Each year, approximately 1,000 Type Ia supernovae erupt in the sky. These stellar outbursts brighten and then fade in such a repeatable pattern that they’re used like “standard candles” – objects so uniformly bright that astronomers can infer the distance to one of them by their appearance.
Our understanding of the cosmos is based on these standard candles. Consider two of the greatest mysteries of cosmology: How fast is the universe expanding? And why is this rate of expansion accelerating? Efforts to understand these two problems are mainly based on distance measurements made using type Ia supernovae.
Yet researchers don’t fully understand what triggers these eerily uniform explosions – an uncertainty that worries theorists. While they can occur in multiple ways, tiny inconsistencies in their appearance could corrupt our cosmic measurements.
Over the past decade, support has grown for one particular story about what triggers Type Ia supernovae – a story that traces each explosion to a pair of dim stars called white dwarfs. Now, for the first time, researchers have succeeded in recreating a Type Ia explosion in computer simulations of the double white dwarf scenario, giving the theory a critical boost. But the simulations also produced a few surprises, revealing how much we still have to learn about the engine behind some of the most significant explosions in the universe.
Blow up a dwarf
For an object to serve as a standard candle, astronomers must know its luminosity or inherent luminosity. They can compare this to the brightness (or brightness) of the object in the sky to determine its distance.
In 1993, astronomer Mark Phillips plotted how the luminosity of Type Ia supernovae changed over time. Importantly, nearly all Type Ia supernovae follow this curve, known as the Phillips relationship. This consistency, as well as the extreme luminosity of these explosions, visible billions of light-years away, make them the most powerful standard candles available to astronomers. But what is the reason for their consistency?
A clue comes from the unlikely element nickel. When a Type Ia supernova appears in the sky, astronomers detect a flood of radioactive nickel-56. And they know that nickel-56 comes from white dwarfs — dim, extinct stars that retain only a dense Earth-sized core of carbon and oxygen, enveloped in a layer of helium. Yet these white dwarfs are inert; supernovas are anything but. The puzzle is how to go from one state to another. “There’s still no ‘How do you do that?’,” said Lars Bildsten, an astrophysicist and director of the Kavli Institute for Theoretical Physics in Santa Barbara, Calif., who specializes in Type Ia supernovae. “How do I blow it up? »
Until about 10 years ago, the prevailing theory held that a white dwarf would siphon gas from a nearby star until the dwarf reached critical mass. Its core would then become hot and dense enough to trigger a runaway nuclear reaction and explode as a supernova.
Then in 2011, the theory was overturned. SN 2011fe, the closest Type Ia found in decades, was spotted so early in its outburst that astronomers had a chance to search for a companion star. None were seen.
The researchers turned their interest to a new theory, the so-called D6 scenario – an acronym for the “dynamically driven double degenerating double detonation” tongue twister, coined by Ken Shen, an astrophysicist at the University of California at Berkeley. The D6 scenario features a white dwarf trapping another white dwarf and stealing its helium, a process that releases so much heat that it triggers nuclear fusion in the first dwarf’s helium shell. The molten helium sends a shock wave deep into the dwarf’s core. It then explodes.
