Supernovae occur when stars detonate, blasting their contents throughout the Universe. This doesn’t just happen with massive stars at the end of their lives; white dwarf stars (which are technically already dead) can end with a bang as well. This latter scenario is known as a Type Ia, and is fairly common. Researchers with NASA’s Spitzer Space Telescope have identified a possible new trigger behind these explosions. The results of the study, which was led by Brian Williams of NASA's Goddard Space Flight Center, have been submitted to the Astrophysical Journal.
The observations of Williams’ team revealed a Type Ia explosion that happens very rarely, where the white dwarf acts like a stellar zombie. The dead star accumulated gas from a different star, siphoning its life away. Astronomers now have to determine the sequence of events which cause the explosion to happen.
"It's kind of like being a detective," Williams said in a press release. "We look for clues in the remains to try to figure out what happened, even though we weren't there to see it.”
Main sequence stars like our Sun are fueled by nuclear fusion, which releases a tremendous amount of energy while building elements that are progressively more massive, up to a point. Stars with higher mass will fuse iron, which requires more energy to produce than it puts out. These types will suffer a core collapse supernova, leaving a neutron star behind. Less massive stars eject their outer layers, which will go on to form planetary nebulae, while insides become the white dwarf remnant.
Type Ia supernovae are fairly consistent, which allows researchers to use the information when calculating size the Universe, as well as the changes caused by expansion. They have long been thought to occur from the collision of two white dwarfs, but over the last decade or so, there have been other scenarios theorized, most notably involving the white dwarf cannibalizing another star until it induces an explosion.
In 1604, Johannes Kepler and other astronomers observed a supernova that is believed to have come from a white dwarf that had an old red giant companion nearby. That supernova was named after Kepler. Over 400 years later, Williams’ team discovered only the second supernova remnant that was likely formed under similar conditions. The supernova is named N103B, and occurred 160,000 light years away. The N103B remnant is about 1000 years old, and resides in gas and dust that is believed to come from a companion star. Sadly, there is no information in the historical record to indicate that the supernova was observed by astronomers.
"It's like Kepler's older cousin," Williams explained. "The region around the remnant is extraordinarily dense.”
The researchers believe that in order for this type of supernova to occur, a white dwarf is orbited by its large, old companion star. As stars age, they slough off some of their outermost material. It is assumed that this material is then “eaten” by the white dwarf, which then becomes more massive. Eventually, it becomes too massive and the gravitational pressure on the core causes it to detonate into a violent supernova.
If this assumption is correct, the researchers believe that it is very uncommon and that white dwarf collisions cause most Type Ia explosions. As for the actual mechanism that causes the white dwarf to crumble and ultimately explode, researchers aren’t quite sure. The Spitzer team will continue to study supernovae in hopes of finding those answers.