White Dwarfs: New Model Explains How Binary Systems Of Dead Stars Go Supernova

The observable universe is filled with Type Ia supernovae — events that occur in a binary star system, in which one is always a white dwarf. Since the chain reaction that triggers these supernovae always happens in the same way, Type Ia supernovae, no matter where they are, always explode with the same brightness — making them one of the “standard candles” astronomers use to gauge distance in the cosmos.

“Type Ia supernovae are extremely important objects in physics, best known for their role in revealing that the expansion of the universe is accelerating,” Saavik Ford, a research associate at the American Museum of Natural History’s department of astrophysics, said in a statement released Tuesday. “The problem is that people do not agree on exactly how Type Ia supernovae come to be.”

In order to understand how these massive explosions take place, researchers at the museum have, in a new study published in the Monthly Notices of the Royal Astronomical Society, detailed a mathematical model that describes the behavior of a binary star system composed of two white dwarfs.

“The simplest way to create a Type Ia supernova is to run two white dwarfs into one another,” Ford said. “In our local universe, there are very few white dwarf binaries that are close enough to collide. Yet we see lots of supernovae lighting up our universe, so we know that something else is probably going on to cause those explosions.”

White dwarfs are extremely dense objects that are created when a star that is not massive enough to become a neutron star nears its end. A white dwarf, which is what our sun would become in roughly 5 billion years, can eventually cool down enough to become a black dwarf — an object that is still hypothetical — or, if it is part of a binary system, explode in a Type Ia supernova.

In this particular study, the researchers used their mathematical model to describe the behavior of “inspiralling” white dwarfs in a binary system of two such objects. When two white dwarfs orbit each other, the constant tug-of-war drains away energy from their orbit, and, as a result, the two objects begin moving closer to each other.

In certain cases, when the frequency of the tugging matches the oscillation frequency of at least one of the white dwarfs, resonance can cause the star to explode even before it touches its companion.

“Basically, we’ve proposed that if you have two white dwarfs spiraling towards each other and you shake one of them the right way for long enough, one will either blow up or you’ll bring the objects closer together faster for an eventual detonation,” co-author Barry McKernan, also a research associate at the museum’s department of astrophysics, said in the statement. “If we’re right, LISA may be able to see glitches in the gravitational waveforms coming from some of the nearest white dwarf binaries.”

LISA — Laser Interferometer Space Antenna — is the name of the space-based gravitational wave detector that the European Space Agency plans to launch in the 2030s. The space-based observatory would hunt for gravitational waves — ripples in the fabric of space-time, created when massive objects such as black holes, neutron stars, or even white dwarf stars collide — far removed from any ground-based interference.