Researchers have Measured all the Starlight in the Observable Universe

Researchers have Measured all the Starlight in the Observable Universe

4×1084 photons of light have been emitted by stars into space since the Big Bang.

Our universe has been creating stars for billions of years. According to an estimate, there are around trillion-trillion stars in 2 trillion galaxies of our universe. Despite all the technological advancement, astrophysicists haven’t been able to trace much about star formation. Having said that, things might change pretty soon as a team of researchers from the Clemson University examined the data from the Fermi Gamma-ray Space Telescope of NASA to determine the history of star formation in our universe. Marco Ajello, the Lead Author of the paper who is an Assistant Professor in the Department of Physics and Astronomy, elaborated their work by saying,

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. Most of this light is emitted by stars that live in galaxies. And so, this has allowed us to better understand the stellar-evolution process and gain captivating insights into how the universe produced its luminous content.

It is virtually impossible to quantify the amount of starlight ever produced, in simpler terms, due to the presence of several variables. However, the latest analysis of the data from the Fermi Gamma-ray Space Telescope did tell us that nearly 4×1084 photons have been emitted by stars into space till now. This powerful observatory of NASA has provided huge amount of information about Gamma rays and their interaction with the Extragalactic Background Light (EBL), a cosmic fog containing all the visible, infrared, and ultraviolet light emitted by stars and their surrounding dust. The researching team examined gamma-ray signals from 739 blazars in their research, which were captured in almost 9 years.

A map of the Observable Universe. Credit:

The word ‘Blazar’ is used to refer to a galaxy who have a supermassive black hole, which releases narrowly collimated jets of energetic particles that jump out of their galaxies and travel across the cosmos approximately at the speed of light. Whenever one of these jets points directly at Earth, it can be detected irrespective of its distance to our planet. The gamma-ray photons will eventually collide with the cosmic fog which will leave observable imprints on it. Due to this, the team of Ajello was able to determine the density of the fog not just at a given time in the history of the universe but also at a given place. Ajello mentioned that in the following words:

Gamma-ray photons traveling through a fog of starlight have a large probability of being absorbed. 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.

Scientists have been studying the star formation history of our universe for decades but they always faced a massive hurdle in their research. Some of the galaxies were too far away (too faint) for even the most advanced telescopes. Consequently, researchers were forced to fall back to estimation in order to determine the starlight produced by these distant galaxies.

Contrary to that, Ajello’s team succeeded in overcoming this issue by contemplating that the starlight escaped from all the galaxies eventually becomes a part of the EBL. Therefore, there is no need of estimating the light-emissions from ultra-distant galaxies. Vaidehi Paliya, a member of the researching team who analyzed all the blazars alongside Ajello, said,

By using blazars at different distances from us, we measured the total starlight at different time periods. We measured the total starlight of each epoch — one billion years ago, two billion years ago, six billion years ago, etc. — all the way back to when stars were first formed. This allowed us to reconstruct the EBL and determine the star-formation history of the universe in a more effective manner than had been achieved before.

The detailed knowledge about star formation will help scientists to explore other aspects of astronomical study like galaxy evolution, cosmic dust, and dark matter. In addition to that, it will help future missions to look for the earliest days of stellar evolution and the James Webb Space Telescope of NASA will have a massive role to play in this regard. Ajello referred to the ultimate goal of their research by saying,

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.

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