Hi -- that is an excellent question! The amount of sunspots, a measure of solar activity, increases to a point where there are lots of sunspots (Solar Maximum) and then decreases to a point where there are few if any (solar minimum). This sunspot cycle is actually closer to 11 years than 13, but it does vary! This century it has been more like 10.5 for each cycle, and there are occasionally times when it is much longer -- between 1645 - 1715 there were very few sunspots at all, so there was a really extended Solar Minimum (called the Maunder minimum). This coincided with a Little Ice Age in Europe, so we are definitely sensitive to the Sun's vicissitudes. The question of why there is a cycle at all (and why it can vary so much) is a subject of active debate among scientists. The Sun is powered by a dynamo-- this just means mechanical energy is converted into electromagnetic energy -- just like in power plants. In the Sun, electrically charged particles are moving around and make electrical currents, which then generate magnetic fields. The way the flows and fields interact is cyclical - hence the sunspot cycle (sunspots are just concentrated magnetic fields at the surface of the Sun), but complicated, and scientists are workng very hard trying to understand (and predict!) why and how the length of the cycle changes. See http://www.windows.ucar.e...ity/solar_cycle.html for more info on the solar cycle and http://www.cora.nwra.com/...eos/text/dynamo.html for more info on dynamos cheers, Sarah