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Scientists have now traced all the starlight across

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ninety percent of cosmic history, thanks to NASA's Fermi Gamma-ray

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Space Telescope. Fermi studies the universe using gamma rays, 

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the highest-energy form of light. Over the last 10 years,

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Fermi has measured gamma rays from nearly 2,000 blazars.

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These galaxies host supermassive black holes and produce

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jets of high-speed particles. Gamma rays from these blazars

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interact with starlight produced throughout the cosmos. Even after

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stars burn out, their light continues to travel across the universe, 

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forming the extragalactic background light, or EBL.

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When a gamma ray collides with starlight, it transforms into two

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particles, an electron and a positron, in accordance with Einstein's

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famous equation. These collisions weaken a blazar's gamma-ray

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signal like fog dulling a car's headlights. The further back scientists

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look, the greater the EBL's dimming effect. A new study

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of the EBL peered back through 12 billion years of starlight,

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confirming that star formation in our universe peaked about 

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10 billion years ago. Stars create most of the light in the universe

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and many of its heavy elements, like silicon and iron. 

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Understanding how our cosmos came to be depends in large part on 

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understanding how stars evolved. Thanks to Fermi,

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we're one step closer. [Music]

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[Music][Beeping]

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