The life of a star is so long, millions to billions of years, and the distances between them so vast that the sky- the universe - appears to be constant. The heavens were to the ancient, a place of perfect harmony and stability. But there are moments when that changes, when you see things happening in real time in the human sense, from fractions of a second to weeks.
Novae are such events, the non-destructive explosion and expulsion of the outer layer of a star -- equivalent to the mass of a planet -- at velocities from thousands to tens of thousands of kilometers per second. These have been known since the first systematic star catalogs of the ancient Chinese, Greek, and Persian astronomers, and they have been known to be eruptive, indeed explosive, events for nearly a century. Yet how they explode and what processes of nuclear elemental synthesis and pollution of the galactic interstellar medium happen during the event remain open questions.
In a paper appearing in the current issue of “Nature” the problem has been tackled using a new three dimensional model. The research has been carried by a collaboration headed by Jordi Casanova and Jordi José (Universitat Politecnica de Catalunya and Institut de Estudis Espacials de Catalunya), with Steven Shore (Department of Physics, University of Pisa), Enrique Garcia Berro (Institut de Estudis Espacials de Catalunya) and Alan C. Calder (Stony University of New York - Stony Brook). The fantastic advance in computational power now permits the extension to full nuclear-dynamic calculations that have just begun to use this vast potential.
Imagine two stars that orbit at a mutual distance no farther than that between the Earth and Moon, each of which has the mass of the Sun, one of which is a million times denser than the Earth and about the same size, and the other is distorted by the mutual tide and dumps mass onto the compact object. This is the description of a cataclysmic binary system, so named because the accumulation of mass by the compact star, called a white dwarf, proceeds through a disk in which matter from its companion accumulates and through which it ultimately gets transferred inward. The density, hence the gravitational pull, of the white dwarf is so great that as mass piles up, it drastically compresses and, if it reaches a critical temperature of a few hundred million degrees, instigates a runaway nuclear reaction by fusing hydrogen and carbon nuclei. If this happens in the core of a star it isn't a problem, the overwhelming weight of the layers above the core contain the reactions and the star can find an equilibrium. If, in contrast, the reaction is ignited on the surface, nothing constrains the matter and it blows off into space with a velocity about one percent the speed of light. The expanding, hot material continues to be illuminated by the now inflated white dwarf and the binary appears as a star whose brightness has increased by a million-fold. This is a nova explosion, the most extreme hydrogen bomb in nature in which the star radiates as much energy in a matter of minutes as the Sun has during the whole of human history.
But there another, even more spectacular end that may await the white dwarf. We know that such stars have an upper limit to their masses because of how their interiors are structured by the interplay of pressure and gravity, about 50% greater than the Sun. If one of these stars, near its limit, continues to accrete matter, it may be pushed over the limit and collapse. The ensuing event, thought to be a type Ia supernova, is bright enough to see halfway across the present universe and is one of the "standard candles" of cosmology. The problem has been, for decades, that we don't know how such explosions originate, what their progenitor stars look like, despite the remarkable understanding we have of how the event later develops (and can be used o determine distances to their host galaxies). It is quite possible that novae are the seat for such events but in order to understand how we must have a more complete picture of the explosion and how much mass is lost or retained during each event.
The paper appearing in Nature is a theoretical calculation of the time development, about 500 seconds, of the accreted layer from the moment of ignition of the nuclear reactions to the point of expansion and the start of the explosive ejection of the envelope.
This has been made possible through a combination of computer technology (the calculation requires massive parallel processing machines such as the Marenostrum in Barcelona and CINECA in Bologna) and improvements in the handling of the structure. This is the first time a fully consistent nuclear hydrodynamic calculation, one that includes all of the fine structure and turbulence that is known to accompany such ignitions, can be included. The result is that the material from the interior of the accreting white dwarf is mixed by chaotic buoyant motions in the form of fingers and vortices, into the burning zone in which it fuels and accelerates the runaway, much as pouring fuel on a forest fire produces a firestorm. The computation followed what happens in a cube about the size of Toscana on a side through the stages when the first mixing occurs. Surprisingly, the medium retains considerable structure of the sort later seen in the spectrum and structure of the expelled gas.
The actual moment of the explosion is hidden from us by the surrounding disk and the deep layer that has accreted before the ignition so we can only proceed through calculations to understand what happens. But because this happens like a detonation of a bomb, the nuclear waste products, synthesized during the runaway, are mixed into the ejected layers without further modification. This is a way to determine, indirectly but uniquely, the dynamics of the nuclear explosion. If the temperature reaches above one billion degrees or falls below a few hundred million, it changes the final mix of elements and all of this depends on the mass of the white dwarf alone. So we can use the final products as a probe of the underlying white dwarf and even determine its ultimate fate, if it throws off more mass than it accumulates between explosions and eventually reduces to a low, stable mass, or grows in mass with each event and ultimately arrives at the stability limit.
The calculations we report now in Nature are the first step to realistically simulating the explosion, without imposing artificial constraints on the dimensionality (for instance assuming two dimensions or just looking at average properties) and the first to produce the variations in nuclear products that have for so long been a mystery.
The depth of the mixing is greater than previously thought and the intensity of the explosion is greater. So it appears we are finally on the road to understanding whether these systems, among the most promising of the progenitors for the cosmologically essential type Ia supernovae, can ultimately end in such a catastrophic explosion.