). –  chris Apr 7 at 18:03
    
@chris Andromeda is the third closest galaxy. There are about 100 Messier objects that are farther away and many of them are galaxies. –  LDC3 Apr 8 at 0:45
    
@LDC3 sure, but I thought the issue was one one could see with the naked eye? –  chris Apr 8 at 6:10

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