New type of cosmic explosion discovered in 'dead' stars

 An entirely new type of cosmic explosion, called a micronova, has been discovered by astronomers. The event is not predicted by current models of stellar evolution and may require a new theory to explain how white dwarfs experience these tiny explosions at their magnetic poles.

White dwarfs are remnants of stars that no longer undergo nuclear fusion and therefore no longer produce energy. In fact, these are highly dense stellar cores, with a mass comparable to that of the Sun, but with a volume comparable to that of the Earth. They may have intense brightness, but only due to high temperature, and not due to nuclear fusion processes.

Our own Sun, for example, is destined to become a white dwarf, as are stars of less than 10 solar masses. When the "fuel" of hydrogen and helium runs out, these stars eject their upper layers, forming a nebula with a hot, dense core at the center.

However, if a white dwarf is close enough to another star in a binary system, it can suck up its material and gain enough mass to go back into runaway nuclear fusion and eventually explode. These events are known as nova and occur over the entire surface of the object.

 What astronomers didn't know is that these explosions can also occur at the poles of a white dwarf. They are much smaller than a nova explosion, but they are still powerful — a single explosion of this type can burn material equivalent to 3.5 billion Great Pyramids of Giza!

For an explosion like this to occur at a white dwarf's magnetic poles, it needs to be part of a binary system, and close enough to its companion to steal its gaseous material. This material forms an accretion disk around the white dwarf, and as it falls to its surface, some of the gas is directed toward the poles by the object's magnetic field.

Astronomers found these explosions when analyzing data from TESS (Transiting Exoplanet Survey Satellite), NASA's "exoplanet hunter". They appear in visible light and only last a few hours. There were three micronovas: two in known white dwarfs and a third that required further observations.

Using ESO's Very Large Telescope (VLT) X-shooter instrument, the team was able to confirm that the unknown object was also a white dwarf. This means that micronovas can be quite common in the universe, but difficult to detect due to their short duration.

The study authors now want to capture more of these events, which should result in a survey of how often they occur in the universe. This can be accomplished with a larger set of instruments and archived data from telescopes that could have detected micronovas without anyone noticing.

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