There are more ways for stars to explode than we thought, with the latest earning the name micronova. These small but powerful stellar explosions have a lot in common with their larger counterparts but occur only over stars' poles.
In 1572, a never-seen-before star appeared in the sky, which came to be known as a “nova” or new star by Danish astronomer Tycho Brahe. Tycho's nova changed astronomy as it revealed the "heavens" were not in fact unchanging for all time. As astronomy developed, humanity learned such novas happen on a regular basis, but rather than new stars, these are old ones that have suddenly become much brighter. Differences in the extent of brightening led to a distinction between ordinary novas and supernovas.
In Nature, a team from the European Southern Observatory now describes a third category, which they called micronovas by comparison. By ordinary human standards, however, these events are anything but micro.
Novas and Type 1a supernovas are both the results of interactions between white dwarfs and companion stars (other forms of supernova have very different origins). White dwarfs are so dense they can draw material off more diffuse neighbors. When so much material accumulates on a white dwarf it crosses a threshold of 1.4 solar masses it will explode as a supernova. In other cases the material forms a shell on the white dwarf, leading to a more modest, but still very powerful, explosion where large amounts of newly acquired hydrogen fuses to helium in the space of a few weeks.
In certain cases, however, the paper reveals, white dwarfs with powerful magnetic fields funnel the incoming material towards their poles. The process is comparable to the way charged particles from solar explosions are channeled towards Earth's magnetic poles, creating aurora, but the scale is immensely larger. This leads to an explosion similar to an ordinary nova, but restricted to small parts of the star, and therefore less energetic still.
"For the first time, we have now seen that hydrogen fusion can also happen in a localized way. The hydrogen fuel can be contained at the base of the magnetic poles of some white dwarfs, so that fusion only happens at these magnetic poles," Profesor Paul Groot of Radboud University said in a statement. "This leads to micro-fusion bombs going off, which have about one-millionth of the strength of a nova explosion, hence the name micronova."
Despite its name, a micronova can involve 20,000 trillion tonnes of material undergoing a nuclear fusion in the space of a few hours. Bewildering units of measurement seem to be all the rage at the moment (half a giraffe anyone?) so the authors describe this as 3.5 billion Great Pyramids of Giza. Here at IFLScience, we're going to think of it as 100,000 Mount Everests. Burning that much material causes the star to become at least three times brighter for a period of up to 10 hours.
"The phenomenon challenges our understanding of how thermonuclear explosions in stars occur. We thought we knew this, but this discovery proposes a totally new way to achieve them," Dr Simone Scaringi of Durham University said. "It just goes to show how dynamic the Universe is. These events may actually be quite common, but because they are so fast they are difficult to catch in action."
Brief dramatic brightening of stars has been recorded several times over the last 40 years without astronomers being able to explain the cause until now. The frequent repeated measures of stellar brightness recorded by the Transitting Exoplanet Survey Satellite (TESS) allowed the team to not only find three more, but study their sources.
Two of the three outbursts TESS detected were known white dwarfs, one heavily studied in the past, but in order to confirm their explanation the team needed to prove the third micronova was also a white dwarf, which they did using the Very Large Telescope.