When I joined Phenomena, Carl Zimmer asked: What obsesses you? Among my obsessions, I answered, are type 1a supernovae. Here we go.
How can an astronomical object of such crucial cosmological importance remain so fundamentally mysterious?
When a runaway thermonuclear explosion rips through a white dwarf star and blows the star to bits, it’s called a type 1a supernova. These explosions are incredibly violent and incredibly bright, sometimes outshining entire galaxies. Thought to occur about once every two centuries in a galaxy like the Milky Way, these stellar cataclysms are relatively frequent events.
The star doing the exploding is a white dwarf with a fairly standard mass, so the supernova’s brightness is predictable. And because luminosity decreases with distance, scientists can use the difference between an explosion’s observed and predicted brightness to determine how far away the blazing starstuff is. That characteristic has led to type 1a supernovae being called “cosmic mile markers” and “standard candles.”
In the late 1990s, distance measurements based on type 1a supernovae revealed that the expanding universe is accelerating. In other words, it’s flying apart more quickly now than it was billions of years ago. Scientists still don’t know exactly what’s going on, but they attribute the phenomenon to an enigmatic thing called dark energy. The discovery represented a fundamental shift in cosmology and earned the Nobel Prize in physics in 2011.
But here’s the thing: Despite their crucial cosmological importance, type 1a supernovae are still very much a mystery. As astronomers study more and more of them, it’s becoming increasingly clear just how non-standard these explosions actually are – and how little we really know about them.
“They’re standardizable candles, not standard candles,” astrophysicist Brad Tucker told me a bit ago, while I was working on a feature describing type 1a supernovae for the Proceedings of the National Academy of Sciences. Tucker splits his time between UC Berkeley and the Australian National University.
“These are very powerful tools in cosmology,” he said. “But we really don’t know what’s going on with them.”
It’s true. The uncertainties swirling around these fascinating explosions are kind of astonishing. Here are a few.
1. Until now, there was no proof that white dwarfs were doing the exploding.
For starters, we didn’t have solid observational evidence pointing to white dwarfs as the culprits behind type 1a supernovae until earlier this year, as reported yesterday in the journal Nature. Decades of solid theoretical work (and circumstantial evidence) suggested as much, but the observations weren’t there to back it up.
But in January, a star exploded in the Cigar Galaxy. Essentially next door at only 11.5 million light-years away, it was the closest type 1a supernova to Earth in four centuries. Chemical signatures in the billowing debris cloud revealed that supernova 2014J, as it’s called, is a type 1a supernova. Because the explosion was so nearby, astronomers were able to detect gamma-rays coming from the debris, a type of radiation that hasn’t been observable in other type 1a supernovae.
Simulations of type 1a supernovae predicted that exploding white dwarfs would produce gamma-rays, but the particles don’t normally make it to Earth.
Yet earlier this year, a team of scientists observed gamma-rays coming from 2014J with the European Space Agency’s INTEGRAL satellite.
Using pathways that explain how gamma-rays are formed from the synthesis of iron, cobalt, and nickel, the team worked backward from the detection and determined what kind of star had exploded. It was a white dwarf star, scientists said, with about 1.4 solar masses of material stuffed into it.
This is the most solid observation to date that implicates detonating white dwarfs in the production of type 1a supernovae, and one that astronomers have been eagerly awaiting, for a long time.
“The importance of this discovery is not because something new/unknown was discovered, but we had an observation of a long-standing theory that had no real evidence,” Tucker wrote to me a few days ago. “Knowing that our fundamental physics is correct is an important thing, especially given how all our other ideas (progenitors systems, mass, donor stars) don’t seem to be working!”
(It’s worth mentioning that another recent nearby supernova – 2011fe, which exploded in the Pinwheel Galaxy in 2011 – provided fairly good evidence for a white dwarf being the progenitor star. Those constraints came from the PIRATE telescope in Mallorca, which serendipitously observed the supernova several hours after it exploded. Because the billowing debris cloud wasn’t yet visible, astronomers concluded the explosion must have been the work of a white dwarf. But there was still wiggle room. “It’s the most compelling, by a reasonable margin,” astronomer Alexei Filippenko of UC Berkeley said of the evidence late last year, when were discussing this point for the PNAS feature. That was before 2014J exploded.)
