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Unsettled Mechanism of Supernova Detonation Gets a New Twist

qq009Type Ia supernovae, often used to calibrate cosmological measurements, may arise from merging white dwarfs, after all... When stellar cataclysms known as type Ia supernovae flare up far across the universe, their brightness and consistency allow astronomers to use them as so-called standard candles to measure cosmological distances. Just over a decade ago, two teams used the supernovae to show that the universe is accelerating in its expansion due to the influence of dark energy, a shocking discovery that thrust type Ia supernovae into the astrophysical limelight. But how exactly did these cosmic mileposts come to be?

A type Ia supernova arises from the explosion of an ultradense stellar remnant known as a white dwarf, but it is less than clear how the white dwarf comes to ignite in a thermonuclear blast. The traditional view held that a white dwarf, locked in a binary pairing with another star, sucked matter from its companion, growing ever larger in size until it could no longer support its own weight. Once a white dwarf reaches the Chandrasekhar limit, roughly 1.4 times the mass of the sun, it contracts and explodes in a massive blast. But a new study presents evidence that, for at least one kind of galaxy, the binary-accretion model should not be more than a minor contributor to the observed type Ia supernovae population. Marat Gilfanov, an astrophysicist at the Max Planck Institute for Astrophysics (M.P.A.) in Garching, Germany, and the Space Research Institute in Moscow, along with M.P.A. graduate student Ákos Bogdán looked at elliptical galaxies for x-rays expected to arise during the accretion process. In a study published in the February 18 issue of Nature, Gilfanov and Bogdán report that they found just a fraction of the x-rays expected from white dwarfs accreting matter from their neighbors. (Scientific American is part of Nature Publishing Group.) The standard path to type Ia supernovae, the study's authors wrote, should have produced 30 to 50 times the x-rays observed, indicating that accreting white dwarfs account for less than 5 percent of the explosions.


As a white dwarf draws off hydrogen-rich material from a binary companion over millions of years, Gilfanov says, it experiences a steady process of nuclear fusion on its surface that gives off tremendous amounts of radiation. That radiation should be detectable in the x-ray band, although interstellar gas and dust would absorb some of it. That is why the researchers focused on elliptical galaxies, which have less obscuring material than spiral and irregular ones. Gilfanov says they are now working on characterizing the type Ia progenitors in other galactic types, such as the spiral cousins of our own Milky Way.

Andrew Howell, a staff scientist with the Las Cumbres Observatory Global Telescope Network in Santa Barbara, Calif., says that alternative origins for type Ia supernovae are becoming more compelling. "The evidence has been building for years that the classical paradigm, the single-degenerate scenario, is not enough to explain every type Ia that we see," Howell says. The favored alternative at present is the so-called double-degenerate scenario, in which two white dwarfs locked in a binary pairing spiral inward and merge, triggering an explosion. Such an explosion, which would have more fuel to burn than a single detonated white dwarf, might explain certain bright supernovae that appear to be powered by an object above the Chandrasekhar mass.

Howell says that these mergers have been less favored because it is difficult to make them work in a three-dimensional computer model, although recent work has offered promise. "Nature is telling us that these mergers happen, but we're not smart enough yet to figure out how this happens," he says.

The use of type Ia supernovae for cosmic distance measurements does not depend heavily on knowing the mechanism by which they detonate, so the new work will not unseat dark energy as a widely accepted component of the universe. But Howell notes that the mix of supernova brightnesses changes as astronomers look farther across the universe and, by extension, further back in time. Understanding the progenitors of the explosions might help unravel their evolution through cosmic time.

And Gilfanov says that resolving the underlying astrophysics of type Ia supernovae would help make standard-candle measurements more precise. "If we want to go from 10 percent to 1 percent [uncertainty] in measuring cosmological parameters," he says, astronomers need a better understanding of why white dwarfs explode in supernovae. "Dark energy will not go away, and the concept of standard candles will not go away," Gilfanov says. "It just gives us a better understanding and a better set of tools."


Birth of a Supernova Type 1a

Type Ia supernovae originate in binary star systems—systems where two stars orbit each other. In one possible scenario, there are two stars approximately the same size—let's say about the size of our sun. At a certain point in their lifetimes, each star will expand into a red giant, then turn into a white dwarf.

Most of the heat that a star generates comes about through nuclear reactions in its core. The reactions are triggered when the high pressure and temperature within the star cause hydrogen atoms to fuse together to form helium atoms. When a star this size runs out of hydrogen, its core begins to contract and its outer layers expand. The star becomes a red giant.

