One of the most recent supernova explosions close enough to Earth for us to see the after-effects has shaken theories on what causes this important category of stellar explosions. Light from the remnants of the “new star” of 1572 appears to contradict theories on the combination of stars that produce Type la supernovae.  

The galaxy has really not been playing fair by astronomers. If there have been supernova explosions in the Milky Way since the invention of the telescope, they have been hidden behind so much dust that we missed them. On the other hand, in the 37 years before Galileo turned his spyglass to the skies, two new stars, which have since been shown to be supernovae, were spotted. The study of the debris left behind from these events is giving astronomers some compensation for not getting a front row view of a supernova's peak.

SN 1572, also known as Tycho's supernova, was the first evidence showing that change occurred beyond the Earth's atmosphere. When it burst into the heavens it was even brighter than Venus. It was still visible a year later, and measurements of its parallax proved it to be very distant from the Earth.

Dr Tyrone Woods of Monash University, Australia told IFLScience that more than four centuries after the event, astronomers categorized the explosion as a Type Ia supernova. They achieved this by detecting light from the original event bouncing off dust, which was lying at just the right distance for the reflection to be reaching Earth now.

Type Ia supernovae are used to measure the universe through their uniquely useful trait of having a consistent intrinsic brightness. The favored theory for their formation is that a white dwarf and main sequence star orbit each other so close together that gas is drawn from one to the other until the dwarf gains so much gas it explodes. If this is the case, the white dwarf should become very hot as it builds towards exploding, releasing enough intense ultra-violet radiation to ionize any nearby hydrogen gas in the process.

However, in Nature Astronomy, Woods reported observations of the interactions between the shockwave from SN 1572 and the surrounding hydrogen gas. This included a particular shade of red light that is only seen when fast-moving material interacts with hydrogen that hasn't been ionized.

The absence of ionization suggests that SN 1572 did not undergo the processes prior to explosion that you tend to find in most textbooks on type Ia supernovae. This lends support to an alternative theory, known as double degeneracy, which suggests that these explosions are actually the result of two white dwarfs (or degenerate stars) merging.

The double degeneracy theory was initially dismissed because it seemed improbable that white dwarfs in such close orbits would be common enough to account for the explosive events. However, Woods told IFLScience that SN 1572 “was either double degeneracy or something we have not thought of.”

Woods added that Type Ias have been shown to be diverse enough to have more than one source, but his work builds the case that double degeneracy is the norm, not the exception.

“Our result provides a benchmark against which all future models of Type Ia supernovae must be compared,” Woods said in a statement. He plans to study the remnants of several other nearby historical supernovae, such as the fabulously bright SN 1006, to see if this can be confirmed.