This photo, taken in 2007 by STEREO, provided NASA with one of the first 3-D images of the sun. Experts hope that this new way of imaging will allow a greater understanding of solar physics and space weather. As the sun has no solid surface and consists primarily of plasma gas, it rotates faster at its equator than it does at its poles. The energy of the sun is caused by nuclear fusion reactions within its core. The sun is highly magnetic and has areas of highly concentrated magnetic fields. These concentrations are what cause sunspots and solar flares.