Even on earth, the average human eye can make out a candle flickering on a dark night from up to 3 miles away. In fact if you manage to raise yourself about 20 meters above the ground, altering your view on the curve of the earth, you can see objects in the horizon that are almost 7 miles away.
However, with no horizon to worry about, when we look up into the sky we can see objects much much further away. For instance the moon is roughly 239000 miles away while the sun is a huge 93 million miles away. The furthest object we can see in our own solar system without help is Saturn which clocks in at just under 1 billion miles away, a journey which would take the average rambler about 91,000 years to complete (accounting for a sandwich break each day).
Our brightest star
When it comes to stars, the brightest ones can be seen from light years away. A light year is the measurement astronauts use to measure distance in space, based on how long it takes light to travel that space. It’s basically the same as when you your friends you are about 10 minutes away, although light tends to be a bit more accurate about it.
The furthest star we can see without any assistance, named Deneb, is about 1500 light years away from earth. That means that if the star was looking right back at us, it could be watching the election of the 52nd pope sometime in the 6th Century.
Looking even further
But the very furthest we can see without any help at all is actually a huge collection of stars and planets. On a moonless night, even on not particularly dark night, the Andromeda galaxy can be seen from earth as a distant glob of light. In reality it contains one trillion stars, at least twice as many as our own milky way.