You surely expect that now we will go about explaining what a tangent is, how human sight works and how we calculate the distance to the horizon. But no. We all know perfectly well that the most distant objects that can be seen with the naked eye have to be in space.
Table of contents – Farthest Naked Eye Object
In that case, let’s go straight into space, far beyond the limits of our solar system.
Since we live in a galaxy that is about 100,000 light-years across, at first glance we might deduce that the farthest object that can be seen with the naked eye should be some star at the other end of the Milky Way. But we would be very wrong, because, as you may have noticed, in the night sky we “only” see a few thousand stars, not the hundreds of billions that our galaxy actually contains.
This is because light loses intensity with distance exponentially (quadratically, being more concrete), so that the light from most of the stars in the galaxy is too weak when it reaches Earth for our eyes to detect. In fact, almost all the stars that we see in the night sky with the naked eye are in a radius of about 1,500 light years around the Earth.
So, in order to answer the question we raised in the beginning, we should actually ask what is the most distant star visible with an unaided eye?
What is the farthest star visible with the naked eye?
Well, the answer to this question is not straightforward as one might think. There are some stars that are especially bright that we can see without the help of any instrument, even if they are at a greater distance. In this StackExchange thread they have been mulling over the matter and it is possible that the farthest star that can be seen with the naked eye is AH Scorpii. It is a red supergiant variable star with a diameter 1,411 times that of the sun and a luminosity 330,000 times greater. In principle, this star is located about 7,400 light years away, but it could be a little closer or further away because the measurement has a certain margin of error.
At a similar distance (7,500 light years) is Eta Carinae, a star that is not visible to the naked eye today, but which, in 1856, underwent an increase in brightness that made it the second brightest star in the sky during three days. Taking these figures into account, it seems that the most distant stars that we can see with the naked eye should be between 7,000 and 8,000 light years away.
But this figure matters little to us in the context of today’s question because there is an object that is far more distant than any star in the Milky Way and that can be seen with the naked eye: the Andromeda galaxy, at 2.4 million light years.
Now, you might say, “what are you talking about? I have not seen the Andromeda galaxy with the naked eye in my life!”
True. You will need a very clear sky and you have to be away from the lights of the city to be able to see it. If one day you manage to get to such a place, we recommend that you download an application called Google Sky that will help you locate the Andromeda galaxy in the sky. And, if you find it, in principle you will be observing the farthest object that can be seen with the eye.
Can we observe a supernova in a distant galaxy with the naked eye?
Now, although with the information provided above we have covered the topic of this article, while researching we have found an interesting detail. It also complements the answer provided quite well.
It turns out that in 1885 a supernova that took place in the Andromeda galaxy was detected. In short, supernovae are the final explosions with which the most massive stars end their lives. In fact, these events are so energetic that the brightness of a single supernova can rival that of the rest of the stars in the galaxy to which it belongs combined.
And, although the supernova of 1885 could not be observed with the naked eye, this has led us to wonder whether a supernova has ever been so bright that it could be seen from Earth with the naked eye, despite the fact that the explosion took place in an incredibly distant galaxy?
It appears that, yes!
This question led me to the so-called gamma-ray bursts, which are the brightest electromagnetic events in the universe and usually consist of an intense and rapid gamma-ray “flash” followed by a residual glow at all wavelengths, including the visible one. These events can last from a few milliseconds to several tens of minutes and it is believed that their duration depends on the type of phenomenon that produces them. The longest bursts would be the result of the collapse of extremely massive stars in the form of hypernovas, while the shorter ones could be produced by the merger of neutron stars or black holes.
In any case, these events are so extremely bright that they are visible from even greater distances than supernovae. And indeed, it appears that the glow from a gamma-ray burst that occurred no more and no less than 7.5 billion light-years away could be observed with the naked eye from Earth for 30 seconds on March 19, 2008. Therefore, at a distance more than 3,000 times greater than that which separates us from the Andromeda galaxy, this outburst, given the epic name of GRB 080319B, is probably the most distant event that has been seen with the naked eye.
Given we were trying to answer the question of what is the most distant object that can be seen from Earth with the naked eye, this additional information is totally irrelevant. Also, these outbursts last a very short time, so we can’t go outside at night and watch them whenever we feel like it.
A gamma ray burst is not a structure that lasts in time, like a star or a galaxy. But it is also true that, as far as we know, the light emitted by the GRB 080319B outburst is the farthest visible radiation that a human retina has ever detected. Assuming someone was looking up at the sky in the right direction during the 30 seconds the event took place, sure.
Anyway, this is the reason why we had covered our backs by using the term “thing” in the title. It gives us enough ambiguity to end the post by saying that, although Andromeda is the most distant object that can be seen with the naked eye, there are very brief and distant celestial phenomena that can potentially be seen from Earth with the naked eye, although it cannot be predicted when they will take place. Does that conclusion sound reasonable to you?