When we look up at the night sky, it's dark yet filled with pinpoints of light. It comes to our eyes from nearby planets and faraway stars. But it's worth remembering that the light we see from planets, and our moon (or other planets' moons) is all reflected. The only original light comes from stars, like our sun, and that bounces off surfaces, creating more points of light.
Recently, all that starlight was added up.
"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 study author and Clemson University astrophysicist Marco Ajello told CNN. "Most of this light is emitted by stars that live in galaxies. Every single star that has existed has contributed to this emission, and we can use it to learn all the details about star formation and evolution and galaxy evolution."
The grand total? Published in the journal Science, the number is huge, so it's expressed in a powers-of-ten notation: It's 4x10^84.
That's 4 followed by 84 zeros of particles of visible light (photons), which comes from the estimated 2 trillion galaxies and the trillion trillion stars (not a type-o!) within them.
To get to the starlight quantity number, scientists had collected data for nine years from NASA's Fermi Gamma-ray Space Telescope. But that scope didn't just scan around and collect light output information, since light from distant stars is too hard to measure from Earth. Instead, the Fermi telescope gathered information on gamma rays. This super-energetic form of light is easier to measure than visible light since the energy is a billion times higher.
But Fermi wasn't gathering gamma rays directly either. Rather, it looked at how those gammas reflect, refract and absorb into the extragalactic background light of the universe, which Ajello refers to as "fog" below, and NASA characterizes as "a cosmic fog composed of all the ultraviolet, visible and infrared light stars have created over the universe’s history."
This map of the entire sky shows the location of 739 blazars used in the Fermi Gamma-ray Space Telescope’s measurement of the extragalactic background light (EBL). The background shows the sky as it appears in gamma rays with energies above 10 billion electron volts, constructed from nine years of observations by Fermi’s Large Area Telescope. The plane of our Milky Way galaxy runs along the middle of the plot. (Photo: Courtesy NASA/DOE/Fermi LAT Collaboration)
This type of measurement allows scientists to see back in time.
"Gamma-ray photons traveling through a fog of starlight have a large probability of being absorbed," Ajello told CNN. "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."
It may sound like a game of telephone, but finding these echoes of what happened a billion years ago, 2 billion years ago, and beyond is what's necessary to determine how much light has shone in the past.
And of course, the light continues on from moment of calculation. It's estimated that seven new stars are born in the Milky Way galaxy every year — just a few small buckets to add to that sea of light above you.