2. The Chandrasekhar Limit sometimes isn’t.
Secondly, there’s a well-known fact that commonly appears when people write about type 1a supernovae: That white dwarfs explode when they get to be about 1.4 times as massive as the sun – a mass known as the Chandrasekhar Limit. All of this is true, but it’s not the whole story.
White dwarfs are incredibly dense, dead stars, formed from the collapse of stars that were once very much like the sun (and yes, our sun will become a white dwarf). They’re about the size of Earth, but with a sun’s mass of material squeezed in. Most of the time, white dwarfs can happily exist in this state for billions of years.
That’s because the white dwarf’s intense, crushing gravity is counteracted by a quantum mechanical thing called electron degeneracy pressure, which basically prohibits electrons from being shoved any closer together. In other words, degeneracy pressure prevents the star from collapsing further. (Degeneracy pressure is the reason why white dwarfs are called degenerate stars.)
These two competing forces can keep a white dwarf stable forever, as long as it doesn’t get too massive. But if the star gains enough material and exceeds about 1.4 solar masses, gravity normally wins and degeneracy pressure fails and all hell breaks loose.
That’s the Chandrasekhar limit. And it’s important, but it’s not the only thing that lights up a supernova.
Something else happens at around 1.4 solar masses, and it turns out that this is a crucial part of exploding the star: At this point, the star is massive enough to begin fusing carbon – and that is what ignites the runaway thermonuclear reaction.
“Fusion happens in a flash,” astronomer Robert Kirshner of the Harvard Smithsonian Center for Astrophysics writes this week in Nature. “A thermonuclear flame rips through the white dwarf, fusing carbon into heavier elements with a sudden release of energy that tears the star apart.”
Many white dwarfs are made from a ton of carbon and a lot of oxygen. So when carbon ignites, it just keeps going and the star is blown to bits. That this happens around the Chandrasekhar limit is kind of a coincidence, says astronomer Ryan Foley of the University of Illinois, Urbana-Champaign. “The Chandrasekhar limit is a red herring. It’s a physical thing that’s very, very important, but for type 1a supernovae, it’s not the most important thing,” he told me, when we were talking about supernova 1as for the PNAS feature. “You have to have the carbon ignition, somewhere near the center of the star.”
AND ANOTHER THING ABOUT THAT: Astronomers have evidence for type 1a supernovae born from white dwarfs that exceeded the Chandrasekhar limit. By a lot. These type 1a explosions, called super-Chandras, are so ridiculously, anomalously bright, and kick out so much radioactive nickel, that they could only come from a bulked up, beefy dwarf star – something with 1.6 or 1.8 or even more than two solar masses of material.
So there are mechanisms that allow white dwarfs to bypass the Chandrasekhar limit (astronomers theorize that something like a very rapid spin rate might help the star avoid catastrophic collapse) and produce explosions that are anything but standard.
Conversely, there are type 1a supernovae that are ridiculously, anomalously dim. These mini-supernovae, discovered in 2013, are called type 1ax explosions. Astronomers have spotted about 30 of them.
Two weeks ago, Foley and his colleagues reported in Nature that they’d found the progenitor system for a type 1ax called SN 2012Z: A white dwarf, paired with a bright blue helium star companion. The dwarf snagged some material from its gassy blue friend, the team says, but didn’t gain enough mass to completely explode. So instead of a bang, the star let out a whimper.
Scientists sorted this out by staring at old Hubble Space Telescope images of the supernova’s home and determining which stars were involved in the eventual conflagration. It’s the first time anyone has been able to so precisely see a progenitor system before it blew up.
Astronomers affectionately refer to the collection of anomalous supernovae as “weirdos.” By studying the weirdos, they hope to better understand the normal type 1as, the explosions that are useful for things like cosmology.
“By identifying and studying the extreme cases, we can hope to learn not only about these interesting weirdos, but the weirdos might end up teaching us something about the more normal ones,” Filippenko said.
3. We don’t know who’s helping to kill the dwarf.
But is there a type of normal supernova 1a? Well…yes. Kind of.
