The Kepler Supernova

October 17

On this date in 1604, Johannes Kepler witnessed the last supernova observed in the Milky Way. Kepler had figured out the laws of planetary motion, and he knew the night sky very well, so he was surprised to see a very bright object in the western sky one night. Kepler thought he was witnessing the birth of a star, but a supernova is actually an explosion that signals the star's death. The exploding star had first been noted in northern Italy about a week before, but Kepler, who lived in Prague, was unable to see it until October 17, due to cloudy weather. He began studying it in earnest, recording observations of it for more than a year. The supernova was so bright that it was visible during the day for three weeks. The telescope wouldn't be invented for a few more years, so all of Kepler's observations were made by the naked eye. He eventually wrote a book about it, which he called De Stella Nova (1606). In the book, he compared the supernova to the Star of Bethlehem, and wondered if it might convert the American Indians to Christianity.

Though astronomers have since observed several supernovae in other galaxies, this one — known as SN 1604, Kepler's Supernova, or Kepler's Star — is the last supernova observed by the naked eye in our own galaxy, the Milky Way. Kepler's Supernova was closer to the center of the spiral galaxy, less than 20,000 light years away from our location out on one of the arms. Scientists estimate that supernovae occur in the Milky Way roughly every 50 years, but there is no observational record of them. Cosmic dust obscures our view.

Astronomers are still studying the supernova's remnants with the help of NASA's three Great Observatories: the Hubble Space Telescope, the Chandra X-Ray Observatory, and the Spitzer Space Telescope. The shockwave of gas and dust is still spreading through space at a speed of 400 million miles per hour. In 2013, astronomers completed their post-mortem of the star, and determined its cause of death. Two stars — a binary system — were orbiting around each other. The smaller of the two — a white dwarf — began to accumulate matter from the larger star, a red giant. The white dwarf became heavier and heavier until it could no longer support its own mass, and burned up in a spectacular explosion.