When supernova explodes the illumination sphere around it grows in radius with the speed of light.
We can measure increase of illumination on surrounding nebulae and other objects.
What do you think, when sphere of illumination radius grows about 0.0012 degrees per day, how far was the supernova when exploded?
Light travels 26 * 109 km in one day (26 * 1012 m).
If it was 0.0012 degrees, then we can calculated distance X like this:
tg(0.0012) = 26 * 1012 m / X
X = 26 * 1012 m / tg(0.0012) = 1.24 * 1018 m , which is 131 light years away.
Hehe, dare you to fit that under the dome.