The red giant's core contracts, which generates enough heat to fuse its helium atoms into carbon. The nuclear fusion in turn creates other elements, including oxygen. Eventually, radiation generated within the star's core pushes the outer layers of the red giant away. Only the dense core remains: the star has become a white dwarf.

In time, the other star evolves into a red giant. As its outer layers expand far out from the core, it becomes less and less dense. Now closer to the white dwarf, the gaseous material from the red giant feels the gravitational pull from the white dwarf and gets sucked in by its neighbor.

Now being fed by its companion star, the white dwarf pulls more and more stellar material into itself. When the mass of the white dwarf reaches 1.4 solar masses (40% more mass than that of our sun), a runaway nuclear chain reaction causes the entire white dwarf to explode. Because Type Ia supernovae always explode at 1.4 solar masses, they all have more or less the same characteristics, including how brightly they shine.

The light from the explosion is 15 billion times brighter than the sun (this is why we can see them from halfway across the universe). The white dwarf is completely obliterated. And what about the red giant? The outer atmosphere is swept away and, with the companion that held it in its orbit obliterated, the core of the red giant shoots off into space.


Supernovae Back Einstein's "Blunder"

When Albert Einstein was working on his equations for the theory of general relativity, he threw in a cosmological constant to bring the universe into harmonious equilibrium. But subsequent observations by Edwin Hubble proved that the universe was not static. Rather, galaxies were flying apart at varying speeds. Einstein abandoned the concept, calling it the biggest blunder of his life's work.

Observations in the 1990s, however, proved that the universe was not only flying apart, it was doing so faster and faster. This seemed to point to a dark energy filling space that actually repelled ordinary matter with its gravity, in contrast to all other known stuff, including dark matter. A number of theories have been developed to explain what this dark energy might be, including Einstein's long discarded cosmological constant.

Now new observations from an international team of astronomers seem to show that dark energy is like the cosmological constant, unvarying throughout space and time. By measuring the distances to 71 far-off supernovae, the scientists were able to ascertain with a high degree of confidence that the effect dark energy exerts on supernovae light does not vary with distance. The researchers also plugged this data into a so-called equation of state, which measures the relationship between pressure and density, and found that dark energy must be less than -0.85--awfully close to Einstein's cosmological constant at -1. "Our observation is at odds with a number of theoretical ideas about the nature of dark energy that predict that it should change as the universe expands and, as far as we can see, it doesn't," says team member Ray Carlberg of the University of Toronto. The results will be published in a future issue of Astronomy & Astrophysics.

The 71 supernovae observations are the results of just one year of an ongoing Supernova Legacy Survey being carried out using telescopes throughout the world, such as the Canada-France-Hawaii Telescope in Hawaii and the Very Large Telescope in Chile. Data collected over the next four years should improve the precision of the finding and help researchers determine more about the enigmatic nature of dark energy that seems to constitute at least 70 percent of the universe.

But the finding brings to the fore another question: the so-called cosmological coincidence. Observations like this one seem to prove that regular matter and dark energy have similar densities at precisely this moment in time, even though the density of matter has been declining steadily since the big bang. Even Einstein couldn't answer why that would be.

How Quantum Effects Could Create Black Stars, Not Holes

Key Concepts

* Black holes are theoretical structures in spacetime predicted by the theory of general relativity. Nothing can escape a black hole’s gravity after passing inside its event horizon.
* Approximate quantum calculations predict that black holes slowly evaporate, albeit in a paradoxical way. Physicists are still seeking a full, consistent quantum theory of gravity to describe black holes.
* Contrary to physicists’ conventional wisdom, a quantum effect called vacuum polarization may grow large enough to stop a hole forming and create a “black star” instead.

Black holes have been a part of popular culture for decades now, most recently playing a central role in the plot of this year’s Star Trek movie. No wonder. These dark remnants of collapsed stars seem almost designed to play on some of our primal fears: a black hole harbors unfathomable mystery behind the curtain that is its “event horizon,” admits of no escape for anyone or anything that falls within, and irretrievably destroys all it ingests.

To theoretical physicists, black holes are a class of solutions of the Einstein field equations, which are at the heart of his theory of general relativity. The theory describes how all matter and energy distort spacetime as if it were made of elastic and how the resulting curvature of spacetime controls the motion of the matter and energy, producing the force we know as gravity. These equations unambiguously predict that there can be regions of spacetime from which no signal can reach distant observers. These regions—black holes—consist of a location where matter densities approach infinity (a “singularity”) surrounded by an empty zone of extreme gravitation from which nothing, not even light, can escape. A conceptual boundary, the event horizon, separates the zone of intense gravitation from the rest of spacetime. In the simplest case, the event horizon is a sphere—just six kilometers in diameter for a black hole of the sun’s mass.