A sunspot group aimed squarely at Earth has grown to 20 times the size of our planet and has the potential to unleash a major solar storm. The amorphous mix of spots, together called Number 652, has been rotating across the Sun and growing for several days. On Friday, it sat at the center of the solar disk. Sunspots are areas of intense magnetic energy, cooler and darker than the surrounding surface of the thermonuclear furnace. Sometimes the magnetic fields let loose and huge amounts of radiation and charged particles are hurled into space. The Sun's last bout of intense...