Astronomers have detected a previously unknown type of stellar explosion called a micronova involving thermonuclear blasts at the polar regions of a type of burned-out star called a white dwarf after it has siphoned material from a companion star.

The researchers said on Wednesday a micronova is by far the least powerful type of star explosions now known — less energetic than a blast called a nova in which a white dwarf’s entire surface blows up and tiny compared to a supernova that occurs during the death throes of some giant stars.

Micronovae are observed from Earth as bursts of light lasting about 10 hours. They were documented on three white dwarfs — one 1,680 light years away from Earth, one 3,720 light years away and one 4,900 light years away. A light year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km). “The discovery was an unexpected surprise. It goes to show just how dynamic the universe is. These events are fast and sporadic. Finding them requires looking at the right place at the right time,” said astronomer Simone Scaringi of Durham University in England, lead author of the study published in the journal Nature.

White dwarfs, among the densest objects in the universe, result from the collapse of a dying star’s core. They have the mass of our sun but are about the size of Earth in diameter.

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