White dwarfs are the smoldering remains of stars like the Sun. Right now, the nuclear fusion of hydrogen in the Sun’s core produces energy that pushes back against gravity and keeps the Sun from falling in on itself. Eventually, though, the Sun will run out of fuel for fusion and collapse until it’s about the size of Earth. At that point, the Sun won’t be able to get any denser without forcing its electrons past a limit set by quantum mechanics. When a star enters this stable, ultra-dense phase, it becomes a white dwarf.
On its own, a white dwarf won’t make a nova. It has to be in a binary system, meaning it and another star orbit one another. If the other star still has plenty of hydrogen, and if it is not too far from the white dwarf, then the white dwarf’s gravity will skim off some of the other star’s outer layers. Over time, this fuel can accumulate on the surface of the white dwarf and heat up. When the fuel gets hot enough, it will ignite in a rapid chain reaction, releasing a giant burst of radiation and throwing off ejecta at thousands of kilometers per hour.
Though this explosion is very energetic, it doesn’t destroy either star. The same pattern of build-up and burst can repeat again and again. T Corona Borealis, in particular, repeats its nova cycle roughly every 80 years. The system consists of a white dwarf and a red giant star located about 800 light-years from Earth.
What’s the difference between a supernova and a nova?
A supernova is a much bigger stellar explosion that does not happen more than once to the same star. Some supernovae are triggered when a star runs out of fuel for nuclear fusion and collapses, while others are caused by white dwarfs that accumulate enough mass. Regardless of the exact path, the source of a supernova has to be so massive that its gravity overcomes the quantum mechanical pressure that would keep a white dwarf stable. And while a nova leaves its star system intact, a supernova results in a neutron star or black hole, or completely destroys whatever object triggered it.
Supernovae are also rarer than novae. The last supernova to happen in the Milky Way was in 1604, while the last nova was in 2022, though it was too dim to see with the naked eye.