every year, roughly 1,000 type Ia supernovae explode in the sky. These stellar explosions are used as a “standard candle” because they brighten and then fade in a repeatable pattern. This is a uniformly bright object that astronomers can infer the distance to any of them from their appearance.
Our understanding of the universe is based on these standard candles. Consider two of the biggest mysteries in cosmology: What is the rate of expansion of the universe, and why is it accelerating? The effort relies heavily on distance measurements made using type Ia supernovae.
But researchers don’t fully understand what causes this strangely uniform explosion. If there are multiple ways they can occur, small discrepancies in how they appear can undermine cosmic measurements.
Over the past decade, there has been growing support for a particular story about what causes Type Ia supernovae: tracing each explosion to a pair of faint stars called white dwarfs. Now, for the first time, researchers have successfully reproduced a Type Ia explosion in computer simulations of a double white dwarf scenario, giving an important boost to the theory. But the simulation also brought some surprises, revealing just how much we still have to learn about the engines behind some of the most important explosions in the universe.
Dwarven detonation
For a celestial body to function as a standard candle, astronomers must know its intrinsic brightness, or luminosity. They can compare it to the brightness (or thinness) of an object they see in the sky and calculate its distance.
In 1993, astronomer Mark Phillips plotted how the luminosity of Type Ia supernovae changed over time. Importantly, nearly all his type Ia supernovae follow this curve, known as the Phillips relation. This consistency, along with the extreme luminosity of these explosions, which are billions of light-years away, makes them the most powerful standard candles astronomers have. What is?
The hint 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 a white dwarf star. White dwarfs hold only a dense, Earth-sized core of carbon and oxygen, covered by a layer of helium. But these white dwarfs are inert. A supernova is nothing. A puzzle is a way to move from one state to another. Astrophysicist and Type Ia supernova expert Lars Birsten, director of the Kavli Institute for Theoretical Physics in Santa Barbara, California, said: “How do you make it explode?”
Until about a decade ago, the prevailing theory was that white dwarfs siphoned gas from nearby stars until they reached critical mass. Its core becomes hot and dense enough to trigger a runaway nuclear reaction and explode into a supernova.
In 2011, that theory was debunked. SN 2011fe is the closest relative to her type Ia found in decades and was discovered early in the explosion, giving astronomers a chance to search for a companion star. I didn’t see anything.
The researchers have developed a new theory, the so-called D6 scenario, an acronym for “dynamically driven double degenerate double explosion,” a tongue twister coined by Ken Shen, an astrophysicist at the University of California, Berkeley. I turned my attention to The D6 scenario proposes that a white dwarf captures another white dwarf and steals its helium. This is the process that releases enough heat to cause nuclear fusion in the helium shells of the first dwarfs. The merging helium sends a shockwave deep into the dwarf’s core. Then explode.
