Every year, around 1,000 type Ia supernovae explode in the sky. These bursts of stars brighten and then fade in such a repetitive pattern that they are used as “constant candles”—objects so bright that astronomers can tell the distance to one of their appearances.
Our understanding of astronomy is based on these candles. Consider two of the greatest mysteries of astronomy: How is the universe expanding? And why is the increase increasing? Attempts to understand both rely heavily on distance measurements made using type Ia supernovas.
However, researchers do not know exactly what causes these unusual explosions—a mystery that worries experts. If there are a number of possible ways, a seemingly minor discrepancy would be detrimental to our astronomical measurements.
In the past decade, support has grown for a particular issue related to the origin of type Ia supernovas – a story that traces every explosion to two faint stars known as white dwarfs. Now, for the first time, researchers have successfully reproduced a Type Ia explosion in computer simulations of the white dwarf event, giving the theory a big boost. But the simulations also did wonders, revealing a lot we have to learn about the engine that caused the most powerful explosion in the universe.
Dwarf explosion
In order for an object to be a stable candle, physicists must know the light from which it was born. They can compare this to how the object appears brighter (or darker) in the sky to increase its distance.
In 1993, astronomer Mark Phillips worked out how the luminosity of type Ia supernovae changes over time. In short, almost all type Ia supernovae follow this pattern, known as the Phillips relation. This consistency—along with the extreme brightness of these bursts, which can be seen billions of light-years away—makes them the most powerful candles available to astronomers. But why their consistency?
The idea comes from the unexpected element of nickel. When a Type Ia supernova appears in the sky, astronomers notice that radioactive nickel-56 is flooding out. And they know that nickel-56 comes from white, star-like stars, which only have a core of gas and the dense atmosphere of Earth, covered by a layer of helium. However these white dwarfs are inert; supernovas and everything. The puzzle is how to get from one area to another. “There’s still no good word for ‘How do you do this?'” said Lars Bildsten, an astrophysicist and director of the Kavli Institute for Theoretical Physics in Santa Barbara, California, who specializes in Type Ia phenomena. “How do you get a bang?”
Until about 10 years ago, the prevailing theory was that the white dwarf took in gas from a nearby star until the star reached its critical mass. The core is so hot and dense that the nucleus can explode and explode into a supernova.
Then in 2011, the theory was debunked. SN 2011fe, the closest Type Ia found in decades, was observed so early in its explosion that astronomers had a chance to search for a companion star. Nothing was seen.
The researchers turned their attention to a new theory, called D6—abbreviation for “twisted tongue” that is doubly driven,” developed by Ken Shen, an astronomer at the University of California, Berkeley. The D6 image shows that a white dwarf is engulfing another white dwarf and stealing helium, a process that produces so much heat that it causes nuclear fusion in the first helium shell. The helium fusion sends a charge deep into the core of the darf. Then they detonate.
