Thanks to the laws of physics, the brightness of the Sun is equivalent to the square of the relative distance from it. So if you are now half as close to the Sun as you originally were, the apparent brightness would be a quarter of what it originally was. (1/2)2 = 1/4, see?
This means that the brightness of the Sun drops off dramatically the further away you get from the Sun. The fact that even by the time you get to Pluto it’s still bright is a remarkable testament to the sheer power of our nearest thermonuclear stellar furnace.