Every year around 1,000 Type Ia supernovae erupt across the sky. These stellar explosions light up and then fade out in such a repeatable pattern that they are used as “standard candles,” objects so uniformly bright that astronomers can tell the distance to one from its appearance.
Our understanding of the cosmos is based on these standard sails. Consider two of the biggest mysteries in cosmology: What is the expansion rate of the universe?? AND Why is that rate of expansion accelerating?? Efforts to understand both of these issues rely critically on distance measurements made with Type Ia supernovae.
However, researchers don’t fully understand what triggers these strangely uniform bursts, an uncertainty that worries theorists. If there are multiple ways they can happen, small inconsistencies in the way they appear could be corrupting our cosmic measurements.
Over the past decade, support has been building for a particular story about what triggers Type Ia supernovae, a story that traces each explosion back to a pair of dim stars called white dwarfs. Now, for the first time, researchers have successfully recreated a Type Ia explosion in computer simulations of the double white dwarf scenario, giving the theory a critical boost. But the simulations also produced some surprises, revealing how much more we have to learn about the engine behind some of the biggest explosions in the universe.
detonate a dwarf
For an object to serve as a standard candle, astronomers must know its inherent brightness, or luminosity. They can compare that to how bright (or dim) the object appears in the sky to calculate its distance.
In 1993, astronomer Mark Phillips traced how the luminosity of type Ia supernovae changes over time. Fundamentally, almost all Type Ia supernovae follow this curve, known as the Phillips relationship. This consistency, along with the extreme luminosity of these bursts, which are visible billions of light-years away, makes them the most powerful standard candles astronomers have. But what is the reason for its consistency?
One clue comes from the unlikely element nickel. When a type Ia supernova appears in the sky, astronomers detect a radioactive flood of nickel-56. And they know that nickel-56 originates from white dwarfs, dim, dim stars that retain only a dense, Earth-sized core of carbon and oxygen, enveloped by a shell of helium. However, these white dwarfs are inert; supernovae are the opposite. The enigma is how to go from one state to another. “There’s still no clear ‘How do you do this?’” he said. Lars Bildsten, astrophysicist and director of the Kavli Institute for Theoretical Physics in Santa Barbara, California, who specializes in Type Ia supernovae. “How do you make it explode?”
Until about 10 years ago, the prevailing theory held that a white dwarf would draw gas from a nearby star until the dwarf reached critical mass. Its core would become hot and dense enough to cause a runaway nuclear reaction and detonate in a supernova.
Then, in 2011, the theory was overthrown. SN 2011fethe closest Type Ia found in decades, it was discovered so early in its explosion that astronomers had the opportunity to search for a companion star. none was seen.
The researchers turned their interest to a new theory, the so-called Scenario D6– an acronym representing the tongue twister “dynamically driven double degenerate double detonation”, coined by ken shen, astrophysicist at the University of California, Berkeley. The D6 scenario proposes that a white dwarf traps another white dwarf and steals its helium, a process that releases so much heat that it triggers nuclear fusion in the first dwarf’s helium shell. The fusing helium sends a shock wave into the dwarf’s core. Then it detonates.