Scientists Estimate All The Starlight in The Universe

The evolution of science is truly unbelievable. Recently an astrophysicist was able to add up all the starlight in the universe. Here is the result 4 x 10^84. This represents the number of photons that have escaped from the stars in the history of our universe.

The number was calculated by astrophysicist Marco Ajello from the Clemson University. “It’s basically starlight that ended up everywhere,” Ajello explained. “All the light emitted by stars that is able to escape to space basically becomes this background.” This refers to extragalactic background light, which is the radiation that makes it into the space, instead of reaching the dust that surrounds the stars.


It is not simple to measure extragalactic background light, because there are bright light sources closer to Earth that might outshine it, and because it is spread all around the universe. In order to get to the final numbers, researchers used blazars. Blazars are a kind of galaxy the hides a huge black hole at the center of it and it shoots high energy material.

Blazar photons and extragalactic background light photons interact, but only at a certain energy level. Scientists extrapolate the light that should have been produced at higher energy levels from the light produced at lower energy levels. The difference is calculated and this helps them determine the extragalactic background light.

“Now, the new thing is using that to figure out the cosmic star-formation history. That’s a question scientists have long been wanting to tackle, but so far, they’ve had to do so indirectly and rely on some initial assumptions, which is never ideal. “The problem [with previous estimates] is that because your initial mass function is … it’s really a guess, it’s an initial guess, and that can introduce uncertainty,” Joshi said.

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