We Finally Know How Much Starlight Has Been Emitted In The Visible Universe

Fermi's Five-year View of the Gamma-ray Sky. NASA/DOE/Fermi LAT Collaboration

Studying the universe in gamma rays opens a window into the most energetic processes that take place in the cosmos. But there are difficulties. One particular one is the opacity caused by other forms of light. Gamma-ray photons sometimes slam into less energetic light emitted by stars. Researchers have now used that opacity to establish just how much starlight has been produced in the universe.

As reported in Science, the team measured the attenuation in gamma rays of 739 active galaxies. With that data, they showed that star formation in the visible universe peaked 3 billion years after the Big Bang, a fact in agreement with other studies. They also estimated that about 4x1084 photons have been emitted by stars since the beginning of the universe.

That’s an incredible number. A four followed by 84 zeros. The Sun emits roughly 2x1062 photons per year and has been shining for roughly 5 billion years. That means that you would need 20,000 billion billion Sun outputs to match what has been emitted by other stars so far.

"From data collected by the Fermi telescope, we were able to measure the entire amount of starlight ever emitted. This has never been done before," lead author Dr Marco Ajello, from Clemson University, said in a statement. "Most of this light is emitted by stars that live in galaxies. And so, this has allowed us to better understand the stellar-evolution process and gain captivating insights into how the universe produced its luminous content."

The team used a special type of galaxy called a blazar. Blazars have supermassive black holes at their center, which produce powerful jets that point directly towards Earth. The gamma rays cross a fog of starlight before being detected by NASA’s Fermi telescope. The fog is known as the extragalactic background light (EBL) and gives us an idea of the starlight emitted by galaxies that are too faint to see.

"By using blazars at different distances from us, we measured the total starlight at different time periods," said Vaidehi Paliya of Clemson's department of physics and astronomy. "We measured the total starlight of each epoch – 1 billion years ago, 2 billion years ago, 6 billion years ago, etc. – all the way back to when stars were first formed. This allowed us to reconstruct the EBL and determine the star-formation history of the universe in a more effective manner than had been achieved before."

Previous attempts at measuring the EBL directly had to contend with several foreground light sources that made the measurement more uncertain. This approach has no such limitations and manages to deliver. The work also provides us with important clues about the earliest years of the universe, a time we are yet to probe extensively with our telescopes.

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