Supernovae are among the most spectacular phenomena in our universe, and burn so brightly once they form, that they are frequently used as "standard candles" by astrophysicists to measure distances. But while astronomers discover existing supernovae quite regularly, it is much rarer to witness the initial explosions. It happens every 50 years or so.
But what if we could create a tiny version of this initial explosion in the laboratory? A team of physicists from the University of Toronto and Rutgers University have done just that, and have published their results in the journal Physical Review E.
Take it from the pop group They Might Be Giants: "Oh the sun is a mass of incandescent gas/A nuclear furnace/Where hydrogen is fused into helium/At temperatures of billions of degrees." That constant stream of nuclear reactions is what keeps a star from collapsing. As the hydrogen runs out, fusion slows down, and gravity causes the core to contract, raising temperatures even further, sufficient to give rise to a brief, shorter phase of helium fusion.
What happens next depends on a star's mass. For example, for massive stars (greater than 8 solar masses), this collapse is so violent that it causes a huge, catastrophic explosion. It is in these explosions that all elements heavier than iron are produced. The temperatures and pressures become so high that the carbon in the star's core begins to fuse. This halts the core's collapse, at least temporarily, and this process continues, over and over, with progressively heavier atomic nuclei.
The cores of those supernovae begin to resemble an onion, with layers upon layers of elements — the outermost layer is hydrogen, which surrounds a layer of hydrogen fusing into helium, surrounding a layer of helium fusing into carbon, and so on. In fact, most of the heavy elements in the periodic table were born in the intense furnaces of exploding supernovae that were once massive stars.
Smaller stars (like our sun) gradually cool to become white dwarfs. If a white dwarf that has run out of nuclear fuel is part of a binary system — that is, it has a companion star — it can slowly start to siphon off matter from its partner, adding to its mass until its core reaches high enough temperatures for carbon fusion.
The white dwarf can also merge with another star, although this is extremely rare. If this happens, the white dwarf will begin to collapse, a process which also raises its temperature past the nuclear fusion ignition point. Within a few seconds of ignition, a substantial fraction of the matter in the white dwarf undergoes a runaway reaction, releasing huge amounts of energy in a supernova explosion. (You can watch a nifty video of this process here, courtesy of the University of Chicago.)
That initial explosion is known as a "flame front," and it shoots matter violently away from the star's core in a giant mushroom cloud. This is what the Toronto and Rutgers physicists have created in the lab. Or rather, they've created an analogue of the flame front that resembles a lava lamp.
The physicists filled a metal cylinder with water and glycerol (a sweet thickening agent often used in low-fat recipes), and then injected a water-based solution into the container through a handy tube. A chemical reaction ensued, and the water-based solution became more buoyant, slowly rising to the top of the cylinder. Get the ingredients in the mixture just right, and that chemical reaction resembles the mushroom cloud phenomenon typical of a white dwarf going supernova (pictured, showing the steps as a, b, c, d, e).
That "plume" is a self-sustaining reaction. As the plume slowly rises to the top of the cylinder, it produces a "vortex ring" that detaches from the initial plume. Because the plume reaction is self-sustaining, eventually another vortex ring forms, an then detaches, and so on, until the plume finally reaches the top of its container.
Sure, it's simply a tabletop analogue of the process, but it could still tell scientists a great deal about how supernovae form — particularly that initial explosion. And when they finally do catch a bona fide supernova in the act of forming, we'll know just how good an analogue it is.