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.