What Makes a Supernova Blow?

Astronomers have spotted the telltale gamma rays from a supernova explosion, revealing its innermost secrets.

What makes a massive star go boom? Astronomers have longed suspected it is thermonuclear fusion that destroys the star. Now they have proof: gamma ray emissions detected by Europe's INTEGRAL satellite show evidence of decayed radioactive isotopes flash-baked in the thermonuclear oven of a freshly made supernova.

The exploded star was found by accident eight months ago in the nearby galaxy M82 (pictured top), located about 11 million light-years from Earth. It turned out to be a particular type of supernovae, known as a "Ia", which ramp up to maximum brightness in about three weeks and then slowly begin to dim.

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At their peak, these types of exploded stars pump out as much energy as 4 billion times the energy of the sun, making them good yardsticks for measuring cosmic distances. It was by using these so-called "standard candles" that astrophysicists in 1998 discovered an unknown force, referred to as dark energy, was speeding up the universe's expansion.

Scientists theorized that supernova Ia explosions are triggered by the sudden fusion of carbon and oxygen into heavier elements, such as nickel-56, inside a white dwarf star, making it unstable.

"Fusion happens in a flash," astrophysicist Robert Kirshner, with the Harvard-Smithsonian Center for Astrophysics, writes in an article in this week's Nature. "A thermonuclear flame rips through the white dwarf, fusing carbon into heavier elements with a sudden release of energy that tears the star apart. Fusion stops yielding energy at the element that has the most tightly bound nucleus - in the case of a white dwarf, nickel-56."

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When the exploded remains of the M82 star were found, astronomers moved quickly to find out if the theoretical predictions were right.

"The last type Ia in our galaxy was in 1604," lead researcher Eugene Churazov, with Germany's Max Planck Institute for Astrophysics, wrote in an email to Discovery News.

He and colleagues used the European Space Agency's International Gamma-Ray Astrophysics Laboratory, nicknamed INTEGRAL, to observe the newly found supernova between 50 and 100 days after the explosion. They found a neat chain of chemistry caused by the decay of radioactive nickel isotopes into cobalt and iron. Calculations show the amount of radioactive nickel, the rate of the supernova expansion and the amount of mass produced in the explosion match predictions.

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"We now see directly gamma-ray lines of cobalt-56, which provides unambiguous proof that a thermonuclear explosion is behind Type Ia. This is what we all expected, but it is great to have a proof," Churazov said.

A hunt for clues on the supernova's progenitor (star) is underway using radio, optical and X-ray telescopes, he added.

"Upsetting the conventional wisdom is always a joy in science. You can get prizes for that. But there is also a deep pleasure in showing decisive evidence on an important physical idea that has been used without proof for decades," said Kirschner, who was not involved in the research. "It is a wonderful result."

The research appears in this week's Nature.

Supernova SN 2014J as observed by the Hubble Space Telescope on Jan. 31.

Supernova Plasma Energy

Computer visualization is an essential tool for scientists to gain an insight to how complex physical, biological and chemical phenomena work. From protein structures to the detonation of supernovae, scientists are finding faster, more precise and more powerful means of simulating these systems using supercomputers. One such supercomputer is the Blue Gene®/P housed at the U.S. Department of Energy's Argonne National Laboratory in Chicago where 160,000 computing cores work in parallel to process 557 trillion calculations per second. If you to tried to simulate an equivalent system on your standard home computer, it would take three years just to download the data! Turning that data into a usable model would be an impossible task. Now, using a new technique called software-based parallel volume rendering, scientists at Argonne are able to visualize 3D models of supernovae. In the visualization above, the various plasma energies of the expanding supernova are color coded, allowing the scientists to peer deep into the inner workings of the explosion, providing an invaluable look at this powerful astrophysical event.

Moment of Detonation

In this visualization, the moment of detonation of a Type 1a supernova is modeled. This situation arises when a white dwarf star has accreted mass from a binary partner to a point when gravitational forces overcome the outward electron degeneracy pressure. The star collapses and it is thought that carbon fusion is initiated in the core, creating a supernova. The star is completely destroyed. Around 1-2 × 1044 Joules of energy is released from Type 1a supernovae, ejecting matter and shock waves traveling at velocities of 3-12,000 miles per second (approximately 2-7% the speed of light).

White Dwarf No More

The Type 1a supernova proceeds in the simulation, ripping through the white dwarf star.

Complex Fluid Mechanics

Detailed visualizations of the nuclear combustion inside a supernova. The calculations are based on fluid mechanics, showing how the explosion rips through the star.

Tycho's Nova

Advanced computational methods as being developed at Argonne National Laboratory will help astrophysicists understand how supernovae behave. This is an image of the famous Tycho's Nova (known as SN 1572), the beautiful remnant of a Type 1a supernova.