An international team of astronomers from Europe, Israel and the United States has succeeded in shedding light on the origin of Type la supernovae – powerful nuclear explosions in deep space that allow us to chart the vast distances between galaxies. It is known that a white dwarf star is responsible for creating the distinctive, intensely bright explosion, but the cause of the supernovae are still a topic of hot debate.
Currently, there are two theories. The first is that the white dwarf goes supernova following an impact with another, less massive white dwarf star. This is known as the double-degenerate model as it requires the collision of two stellar bodies.
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The second origin theory asserts that a Type la supernova occurs when the powerful gravity of a dense white dwarf strips material from a partner that could either be a red giant, or a star similar to our own Sun. The white dwarf continues the process until it reaches a point of critical mass, known as the Chandrasekhar limit, after which a runaway nuclear reaction is inevitable. This is the single-degenerate model. Both origin theories could potentially result in what is known as a "standard candle" event, meaning direct observation would be needed to settle the argument.
This family of supernovae are so named standard candles due to the fact that we know to a good level of detail, the amount of light that is thrown out by these nuclear explosions. We can work out its distance from Earth by measuring how much said star dims from what we know to be its true brightness.
However, recent discoveries have shown that our understanding of these events is yet in its infancy, yet each discovery sharpens our ability to use the cosmic markers more efficiently. For example, a revelation earlier this year that there were in fact two subsets of Type la supernovae, has allowed us to isolate particular instances of the explosions that are more reliable as a measuring stick.
The newest observations were made using the intermediate Palomar Transient Factory (iPTF) instrument mounted on the 48-inch (122-cm) Samuel Oschin Telescope atop Palomar Mountain, Southern California. The telescope takes long-term observations of a large patch of sky searching for transient celestial objects. On May 3, the telescope spotted the Type la supernova iPTF14atg, which sits an estimated 300 million light years away from Earth in the galaxy IC831.
It was an exciting find, as Type la supernovae occur only once every few centuries in the Milky Way, making them a rare celestial phenomenon, and this specimen exhibited a characteristic with serious implications to the origin debate. Therefore, soon after the initial detection, ground and orbital assets, including NASA's Swift satellite, were brought to bear on iPTF14atg.
Astronomers were able to get the most out of observations by imaging the supernova in ultraviolet light, a spectrum which has a higher energy than visible light, making it a prime medium for detecting the minutia of the rare stellar event. The observations carried out by Swift detected a pulse of UV light that initially lessened, but then increased as the supernova brightened.
The reading was consistent with the single-degenerate model, which, it was predicted, would see material thrown out from the white dwarf impact its companion star, creating a powerful shock wave that would go on to ignite the stellar body.
Type la supernovae represent a cornerstone of our understanding of the universe, and understanding their creation may allow us to chart the distances between galaxies with a fidelity that is beyond our current capabilities. However whilst the findings represent direct evidence of the single-degenerate model, it is not to say that Type la supernovae are not being generated by the competing double-degenerate model. The universe is a big place, and continued observation will be needed if we are to truly understand the secrets at the heart of the stellar maelstrom that are Type la supernovae.
A paper outlining the team's findings are available from the journal Nature.