Supernova Mystery Solved: Explosion Seemed Brighter Through Galactic Lens
In 2010, a telescope in Hawaii saw something strange. A star had reached the end of its life and exploded in a supernova. When scientists analyzed the image, the supernova appeared 30 times brighter than any others like it. Why was this one so brilliant and all the others so dull?
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A team of Japanese astronomers say they have the answer. And in the process of finding it, they say they've discovered a new method of measuring the speed at which the universe is expanding. In the journal Science, they explain how a previously undiscovered galaxy in the foreground was distorting the image, making this supernova appear much brighter than it really was.
The supernova, dubbed PS1-10afx, had the trademark color and "brightness curve" — the rate at which the brightness intensifies and decays — as a class of supernovae known as Type Ia. "PS1-10afx looked a lot like a Type Ia supernova," says Robert Quimby, of Kavli Institute for the Physics and Mathematics of the Universe, in a statement. "But it was just too bright."
There were two possible solutions to the problem. Either they were witnessing something totally new, or there was something large yet unseen warping gravity and enlarging the supernova. "If there was a massive galaxy in front of PS1-10afx, it could warp space-time to form magnified images of the supernova," said co-author Marcus Werner, a mathematical physicist, in the statement. So they set to work to find the galaxy in the way.
They used a different kind of telescope in Hawaii to soak up light in the direction of the supernova, which had dimmed to invisibility by then. After taking a seven-hour exposure, they analyzed the image and discovered a large, old, and faint galaxy. It was difficult to see against the relative brightness of the supernova's host galaxy behind it. After some calculations the team confirmed the galaxy was causing the gravity lens.
But something else was interesting about their findings. We know that since the Big Bang, nearly 14 billion years ago, the universe has been expanding, with all matter moving away from all other matter. Up to now, our tools for measuring this have been limited to red shift.
When the galaxy's gravity bent the light, it split the beam into distinct parts, which traveled different paths and arrived at different times. The scientists say that the next time this happens, they can use high-resolution imaging to determine the exact timing of each beam. This could give them an exact measurement of how quickly our galaxy is moving away from the supernova's host galaxy — offering a new tape measure for cosmic expansion.
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