For a long time, astronomers figured that since 1as were all so similar, there must be just one way to cook them up. A key step in the process of making a 1a is that the white dwarf has to somehow grab enough material to ignite carbon fusion and explode.
This means that the white dwarf can’t live on its own. It needs to live in close proximity to a stellar friend it can steal from. As the two stars orbit one another, the dwarf’s intense gravity will siphon material from its companion, and it will eventually gain enough mass to detonate.
For decades, astronomers thought that companion star was something like a red giant, something big and gassy that’s easy to steal from. Many textbooks and diagrams still depict this kind of supernova precursor, called a single-degenerate system (because there’s only one degenerate star, the white dwarf).
And there is some recent observational evidence implicating this lethal pairing in type 1a supernovae.
The remnant of a supernova called PTF 11kx contains shells of gas that can only have come from a large, red giant companion, scientists reported in Science in 2012. And the paper published this week about 2014J suggests that it came from a white dwarf-red giant pair.
But as with everything we’re learning about type 1a supernovae, those single-degenerate systems are not the only game in town. Now, many astronomers think those systems produce a minority of type 1a supernovae and that most of the explosions we see come from a different deadly combination.
“Less than 1 percent of all 1a supernovae could be from a companion red giant star,” Tucker said. “It’s a dramatic shift in what we’re thinking. It’s gone from the most popular scenario to one no one wants to touch.”
Recently, evidence has been amassing that suggests type 1as result from a pair of two white dwarf stars (a double-degenerate system). So, instead of being locked in a deadly dance with a red giant, the white dwarf is dancing with another white dwarf.
This kind of progenitor system was discounted years ago because explosion stimulations couldn’t quite make it work – scientists had trouble bringing the white dwarfs close together quickly enough, or couldn’t get the variety of elements produced by the explosion to come out right.
But in the last five or 10 years, that’s changed. And a variety of new observations point toward double degenerate systems as producing type 1a supernovae.
These include studies of supernova remnants where astronomers have failed to find the large ex-companions that should still be visible (this kind of star would survive the supernova and be detectable, whereas a second white dwarf would also be obliterated); surveys that look at the gas outflows produced by type 1a supernovae and find only scarce evidence for the expected bits of red giant; and observations of exploding supernovae that show no evidence for what astronomers call a “shock breakout,” or a kind of flare produced by a big star getting in the way of the billowing debris cloud. (And there’s another recent paper that suggests 2014J came from a precursor system with two white dwarfs.)
Now, it seems, instead of arguing about which of the two progenitors channels is The One, astronomers are debating how many different type 1a progenitor systems exist, and how common each of them is. This bothers some scientists, who are having a tough time reconciling the observed similarities in normal type 1a explosions with the fact that so many different ingredients could be involved.
“Somehow, these completely different stellar systems that have evolved in completely different ways and are completely different at the end of their lives – when the star explodes – make things that are almost identical,” Foley said. “That troubles me a bit.”
Ultimately, it will be really important to sort this out because the more astronomers know about type 1a supernovae, the more accurately they’ll be able to measure cosmic distances. Even the “normal” type 1as – the ones used for cosmology – explode with varying brightness. Their explosions vary depending on what kind of galaxy they’re in, where in the galaxy they are, and, potentially, which progenitor system is involved.
So that whole predictable brightness thing? It’s not quite that simple.
4. Uh-oh. Does this mean dark energy is going away?
Nope. Dark energy and the accelerating universe are here to stay. But this is an era of precision cosmology, and tiny inaccuracies can have a big effect on what scientists understand about the repulsive force that is dark energy.
This is especially true for the supernovae scientists are really hoping to detect – the really, really ancient ones that can help nail down the behavior of dark energy in the very early universe.
“The question is, are those supernovae really the same as the ones nearby? This issue of progenitors really affects that,” said Saurabh Jha, an astrophysicist at Rutgers University.
Ten billion years ago, the universe was filled with a different population of stars than we see today. The matter making up those stars had different chemical compositions. And if those ancient supernova 1as are behaving differently than the more recent ones, scientists need to know about it. Otherwise, imprecise distance measurements will yield an inaccurate understanding of what was happening during these earlier time periods.
Long live the standard candle.