... Callisto's surface shows its age. ... Like the Earth's Moon, Mercury is scarred with craters, testifying to an intense bombardment during the early history of the Solar System. ... This huge ball of stars predates our Sun. ... Red stars are cool, with temperatures of around 3,000 degrees Kelvin (K), while blue stars are hotter and can have temperatures over 30,000 degrees K. Our own lovely yellow Sun's temperature is a comforting 6,000 degrees K. Star Trails in Northern Skies . ... Aurora Astern . ...