Tycho Brahe observed a supernova in 1572. Though the intense bright light fizzled out over 440 years ago, modern day astronomers are still able to study the remnants because reverse shock waves are keeping it lit up. This announcement comes from lead author Hiroya Yamaguchi of the Harvard-Smithsonian Center for Astrophysics and has been accepted for publication in The Astrophysical Journal.

The remnant used to be a white dwarf prior to the explosion, and it ultimately became what is what is known as a 1a supernova. This sometimes happens with white dwarfs in a binary system. As one star has completely used up all of its hydrogen in nuclear fusion, the gravitational pull might absorb material from the neighboring star. If the dying white dwarf becomes too massive, the pressure at the star’s core becomes too great and it causes a colossal thermonuclear explosion known as a supernova. For nearly a year when it was discovered, Tycho’s supernova was one of the brightest objects in the night sky, and was one of the first indications that the cosmos were dynamic.

It is estimated that the star ejected metal-rich material out at speeds up to 11 million miles per hour (5,000 km/s). This created an incredible shock wave that went rippling out through space. Even today, it continues to travel at approximately 300 times the speed of sound. The team found that there is also a shock wave that moves in reverse toward the star at about Mach 1000. The heat generated from the incoming shock wave lights up the gas, allowing astronomers to see it using x-rays. 

The reverse shock waves are responsible for generating light through heated gas that can be detected using the Suzaku spacecraft’s x-ray telescope (XRT). The team also discovered that the heat was not generated through the collision of electrons. Shock waves that don’t involve collisions are very common in space, though some of the most fundamental properties are not very well understood. Through computer modeling, the team found that collisionless heating gets electrons 1000 times hotter than typical Coulomb collisions. This research presents the first real evidence that electrons can become heated so efficiently without collisions in the reverse shock wave of the remnant of Tycho’s supernova.

The team will now look to more recent supernova remnants to study the associated reverse shock wave.