If the Sun's internal energy source, nuclear fusion, suddenly shut down, we wouldn't be able to tell through just visible light. The photons generated by nuclear fusion can take up to million of years to exit the maze that is the inside of our Sun. The Sun would begin to dim, but it would take hundreds of thousands, if not millions, of years to cause a visible difference in our lives. However, solar neutrinos, leave in straight lines from the core. So, if one was detecting neutrinos, it would take about 8 minutes for them to record the lack of new neutrinos from the Sun.