The night sky may be full of stars, but that’s nothing compared with the number of stars filling the universe. For the first time, scientists have actually been able to measure the amount of starlight produced by the observable universe over billions of years.
Are you ready for the amount? It’s almost unfathomable. The measurement, published in the journal Science on Thursday, is in photons, or particles of visible light.
000,000,000,000,000,000,000,000,000,000,000,000,000,000 photons. Or 4×10^84.
This is the amount of light particles that were emitted by stars and escaped into space, the study says.
“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,” said lead study author and Clemson University astrophysicist Marco Ajello. “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.”
Data gathered over the course of nine years by NASA’s Fermi Gamma-ray Space Telescope has revealed this history of star formation.
Consider this: The universe is 13.7 billion years old. About a few hundred million years after the Big Bang, star formation kicked into high gear, which is why there are a trillion trillion stars and 2 trillion galaxies in it.
Total starlight has been difficult to quantify because, outside of our sun and the light of the Milky Way galaxy, the light from other stars appears very dim on Earth. Galaxy surveys have attempted it, but current telescopes couldn’t detect some of the fainter or more distant galaxies and had to rely on estimates. So scientists had to use a more indirect method.
Over the past 10 years, the Fermi telescope, which launched in June 2008, has helped scientists gather data on gamma rays. Gamma rays are photons with an energy greater than 1 billion times the energy of visible light, making them the most energetic form of light.
Fermi allows for gathering data on how gamma rays interact with extragalactic background light, a type of cosmic fog made up of dim starlight.
“It’s like following the rainbow till the end and finding the treasure. That’s what we found,” Ajello said.
The nine years of data on gamma ray signals originated from 739 blazars, galaxies with supermassive black holes that release energetic particle jets across the universe that can reach almost the speed of light. The photons within those jets collide with cosmic fog, which leaves a traceable mark, an imprint with a specific place and time in history.
“Gamma-ray photons traveling through a fog of starlight have a large probability of being absorbed,” Ajello said. “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.”
This works like a timeline of the universe.
“By using blazars at different distances from us, we measured the total starlight at different time periods,” said Vaidehi Paliya, postdoctoral fellow at Clemson, who participated in the study. “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.”
Star formation has not ceased since its peak 11 billion years ago, and seven new stars are born in the Milky Way each year. Because stars, and their formation, recycle energy, matter and elements, they act like the motor of the universe, said Clemson University professor Dieter Hartmann, who also participated in the study.
“Without the evolution of stars, we wouldn’t have the fundamental elements necessary for the existence of life,” he said.
Ajello and his team are already investigating the expansion rate of the universe, known as the Hubble constant. But what about the very beginning of the universe?
“The first billion years of our universe’s history are a very interesting epoch that has not yet been probed by current satellites,” Ajello said. “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.”
When the James Webb Space Telescope launches in 2021, its capabilities could help astronomers with this search.