Svensmark is best known for his 2007 book, The Chilling Stars: A New Theory of Climate Change. So, if changing cloud formation was a factor determining climate, what was driving cloud formation? So, a more active Sun blocks more cosmic rays causing fewer cosmic rays to produce fewer low clouds. Conversely, a less active sun permits more cosmic rays to reach the atmosphere, increasing low-cloud cover and cooling the planet. Cosmic rays are produced largely by supernova remnants, which are concentrated around galactic spiral arms.