Captured at Last: The Tiny Stars that Spark Fierce Supernovae Explosions

A longstanding mystery about the tiny stars that let loose powerful explosions known as Type Ia supernovae might finally be solved. For brief periods, these cataclysmic blasts can outshine an entire galaxy of billions of stars. Astrophysicists want to understand their origins because they are integral to the evolution of galaxies and the study of dark energy.

Type 1a Supernova A visualization of a computer simulation of a Type Ia supernova. Ejected material (brown) from an exploding white dwarf star crashes into its companion star (blue), yielding an ultraviolet pulse of light streaming from the companion star’s location

Evidence has squarely pointed to “white dwarfs” as the source of these cosmic blow-ups. White dwarfs are the aged remnants of stars like our Sun that have contracted into a dense, compact balls about the size of Earth. Yet whether one white dwarf, or two, is needed to spark a Type Ia supernova inferno has remained frustratingly unclear to astrophysicists for decades now.

Two recent studies now offer perhaps the strongest evidence to date that both scenarios are valid, though the evidence hasn’t been easy to come by. Because supernovae are typically so far away, researchers must forensically work backwards from the explosion’s aftermath to pinpoint its culprit star or stars. Now, armed with a new theory about Type Ia supernovae and aided by a fleet of modern telescopes, astronomers have shown that these stellar explosions can occur under more than one set of circumstances.

The Kavli Foundation spoke with four astrophysicists about the significance of nailing down the causes of Type Ia supernovae for learning more about the dynamics of stars, galaxies and even the universe on its grandest scales. 

Top: An artist’s impression of the single white dwarf Type Ia supernova scenario. Bottom: An artist’s impression of the double white dwarf Type Ia supernova scenario

One white dwarf star or two: Which scenario leads to a Type Ia supernova, one of the biggest, brightest explosions in the universe? In the single white dwarf scenario, the dwarf star gravitationally draws matter off of a normal, nearby companion star. That matter piles up on the white dwarf, eventually triggering a runaway thermonuclear explosion. An alternative scenario does not require a normal companion star; instead, two merging white dwarfs sparks a Type Ia supernova.

A new theory, developed by the physicist Daniel Kasen, has offered a fresh way of finding out which scenario, or both, is correct. The theory predicts that the material hurled out from the supernova explosion of a single white dwarf should create an observable ultraviolet flash when it slams into a normal companion star. The absence of that flash, on the other hand, would lend support to the double white dwarf scenario. Recently, astronomers have found examples of Type Ia supernovae with and without Kasen’s ultraviolet flash, suggesting that there is more than one way to unleash this cosmic outburst.

Source: Kavli Foundation