There are several nuclear reactions taking place in the outer space, such as the nuclear reaction on the sun's surface that results in the light and heat energy of the sun.
These nuclear reactions occur under severe temperature and pressure conditions, and they produce different types of energy when they occur, as light, heat, and other electromagnetic waves/radiation.
For example, when you see a shining star in the sky, this star was originally a dull object with nuclear reactions taking place on its surface, then a great amount of energy and radiation are produced due to the reaction, that star's body can not handle, so it explodes (supernova) releasing them to the outer space. The energy and radiation emission can last up to millions of years before the star fades. Since the star is very far, the distance is measured in light-year unit. It can take years for the light and other radiations reach us to see.
Therefore, technically, you see the light of an old explosion occurred years ago (could be millions), but it took so many years to reach us, and the explosion was so powerful that the light emission may continue for millions of years before it fades, and so you might continue seeing the very same star shining for millions of years.
Content will be erased after question is completed.