Astronomers Measure Total Starlight Emitted Over 13.7 Billion Years

Astronomers have measured all the light from all the stars that have ever existed. 'In total, the astronomers estimate, stars have radiated 4x1084 photons (a photon being the smallest unit of light),' reports The Guardian. 'Or put another way: 4,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 photons.' From the report: The astronomers based their calculation on measurements of the extragalactic background light (EBL), a cosmic fog of radiation that has been accumulating since stars first illuminated the dark, vast expanse of space. More than 90% of starlight ends up surviving in this dim backdrop of radiation. The latest observations, collected over nine years by Nasa's Fermi space telescope, use the light from blazars -- super-massive black holes that emit powerful jets of gamma rays -- as beacons to illuminate the cosmic fog.

In total, the team captured signals from 739 blazars -- some relatively close and some extremely distant, whose light was emitted in the ancient universe and has taken billions of years to arrive at Earth. Gamma-ray photons travelling through a fog of starlight have a high chance of being absorbed. So by taking blazars at different distances from the Earth and working out how much of their radiation had been lost along the way, the total starlight at different time periods could be ascertained. The researchers used a computer model to factor in the cosmic fog, which 'is simultaneously being diluted as the universe expands and space itself is stretched out,' the report mentions. 'The measurements suggest that star formation peaked about 11 billion years ago and has been on the wane ever since. About seven new stars are created in our Milky Way galaxy every year.'

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