Colored according to x-ray energy intensity, this supernova remnant’s bluish shockwave bubble is twice as hot as the mottled gaseous debris expanding behind at 10 million degrees Celsius (more at Astronomy Picture of the Day and CXC).

“Tycho’s star” was a supernova that appeared in 1752 CE. According to Robert Burnham, Jr. (1931—93), this “guest star” was first noted by Wolfgang Schuler (?—1575) as early as November 6, 1572, but was seen by many observers throughout Europe and in the Far East, shattering locally held beliefs in the immutable nature of stars. While Tycho Brahe (1546—1601) was not the first to observe the supernova in Cassiopeia, he became known as a respected astronomer after publishing his careful observations about the “new star”—Stella Nova in Latin—two years later (more history and links from Hartmut Frommert and Christine Kronberg). Tycho found it at first as bright as Jupiter, but the supernova soon grew as brilliant as Venus (around —4 magnitude). For about two weeks the star could be seen in daylight, but at the end of November it began to fade and change color, from bright white over yellow and orange to faint reddish light, finally fading away from visibility in March, 1574, having been visible to the naked eye for almost 16 months (more about Brahe’s “acid tongue and silver nose,” the cultural shock of the “new star,” and how supernovae create high-energy radiation from Wallace H. Tucker).