Four thousand octillion octillion octillion — or 4×1084. That’s roughly the number of ultraviolet, visible, and near-infrared photons emitted by all the stars in all the galaxies throughout time, according to a new estimate.
By tallying all these photons of starlight, astronomers have pieced together an updated timeline of how the rate of star formation has waxed and waned over the 13.8 billion years since the Big Bang, astronomers report November 30th in Science. This new timeline jibes with earlier independent estimates: the cosmic star-forming factories were at their most prolific about 3 billion years after the Big Bang and have been gradually slowing down ever since.
The vast majority of starlight that escapes galaxies contributes to a haze of photons known as the extragalactic background light. This light is tricky to measure directly.