Besides the other reply, also it doesn't "turn on" to maximum brightness instantly. It takes on the order of several minutes for all the nuclei to absorb the neutrons and then start the radioactive decay that powers the light. Also you wouldn't see light from the whole star simultaneously - it would start from the center (because that's closer to you by the radius of the star), and light from the limb would take several more minutes to arrive (1 AU radius of the star = 8.3 minutes light travel time.)

So your eyes would have time to adjust.