system
Given the following post, respond as if you're commenting on Reddit:
human
Is it possible there is a super nova we don't see yet within our galaxy, that is close enough to our solar system, that once it appeared would be bright enough to illuminate the night sky for the rest of human existence? 

This of course assumes that the event happened several hundred thousand years ago, and light from the event has not yet reached Earth. Based on a program I saw on TV the other night, it seems quite plausible, though the program did not attempt to paint this scenario at all.
gpt
Supernovae don't last very long. The initial burst of energy can be as brief as only a few hours, depending on the type of supernova, (edit: but will often take weeks/months to burn out entirely).

So, taking IK Peg as an example, which is only 150LY away, we can do a little maths with known values to figure out how bright it'll be here on earth:

IK Peg will be a type 1a supernova, if it does explode, which will have an absolute magnitude of around -19. (Magnitude is a measure of brightness, essentially. Lower values are brighter.) So, we use the formula:

Apparent Magnitude = -5 + Absolute Magnitude + 5log(d)

...where d is the distance in parsecs. 150LY is about 46 Parsecs, we get:

Apparent Magnitude = -5 -19 + 8.31 = -15.69

Now, remembering that with magnitudes, lower numbers are brighter, we can compare this to a few things we're familiar with. The maximum brightness of the moon is magnitude -12.94. The sun is around -26.74. It might not be particularly visible during the day, given the presence of the sun, but at night, it'd easily be the brightest thing in the sky.

If we examine the case of the second closest potential supernova, Spica; this star will end in a type 2 supernova, due to its composition. At a distance of 260LY, this event will have an apparent magnitude of -12.58, which still places it on-par with the moon in terms of brightness.