In 1604, a white dwarf star went supernova. This is quite normal behaviour for a white dwarf star; but this one, at a distance of just 20,000 light-years from Earth, was visible to the naked eye, and documented by astronomers around the world, including German astronomer Johannes Kepler.
Kepler's Supernova, as it came to be known, is still expanding to this day, the guts of the star blasting out into space. And, according to new research, it's not slowing down. Knots of material in the ejecta are moving at velocities up to 8,700 kilometres per second (4,970 miles per second) - over 25,000 times faster than the speed of sound in Earth's atmosphere.
You may be thinking "Duh, space is a frictionless vacuum, things will just keep moving forever", but a cloud of debris could slow down material moving through it. And it was thought that this might be the case for Kepler's Supernova.
That's because, as we now know, Kepler's Supernova was what is known as a Type Ia supernova. These take place when a white dwarf star in a binary system is cannibalising its companion, and accumulates so much mass that it is no longer stable - resulting in a cosmic kaboom.
But not all the material being stripped from the companion star makes its way onto the white dwarf. Instead, it collects into a cloud surrounding the binary system, what we call the circumstellar medium. When the white dwarf goes supernova, it explodes out into this medium.
Due to its proximity and relative recentness, Kepler's Supernova is now one of the most important objects in the Milky Way for studying the evolution of Type Ia supernovae. And a wealth of data going back decades has helped reveal how fast the supernova ejecta is travelling.
A team of astronomers led by Matthew Millard University of Texas at Arlington used images of the supernova obtained by the Chandra X-ray observatory from 2000, 2004, 2006, 2014 and 2016 to track 15 knots of material in the supernova ejecta, observing their changes in position to calculate their velocity in three-dimensional space.
Some of the knots do seem to be decelerating, as expected from interaction with the circumstellar medium.
To the team's surprise, their measurements show that other knots are almost freely expanding, 400 years after the event - and that their velocities, at an average of 4,600 kilometres per second (2,860 mps), are similar to those seen in optical observations of supernovae in other galaxies only days or weeks after the actual explosion.
This suggests that at least some of the supernova material can blast right through the circumstellar medium, without being slowed.
Interestingly, the directions of these knots aren't uniformly distributed. Eight of the 15 knots are moving away from Earth; only two are moving towards it (the direction of the remaining five could not be ascertained).
This asymmetry in direction suggests that the explosion itself may have been asymmetrical; or, there's an asymmetry in the circumstellar medium along our line of sight. It is, however, impossible to know at this point - further study is needed.
The asymmetry, however, can reveal information about the supernova explosion itself. Four of the faster knots are close together, moving in the same direction, and have similar elemental abundances. This, the researchers note, suggests that they originated from the same region on the surface of the white dwarf progenitor.
In all, their findings suggest that the supernova itself could have been unusually energetic for a Type Ia. Measuring the velocities of more ejecta knots over the coming years could help to confirm their measurements and calculations, build a more complete three-dimensional map of the material's distribution, and place constraints on exactly how energetic that explosion could have been.
The research has been published in The Astrophysical Journal.