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All the light of all the stars that have ever existed. It's a quantity of unimaginable magnitude, but now astronomers have put a number on it.
From the oldest stars to the largest galaxies to the largest, an international team has managed to measure the total amount of starlight emitted during the 13.7 billion years of the history of the universe .
"This has never been done before," said Marco Ajello, an astrophysicist at Clemson College of Science in South Carolina and lead author of the paper.
The first stars appeared a few hundred million years after the big bang. Since then, galaxies have dazzled the stars at a staggering rate, and scientists estimate about 1 trillion billion individuals.
In total, astronomers estimate that the stars radiate 4×1084 photons (a photon being the smallest unit of light). Or in other words: 4,000,000,000,000,000,000,000,000,000,000,000 photons.
Astronomers have based their calculations on measurements of extragalactic background light (EBL), a cosmic radiation fog that has been accumulating since the stars first illuminated the vast, dark space.
More than 90% of the starlight eventually survives in this dark radiation environment.
"Today, we are sitting in this sea of light," said Kári Helgason, of the University of Iceland, in Reykjavík and co-author of the paper.
Since it has been masked by the light of nearby stars, the EBL has proved tricky to explore.
The latest observations, collected over the past nine years by NASA's Fermi Space Telescope, use the light of blazars – super massive black holes emitting powerful gamma rays – as beacons to illuminate the cosmic fog. "They are so brilliant that they can shine in almost any observable universe," Helgason said.
In total, the team captured the signals of 739 blazars – some relatively close and others extremely distant – whose light was emitted in the ancient world and took billions of years to go to Earth.
Gamma photons traveling in a stellar light fog are likely to be absorbed. Thus, by taking blazars at different distances from the Earth and calculating the amount of radiation lost along the way, the total light of the stars at different periods could be determined.
"We measured the total amount of starry light of each era – a billion years ago, two billion years ago, six billion years ago, up to a billion years ago. to star formation, "said Vaidehi Paliya, co-author of the newspaper, also from Clemson.
"It's really this distribution that allows us to reconstruct the background light as a function of time," said Helgason.
An additional complication is that as starlight accumulates over time, cosmic fog is simultaneously diluted as the universe expands and the space itself expands. Overall, the fog becomes denser. This phenomenon, as well as other complex phenomena, has been taken into account when using a computer model.
The measurements suggest that star formation peaked about 11 billion years ago and that it has since declined. About seven new stars are created each year in our galaxy.
The results, published in the journal Science, closely match previous indirect estimates of total starlight based on galaxy surveys. "It's really satisfying that two methods give the same answer," Helgason said.
The data also provides new information about the first billion years of universe history, an era that has not yet been surveyed by today's satellites.
"Our measurement allows us to take a look inside," Ajello said. "Maybe one day we will find a way to wind up to the big bang. This is our ultimate goal. "
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