Image credit: F. Roepke & W. Hillebrandt
In the past few days, I've talked about the controversy of what types of stars explode as Type Ia supernovae. But the mystery surrounding these explosions is not limited to what explodes, but also how.
When a white dwarf explodes, it is undergoing an uncontrolled nuclear reaction, turning carbon and oxygen into silicon, iron, nickel, cobalt, and many other elements. In a matter of a second or so, a sun's worth of carbon and oxygen is burned. But there is a question as to how it burns. Burning comes in two flavors. In a deflagration, or what you would call a normal flame, a flame moves more slowly than the sound speed. A good example is a wildfire, which can race across the ground at 60+ miles per hour (more than 100 km/hr), but even seemingly explosive burning, like gunpowder or an automobile engine, is still just a deflagration.
In a detonation, the flame moves faster than the speed of sound. Most high explosives do this sort of explosion. Detonations are more destructive than deflagrations, which is why plastic explosives are used in modern warfare instead of kegs of black powder.
In white dwarfs, the speed of sound is very high. In Earth's atmosphere, it is around 770 mi/hr at Earth's surface. Sound moves faster through dense material. For example, the average speed of sound through the Earth (as measured by earthquakes) is about five miles per second, or 18,000 miles per hour! Even so, this means it can take nearly half an hour for a sound wave to go through the Earth. White dwarfs, on the other hand, are much denser yet. Although the size of the Earth, sound waves can travel all the way through a white dwarf in less than two seconds at a whopping speed of over 14 million miles per hour! So, a white dwarf can completely burn in a matter of a couple of seconds and still not be a detonation.
How can we tell how a white dwarf burns? By what it leaves behind. In a deflagration, the white dwarf burns pretty thoroughly, leaving behind almost no carbon. The same is true of deflagrations on the Earth -- in your car's engine, you burn all of the gas you put into the piston. In a gun, you burn almost all of the gunpowder. Otherwise, you are just wasting fuel! In a detonation, pieces of unburned material are sent flying. So, for a white dwarf, we would expect to see lots of carbon and oxygen left if it detonates.
When we look at the Type Ia supernovae, we usually see a little bit of carbon and oxygen -- not much, but some. Neither the detonation or deflagration models really work! The truth seems to lie somewhere in between -- most of the white dwarf burns "gently," but somehow, somewhere a detonation explosion occurs, adding some extra energy and keeping all of the carbon from burning.
The problem is, it really is difficult for theorists to explain why one type of burning would turn into another type. So, at meetings such as the one last week, scientists argue very heatedly about their latest models. This isn't work that I do, so I am not qualified to make a guess as to which models are more likely to be correct.
The picture above is a model of a white dwarf deflagrating ("gently burning"). The yellow-orange is the unburned white dwarf, while the grey/white indicates where the flame has already burned the white dwarf. You can see that, as the burning continues, the heat causes the white dwarf to get larger. Soon after the phases pictured above, the flame reaches the surface, and the ashes are spewed into space at a speed of 10,000 miles per second!