For over four-and-a-half thousand million years the sun has been pouring out heat and light on our planet. In just one second this vast sphere of boiling gas radiates more energy than Man has used since the beginning of civilisation; energy that comes from the 15-million degree nuclear furnace in the heart of the sun. Until recently the sun was thought to be well understood, but now scientists have begun to question some of the basic theories about how it works. New observations are beginning to throw doubt on the comforting view of our sun as a constant and well-ordered star, and to suggest that the sun, on which all life depends, is not behaving as it should.