For years, astronomers have relied on a valuable observational tool to study the expansion of the universe: a variety of supernova called a Type Ia. These titanic explosions were thought to erupt under conditions that produced a consistent brightness. Using that "standard candle," astronomers could calculate a supernova's distance from Earth and, by detecting stretches in the wavelengths of its light, also track the acceleration of the universe caused by the mysterious force called dark energy. Now, the discovery of two supernova remnants in a nearby galaxy calls the consistency of Type Ia explosions into question.
Supernovae occur when stars with big enough masses--at least 1.4 times that of the sun--have exhausted the supply of fuel that powers the thermonuclear fusion reactions in their cores. When the reactions stop, the stars can no longer push outward against the inexorable effects of gravity. In an instant, they collapse into tiny versions of their former selves, and the recoil from that abrupt and violent process produces the supernovae, which expel huge amounts of matter outward at hypersonic speeds. Up to now, astronomers have identified four types of supernova, including Type Ia, which was thought to occur when a white dwarf star pulled in enough extra material from a binary companion to explode within a billion and a half years of its birth.
Now a team from several universities using NASA's orbiting Chandra X-ray Observatory and the European Space Agency's XMM-Newton spacecraft say they have discovered supernova remnants that harbor the usual products of Type Ia explosions but are considerably brighter and contain much more hot gas than normal. After studying x-ray observations of the remnant clouds, the astronomers concluded that the stars exploded when they were only 100 million years old, instead of the 1 billion to 1.5 billion years old for a typical star producing a Type Ia. One possible explanation, team members reported in the 1 December issue of the Astrophysical Journal, is that both stars grew up in a dense star-forming area and therefore could pull in a great deal of matter easily and quickly. That might have caused them to grow prematurely massive and trigger the supernovae. "We weren't around to see these stars before they exploded," says team member Sean Hendrick of Millersville University in Pennsylvania, "but these x-ray clues tell us that something unusual happened in the case of these two."
Next, says astrophysicist Kimberly Weaver of NASA's Goddard Space Flight Center in Greenbelt, Maryland, scientists will seek the cause of the so-called "prompt Ia" explosions, by plugging the findings into computer models. If confirmed, she says, the discovery could produce an entirely new tool for studying the early universe. "If these truly were younger white dwarfs that produced Type Ia explosions, then we could use them to study the first galaxies, and they could show how stellar evolutionary processes can happen much faster than we think."
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