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NASA astronomers measure all the starlight, ever.

Stars have been twinkling across the universe for about 13.3 billion years. Now, NASA astronomers are using gamma rays from distant blazars to sum all the starlight that has ever shone in the history of the universe. 
NASA representation of a supermassive black hole with a jet streaming outwards
If the jet points at earth, it's called a blazar. Image Credit: NASA.

Up in low Earth orbit, the first 100 to 200 miles of space, the Large Area Telescope on the Fermi Gamma-ray Space Telescope takes a picture of the high-energy gamma rays across the whole sky. Taken every three hours for the past four years, these images have summed to the most detailed gamma-ray map of the universe.

The gamma rays detected by the Fermi telescope originate in distant blazars. Active supermassive black holes are often found at the hearts of elliptical galaxies. As matter is sucked into the black hole, some of it is spewed back outwards in massive jets, which are dominated by high-energy gamma rays. When one of the jets is aimed at Earth, the black-hole powered galaxy is called a blazar.

Millions to billions times the energy of visible light, gamma rays can travel across the universe for billions of light years. During this journey, these high-energy photons pass through all the light that stars have ever emitted throughout the history of the universe. Astronomers call this total-body-of-all-the-light the extragalactic background light.

"The optical and ultraviolet light from stars continues to travel throughout the universe even after the stars cease to shine, and this creates a fossil radiation field we can explore using gamma rays from distant sources," said Marco Ajello at NASA's press conference. Using gamma-ray photons, NASA astronomers have established "the total amount of light, from all of the stars that have ever shone."

Ajello is the lead researcher on the project and a postdoctoral scientist at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University and the Space Sciences Laboratory at the University of California at Berkeley.

As gamma photons jettison from their distant blazar through time and space in the universe, they can collide with the visible and UV photons from ancient stars that make up the extragalactic background light. When a gamma-ray slams into these lower-energy photons, it transforms into an electron + positron pair of particles and the gamma-ray light is lost.

Those gamma-rays that do survive the long journey to Earth interact with the massive tungsten plates in the Large Area Telescope on the Fermi telescope.  The gamma-rays that hit the telescope transform into electron and positron pairs that shoot off along paths determined by the direction the gamma-ray had been traveling during its trip from the blazar jet. Studying the electron and positron paths through the detector, the scientists are able to backtrack the path of the gamma rays to their source.

The location of 150 blazars (green spots) used in the study.
The plane of the Milky Way runs along the middle of the plot.
Image Credit: NASA / Fermi LAT Collaboration
Studying high-energy gamma-rays from 150 blazars distributed across the universe,  Ajello and his colleagues were able to calculate the amount of starlight in the universe.

Gamma-rays from nearby blazars helped the scientist determine how many gamma rays should be emitted by blazars. At more distant blazars, fewer high-energy gamma-rays arrived at the telescope, while the almost no high-energy gamma-rays arrived from the farthest blazars.

The fewer gamma-rays at the detector, the more light the gamma-rays had to pass through to get there. From this data, the scientists were able to describe the density of starlight emitted at different eras in the history of the universe.

To account for the depletion of gamma-rays, Ajello and his team found that the average starlight density in universe must be about 1.4 stars per 100 billion cubic light-years, making the average distance between stars about 4,150 light-years. In addition to measuring the extragalactic background light, the findings, published on November 1st, 2012 in the journal Science, suggest that the earliest stars may have taken longer to form than previously thought.

Fermi telescope detection of gamma-rays
helps define the number of stars that have ever shone in the universe.
Image Credit: NASA's Goddard Space Flight Center

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The measurements of extragalactic background light accomplished one of the primary goals of the Fermi telescope and outlines the era in the universe's history when the first stars began to shine. This research also sets the stage for NASA's upcoming James Webb Space Telescope, which is set to launch in 2018 and will compliment Fermi's theoretical measurements with direct observations of the extragalactic background light in order shed light on the history of our universe.


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