The universe has been making stars for a good 13 billion years or so, and a natural question to ask might be "how many stars have existed in that time?" But now astronomers have taken it several steps further and asked "how much light has been emitted in that time?" Using a new measurement method, the team has apparently managed to quantify all the starlight every produced in the observable universe – and the result is a figure that'll make your eyes water.

Let's get this number out of the way first. According to the team's measurements, the amount of photons emitted by stars in the past 13 billion years is expressed as 4 x 10^84. In English, that's a 4 followed by 84 zeroes. That's 4 septenvigintillion, for those of you playing at home.

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.

Given how mind-bogglingly enormous the universe is, how can astronomers possibly calculate something like that? The researchers sifted through nine years of data from the Fermi Gamma-ray Space Telescope, and analyzed how gamma rays from blazars interacted with the "cosmic fog" to arrive at the total amount of light ever produced.

This cosmic fog, more accurately known as the extragalactic background light (EBL), is made up of all the light emitted by stars across the spectrum of ultraviolet, visible and infrared wavelengths. When gamma rays, the most energetic form of light, pass through this fog, they collide with other wavelengths of light to produce electrons and positrons. By analyzing these signatures from gamma rays from 739 blazars, the astronomers were able to measure the density of the fog at any given place, at any given time in the history of the universe.

"Gamma-ray photons traveling through a fog of starlight have a large probability of being absorbed," says Marco Ajello, lead researcher on the study. "By measuring how many photons have been absorbed, we were able to measure how thick the fog was and also measure, as a function of time, how much light there was in the entire range of wavelengths."

Fermi has been used to study the EBL in the past, but this new project used five times as many blazars as previous attempts. Since they're all located in different parts of the sky and at different distances, the team was able to peer deeper into space – and as a result, further back in time – than ever before.

The study was also able to determine that the universe's star formation peaked between 10 and 11 billion years ago. The team says this new gamma ray map lays the groundwork for future missions, like the James Webb Space Telescope, to peer even deeper into space and time.

"The first billion years of our universe's history are a very interesting epoch that has not yet been probed by current satellites," says Ajello. "Our measurement allows us to peek inside it. Perhaps one day we will find a way to look all the way back to the Big Bang. This is our ultimate goal."

The research was published in the journal Science. The team describes the work in the video below.

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