Astronomers have weird names for things. Blame it on the fact that they started naming stars and planets a long, long time ago, back when they thought the sun went around the Earth. "Planet" means wanderer, even though planetary orbits are completely regular and predictable. A "planetary nebula" has nothing whatsoever to do with planets. And a Type II (aka Type 2, but astronomers insist on Roman numerals) supernova is common, but a Type Ia (1a) supernova is rare, with a bizarre origin story.
Rare, but useful. (We'll get to that.)
A group of astronomers just announced that they've found the perfect Type Ia supernova. The Platonic ideal of Type Ia supernovae: 2011fe.
What makes 2011fe so perfect? For one thing, the supernova spotters (known as the Nearby Supernova Factory) found it almost right away in August, 2011, when the supernova was less than 12 hours old.
“We’d never before seen a Type Ia supernova this early,” said Greg Aldering of the Lawrence Berkeley National Laboratory in a press release. For another, it's right in our galactic backyard – just 21 million light years away – so we got a crystal-clear view. Amateur astronomers could see it through binoculars.
Most of the Type Ia supernovae we've spotted before were far enough away that the light was distorted by interstellar dust. Astronomers can "correct" for those distortions, but in doing so, they have to make a lot of assumptions. That makes the data somewhat less trustworthy. This time, we got an undistorted view, giving us a gold standard against which to measure all future Type Ia supernovae. It's also "remarkably normal," the astronomers found – the light curve fell right in the heart of normal distributions from past Type Ia observations, with no "peculiar" outliers to explain away.
Rollin Thomas, a member of the research team, remembers that as new data arrived from the telescope each night, he would think, “Please don’t be peculiar, please don’t be peculiar.” His wish was granted: “Our measurements showed how remarkably normal 2011fe is," says Dr. Aldering.
The researchers began watching the supernova just hours after it began. Two weeks later, it reached its peak brightness, but they kept watching it, off and on, for another three months, as the light faded away. In the June (current) issue of Astronomy and Astrophysics, the Nearby Supernova Factory is releasing their 32 nights of data on 2011fe. They even made a movie of it.
Remind me: What's a supernova?
Nearly all supernovae are explosions caused by super-giant stars exploding. After a super-giant's core stops fusing, it begins to collapse on itself. As more and more mass falls inward, the atoms are forced to fuse together, resulting in a thermonuclear explosion. Think Nagasaki, but instead of something the size of a softball, it's a star 10 to 70 times bigger than our sun.
That's a Type II supernova.
But there are a few oddballs, known as Type Ia supernovae. Type Ias are also fusion explosions, but they're not caused by the sheer enormousness of the exploding star. In fact, Type Ia supernovae are kicked off by tiny white dwarfs.
When big stars die, they go supernova. When small stars die, their outer layers expand into a planetary nebula (which, remember, has nothing to do with planets) and the surviving core collapses into a white dwarf.
White dwarfs are also known as degenerate dwarfs. (No, not Tyrion Lannister.) These are very old, Earth-sized stellar remnants under so much pressure that it's literally impossible to squish them any further without collapsing the atoms into neutrons.
They're also thieves. If another star wanders close enough to a degenerate dwarf, the dwarf will start stealing mass from it. (If two white dwarfs find each other, they collide in what's called a "double-degenerate" system.) The stolen mass leads to fusion explosions on the white dwarf's surface, called novae.
Remember how astronomers give weird names to things? Novae and supernovae really have nothing in common except for fusion. A nova is a fusion explosion on the surface of a star. A supernova is the explosion of a star.
But a degenerate dwarf can only steal so much. Once it gets to 1.4 times the mass of our sun, it'll collapse into a Type Ia supernova. (Hat tip to Subrahmanyan Chandrasekhar, who calculated the 1.4 solar mass limit, named the Chandrasekhar limit in his honor.)
Of course, these are theoretical models, based on all the Type Ia supernovae observed before. “The 2011fe observations can be used to test these models,” says Aldering. “For 2011fe, the existing models of the double-degenerate scenario agreed best at some epochs, but the single-degenerate scenario was better at others. And for some epochs both agreed very poorly with the data, suggesting these models have a way to go.”
What's so cool about Type Ia supernovae?
Thanks to the Chandrasekhar limit, we know that every single Type Ia supernova starts with a mass of 1.4 stellar masses. All of them. No matter how far away they are.
In astronomy, it's surprisingly difficult to calculate interstellar distances. When you see a pale dot in the sky, is it a supergiant star that's very far away, or a little planet nearby? Ships at sea had the same problem: Is that dim glow a lighthouse on the horizon or a firefly in front of your nose?
Knowing the mass of a Type Ia supernova, we can calculate exactly how much energy it will release over its supernova lifespan. That means we can measure any Type Ia supernova's brightness and back-figure exactly how far away it is. That gives us distances to anything nearby, too.
This prompted astronomers to call Type Ia supernovae "standard candles" – not lighthouses, not fireflies, but perfectly standard candles with a perfectly predictable light pattern.
That's why the astronomers were so excited to find 2011fe and get a crystal-clear view of a Type Ia supernova, right in our backyard: it gave them a "standard candle" light curve that hasn't been subjected to any of the assumptions necessary to correct fuzzy data.
The team hopes 2011fe will answer many questions about Type Ia supernovae, including exactly what causes these titanic thermonuclear explosions. "We've never had data like this," says Aldering. "It’s a dream opportunity to stimulate deeper thinking about these markers of the expansion of the universe.”