Blast from the Past
More than 400 years ago, a nearby star in our Galaxy exploded with a flash of light so bright that we can still see echoes of the event from the surrounding dust clouds. The event is known as "Tycho's supernova", after the Danish astronomer who published detailed observations of it in 1572. In today's issue of the journal Nature, astronomers announced new observations that have for the first time established the exact cause of the explosion.
When a new star appeared in the sky in 1572 -- bright enough to be visible in full daylight -- it instantly attracted public attention. At the time the "sphere of the stars" was believed to be unchanging and eternal, so the event challenged the prevailing view of the universe. Tycho Brahe was one of the first astronomers to make precise observations of the positions of stars and planets, before he famously died of a urinary infection after straining his bladder at a royal banquet as a courtesy to the host. Tycho's observations of the 1572 event were published in a book titled "Stella Nova" (Latin for "new star"), which demonstrated that the object was far beyond the orbit of the moon.
Modern analysis of the historical record suggests that the event was actually an exploding star, now commonly referred to as a "supernova". There are several types of supernova explosions, arising either from the detonation of a white dwarf star (the ember that is left at the end of the life of a regular star like our Sun), or from the gravitational collapse of a very massive star at the end of its life. The available evidence suggested that Tycho's supernova was probably the first type, but it could not distinguish between two possible scenarios that can trigger such an event. In one scenario the white dwarf is in a close orbit with a regular star -- so close that its gravity slowly consumes the mass from its companion until the white dwarf exceeds a critical size and collapses under its own weight. In the other scenario the companion is also a white dwarf, and the two white dwarfs slowly draw closer together until they merge and similarly collapse in a spectacular explosion.
The usual way of deciding what triggered the supernova explosion requires evidence that was not available in 1572. The light from the event is passed through a prism to separate it into its constituent colors (a spectrum), where dark lines appear at specific colors that reveal what the object is made of and how it is moving. For the observations that were announced today, astronomers used the interstellar dust that surrounds Tycho's supernova as a sort of time machine to record the spectrum of the event. Here's how it works: the explosion created a sudden flash that moved out in all directions at the speed of light and reached the Earth in 1572. Now imagine that there was a cloud of dust 200 light years behind the supernova that reflects some of the light of the original flash back toward the Earth. This dust cloud would not be lit up until 200 years after the explosion, and the reflected light would then have to travel another 200 years just to get back to the supernova itself -- so the flash from the dust cloud would arrive 400 years after the original explosion. Using this basic idea, astronomers recorded the spectrum of an illuminated dust cloud near Tycho's supernova and discovered that it looked exactly like an explosion caused by the first scenario -- from a white dwarf consuming a regular star.
It's a wonderful piece of detective work, and by repeating the experiment on dust clouds in every direction around the remnant of Tycho's supernova, scientists hope to eventually measure the three dimensional shape of a 400 year old explosion. Tycho would surely be impressed by our ability to study this blast from the past.
