What a Simple Flame Can Tell Us About Supernovae
"There is not a law under which any part of this universe is governed which does not come into play and is touched upon in these phenomena. There is no better, there is no more open door by which you can enter into the study of natural philosophy than by considering the physical phenomena of a candle."
In 1860, British physicist Michael Faraday gave the last in a series of Christmas lectures at the Royal Institution, on the chemistry and physics of flames, eventually published as a popular book, The Chemical History of a Candle. It seems like such a simple thing, but even today, over 150 years later, scientists understand very little about the complex processes taking place within even a simple flame.
In fact, it's a very active area of research, as evidenced by papers presented at a recent meeting of the American Physical Society's Division of Fluid Dynamics, held November 20-22 in Baltimore, Maryland.
Using detailed computer simulations, physicists have been studying one critical transition in particular that could not only improve safety in industrial settings, but could also shed light into the mysterious processes behind Type 1a supernovae.
There are several types of supernovae. The Type 1a variety doesn't form as the result of the explosion of a massive star. Instead, it develops via the destruction of a white dwarf orbiting a younger binary partner, or two white dwarfs slamming into one another.
In the former case, material is stripped from the binary partner until the material reaches "critical mass" around the white dwarf. At that point, the supernova ignites, burning brightly for a short period until all that's left is remnants, known as nebulae.
That much physicists understand, but the underlying physical mechanism is complex, and remains mysterious. A better understanding of the processes that give rise to Type 1a supernovae would enable us to make more precise distance measurements in space, since these objects are used as standard candles.
Of particular interest is a phenomenon known as deflagration to detonation transition (DDT). Deflagration is essentially a slow burn, or "subsonic combustion," in which a burning substance heats the next layer of cold material until it ignites. That's the process behind internal combustion engines, for example, or the pyrotechnics of fireworks.
Detonation is quite different. It is supersonic, and it spreads through shock compression — in short, it's an explosion, and far more destructive than deflagration. Sometimes a subsonic flame can go supersonic, resulting in a big BOOM! That's the DDT, and it's behind some of the worst industrial accidents in recent history, such as the 2005 Hertfordshire Oil Storage Terminal fire.
"Explosions are most often driven by flames propagating at relatively slow subsonic ," Alexei Poludenko of the Naval Research Laboratory told Physorg.com. "Under certain conditions, however, this 'slow' mode of burning can transition to a completely different regime" — the DDT. "In this case, burning is driven by very fast, strong shock waves that can travel at more than five times the speed of sound."
Poludenko has been running computer simulations of this critical transition with colleagues at Sandia National Laboratory to better understand what gives rise to the DDT.
Among other findings, they discovered that DDT often occurs in confined systems, that is, when walls and other obstacles are present.
If something starts burning in an enclosed space, the pressure will increase, and once it gets high enough, shock waves can form. The walls and obstacles serve to amplify those shock waves, until — BOOM! You get detonation.
But under just the right condition — say, flames fueled by hydrogen-air and methane-air mixtures under atmospheric pressures subjected to intense turbulence — even a flame in an unconfined environment can go from deflagration to detonation.
That's the kind of DDT that might be at work in supernovae, although since those reactions are thermonuclear, it is not identical to what takes place in chemical combustion. Still, researchers at SUNY-Stony Brook think they can learn something about what happens inside supernovae by gaining a better understanding of its chemical analog here on Earth.
Along with Alan Calder of Stony Brook and Dean Townsley of the University of Alabama-Tuscaloosa, Aaron Jackson — now a research associate at the Naval Research Laboratory — devised 3D simulations of a slow-burning flaming subjected to extremely intense turbulence, to see what conditions result in a DDT.
One hypothesis among astrophysicists is that the interior of a white dwarf is also subject to intense turbulence. When the DDT occurs, the white dwarf goes supernova.
You can see this process unfold in the image at the top of this post: four still shots showing a flame spreading outward, first subsonically (the slow burn of deflagration), until it reaches a threshold density, at which point the flame transitions to detonation. Compare that with the image just below it on the right: that's a computer simulation by Argonne researchers of a Type 1a supernova.
It's just a start, Jackson cautions; since there are so many unknown variables about this process, the simulation produced a broad range of possible outcomes. The next step will be to match his simulations with actual observations of supernovae to identify the most likely conditions under which the DDT can occur.
So it turns out Faraday was right, way back in 1860. Careful observation of a deceptively simple process here on Earth could end up teaching us something new and invaluable about the universe.
Image credits: Naval Research Laboratory, NASA, Argonne