Explosions are always messy, and this is especially true when combustion occurs on a cosmic scale.
In space, small, dead stars called white dwarfs occasionally blow themselves to smithereens, producing enough light to drown out entire galaxies. Astronomers use these explosions – called type 1a supernovas – to calculate cosmic distances. In the late 1990s, observations based on type 1a supernovas revealed that the universe is flying apart faster and faster as time goes on. The force behind this acceleration, called dark energy, is still enigmatic, but the discovery earned a Nobel Prize in 2011 and is considered one of the most fundamental in cosmology.
Yet despite their role as the workhorses of the cosmological distance ladder, type 1a supernovas are still poorly understood. Blowing up a white dwarf, both in terms of the detonation physics and the necessary stellar ingredients, is still a scientific conundrum. Now, two studies published this week in Nature are helping solve that ingredients problem, even though at first the observations appear to disagree with one another.
Together, the studies tell “a rich and interesting story about the entire system that leads to supernova of type 1a,” says UC Berkeley astronomer Joshua Bloom. “It’s remarkable how important type 1a supernova were – and still are – for cosmology, given how relatively little we know about the diversity of the progenitor channels.”
White dwarfs are the extinguished corpses of stars that were once very much like the sun. They’re incredibly dense, with a sun’s mass of material stuffed into something the size of the Earth. Left alone, a white dwarf will simply fade to black over billions or trillions of years.
But dwarfs with a starry companion can suffer a different fate. Sometimes, the two stars are close enough for the dwarf’s incredible gravity to begin stealing material from the companion. If the dwarf gets too greedy and gains enough mass, a runaway thermonuclear explosion ignites and blasts the star to pieces.
For a long time, scientists thought white dwarfs must be burgling a large companion star – something like a red giant that’s big and gassy and easy to steal from. Yet pre-explosion observations of nearby type 1a supernovas have failed to identify any red giant companions. Nearby supernova remnants show similarly scarce evidence for giant companions, which would not be destroyed during the explosion, just banged up and tossed from the system.
Other observations have suggested the doomed dwarfs could be dancing with a more or less normal star – a main sequence star, like the sun.
And another possibility, supported by multiple observations but thought to be extremely unlikely up until the last decade or so, is that type 1a supernovas could be the nuclear death spasms produced when two white dwarfs merge. These two-dwarf pairs are known as “double-degenerate” systems, because white dwarfs are stabilized by electron degeneracy pressure.
(There are some even crazier ideas being batted around.)
Different pairs of stars producing type 1a explosions is troubling for some scientists, who wonder how such different starting ingredients can produce such similar explosions. For now, the data suggest that as long as a white dwarf is involved, it’s possible that “type 1as can come from essentially anything,” says Brad Tucker, who splits his time between UC Berkeley and the Australian National University.
A Tale of Four Supernovas
This week’s Nature papers make that even more clear.
One of studies reports that when a type 1a supernova exploded in May 2014, the blast debris crashed into the former companion star. The collision produced a pulse of ultraviolet light that scientists could detect using NASA’s Swift satellite, which swiveled to watch the explosion four days after it started. It’s the first time this type of shock-induced brightening has been seen with a type 1a explosion, says lead study author Yi Cao of Caltech, and indicates that the dwarf’s friend is not another white dwarf. Instead, that companion was probably a bigger, normal star.
The second study used NASA’s exoplanet-hunting Kepler telescope to observe three explosions almost from the moments they ignited in 2011 and 2012.
Those three supernovas, described by the University of Maryland’s Rob Olling, look like type 1a supernovas and are all very far from Earth, between 600 million and 2 billion light-years away. But unlike Cao’s team, Olling and his colleagues saw no brief bump in brightness produced by debris colliding with a companion star. “The data support the idea that the companions are quite compact, with the most likely explanation being double-degenerate systems, two white dwarfs merging,” says Saurabh Jha of Rutgers University, who was not involved in the study.
Apples to Apples
While these results may seem contradictory in that different ingredients are involved, they actually mostly fit together pretty well – and that’s because there’s one more piece of the puzzle to consider.
Over the years, it’s become clear that type 1a supernovas don’t come in just one flavor. Put very simply, some are brighter than normal and others are dimmer. “It’s not just apples,” Olling explains. “There are Granny Smiths and Jonagolds and Braeburns, and we don’t know exactly how many varieties there are.”
The supernova described by Cao, which likely resulted from a white dwarf stealing material from a normal star, is one of those dimmer subtypes. It’s similar to a flavor known as a type 1ax, which is less luminous than your classical kaboom (kind of like a “peculiar cousin,” to borrow Jha’s description, or a “weirdo” to use Tucker’s).
Conversely, the supernovas described by Olling are “normal” type 1a explosions – the type with a predictable brightness that can be used to measure cosmic distances. These likely result from two white dwarfs merging and turning into a giant nuclear bomb.
“Cosmological 1a supernovas do tend to show that they might be doing the same thing, which is great,” Tucker says. “But then there are the weirdoes. I like weirdoes. We all like weirdoes. But cosmology doesn’t like weirdoes.”
So, could it be that two merging dwarfs produce normal type 1a supernovas, while systems with only one white dwarf lead to peculiar explosions?
If yes, then what’s happening is that these observations neatly fall into line. Regardless, not only is it becoming clear that type 1a explosions come from different ingredients, it’s becoming clear that those ingredients don’t all cook the same supernova. And maybe, just maybe, scientists are getting closer to determining exactly how that all works.