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Introduction
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The deeper you gaze out into space, the further you see back in time. That is why a group of astronomers observed the Subaru Deep Field, a patch of the sky the size of the full moon, using the 8.2-m Subaru Telescope in Hawaii. They stared with the telescope at this patch for two nights at a time, collecting the faint light from over 150,000 galaxies that become visible in this small region at such image depth. Why so many galaxies? In order to hunt for supernovae, the colossal explosions of dying stars. The team found a total of 150 supernovae, including 12 of the most ancient ever seen, having exploded 10 billion years ago, with their light reaching us now.

So-called Type-Ia supernovae are major producers of the element iron in the Universe, and are useful for measuring cosmological distances. In this latter context, they were used over the past decade to discover the accelerating expansion of the Universe, for which the 2011 Nobel Prize in Physics has just been anounced. The present team of researchers was able to show that such supernovae exploded 5 times more frequently than today when the Universe was only about one-third its current age. Tracking how the frequency of these explosions changed as the Universe evolved is important for reconstructing the cosmic history of element production, and for understanding the explosions themselves.

The image to the left shows one of the most ancient supernovae ever discovered, having exploded 10 billion years ago, more than 5 billion years before our solar system was formed.

The image below shows 10% of the Subaru Deep Field. Move your cursor over the image to reveal 22 of the 150 supernovae in the sample. Each point of light in the image is a galaxy, consisting of tens of billions of stars (except for the few bright spots that are nearby Milky-Way stars). Every triplet of frames zooms in on one event, showing the galaxy before the explosion, with the supernova in progress, and in a digital "difference image" that isolates the light from the supernova.