How Much Starlight Has Been Emitted Since the Beginning of Time?


Scientists have found a way to measure all the starlight emitted for most of the history of the universe. George Rose/Getty Images

Here's a mind-blowing figure for you: 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.

In case you're wondering what all those digits signify, that's the number of photons — more compactly expressed as 4 x 1084 — emitted by all of the stars in the observable universe, going back to when the 13.7 billion-year-old universe had been around for just a billion years, according to a team of researchers headed by Marco Ajello, an astrophysicist in the College of Science at Clemson University.

That's based on an analysis of data from NASA's 10-year-old Fermi Gamma-ray Space Telescope, which enabled the researchers to compile a history of star formation over most of the universe's lifetime.

The scientists detailed their findings in a paper published on Nov. 30, 2018, in the journal Science, with Ajello as the lead author.

Here's a NASA video about the research:

Measuring starlight for most of the universe's history required considerable ingenuity. As Ajello explains in prepared remarks via email, the total amount of light emitted by stars is comprised of two types. "One is stellar light that survives absorption by dust," he writes. "This is what we measured. The rest is stellar light absorbed by dust and re-emitted in the infrared. We are not sensitive to that. It turns out half of the energy emitted by stars across the history of the universe is re-processed by stars at longer (infrared) wavelengths."

The sky is filled with photons emitted long ago by distant stars — this is called the extragalactic background light, or EBL. Nevertheless, except for the moon and stars from our own galaxy, the sky appears dark to our eyes. According to Ajello, that's because most of the starlight that reaches Earth from the rest of the vast universe is extremely faint — the equivalent of a 60-watt light bulb viewed in compete darkness from about 2.5 million miles away.

As this Science News article explains, to get around that problem, Ajello and his team perused 10 years of data from the Fermi telescope, and looked at the EBL's interaction with gamma rays emitted by distant blazars — black holes that can send powerful streams of radiation out into the universe. The researchers calculated the extent to which the gamma rays from those blazars had been absorbed or altered by collisions with the EBL's photons.

"Blazars emit light across the electromagnetic spectrum, but release most of their energy in the gamma-ray band," Ajello explains. "The Large Area Telescope (LAT) on board of Fermi is able to measure gamma-rays from blazars from 100 MeV (1 million times the energy of visible light) to 1 TeV (1 trillion times the energy of visible light). The pair production process (where two photons produce an electron-positron pair) which absorbed the gamma rays emitted from blazars starts only at energies of ~10 GeV (billion times the energy of visible light). So below this energy we observed the true, un-absorbed, blazar output, but above this 'threshold' more and more photons from the blazars are absorbed till the point (if you increase the energy enough) you don't see the blazar anymore."

"We look for this transition from zero percent absorption to 100 percent absorption as a function of energy," Ajello continues. "The energy at which the transition starts and how fast it goes from zero percent to 100 percent measure the energy of the EBL photons and how many of those are out there. The more there are the quicker is the zero 100 percent (absorption) transition."

Ajello describes tracking the EBL as the astrophysicists' equivalent of "following the rainbow and discovering a pot of gold. The EBL is the rainbow and its knowledge can finally disclose a lot of useful information."

Ajello explains that the total amount of light emitted by stars is comprised of two types. "One is stellar light that survives absorption by dust (this is what we measured). The rest is stellar light absorbed by dust and re-emitted in the infrared (we are not sensitive to that). It turns out half of the energy emitted by stars across the history of the universe is re-processed by stars at longer (infrared) wavelengths."

The researchers' technique enabled them to see the history of star formation in the universe, which they found had peaked about 3 billion years after the Big Bang and has slowed dramatically since then, according to a Washington Post article on the work.

The count doesn't include the amount of starlight emitted in the first billion years of the universe's existence. "This is an epoch we can't really probe yet," Ajello explains. That's one reason he and other scientists are looking forward to the 2021 launch of the James Webb Space Telescope, which NASA says will be sufficiently sensitive to detect the first stars.


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