*Is there a limit on how far a human can see?*
You might be surprised at the answer here. It turns out that there’s actually no practical limit on how far away we can see – if we’re looking at objects brighter than their backgrounds.
That’s why – with the eye alone – we can see stars that are hundreds or even thousands of light-years away. The farthest object commonly seen with the unaided human eye isn’t a star, though – instead, it’s an entire galaxy of stars. The Andromeda Galaxy is more than two million light-years away. It’s one of only a few objects outside our own Milky Way galaxy that we can see without the aid of telescopes.
The brightness of objects we can see with the eye is measured on a scale between one and six – the faintest objects visible to the eye are said to be 6th magnitude. On that scale, the Andromeda galaxy has a brightness of 3.4 – so although it’s the most distant object we can see, it’s not the faintest. The light of the dimmest stars visible is about like that of a candle seen at night from ten miles – or about 16 kilometers – away.