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Fermi telescope measures foggy light from ancient stars

Elizabeth Howell, News Writer
Nov 6, 2012, 8:00 UTC

Sen—Astronomers, using gamma rays and foggy light from old stars, have mapped starlight in the universe to the best precision ever.

New measurements from NASA's Fermi Gamma-ray Space Telescope established the total amount of starlight from all stars that have ever sent forth light in the universe.

Gamma rays, the most active form of light, originate in hot areas of the universe. Fermi's Large Area Telescope specializes in measuring these rays across the entire sky, producing maps every three hours.

"The optical and ultraviolet light from stars continues to travel throughout the universe even after the stars cease to shine," stated Marco Ajello, the lead scientist of the project.

"This creates a fossil radiation field we can explore using gamma rays from distant sources," added Ajello, who holds positions at California's Stanford University and the University of California at Berkeley.

Ajello and his colleagues focused on a phenomenon known as extragalactic background light - casually referred to by NASA as a "cosmic fog" - which is defined as the total sum of starlight known in the universe.

The team looked at gamma rays from 150 galaxies generated by black holes, also known as "blazars".

In a black hole, matter falls inwards. Some of it is generated back, at nearly the speed of light, in jets pointing opposite ways. Blazars have one of the jets turned towards Earth.

The individual energies of the observed blazars were greater than three billion electron volts, about a billion times more energetic than the visible light that humans see with our eyes.

Blazars produce gamma rays that head towards Earth. These rays have to make their way through a "fog" of visible and ultraviolet starlight produced by all the stars that ever shone in the history of the cosmos.

Timeline of cosmic history

A timeline of the universe's history, including the formation of blazars. Credit: NASA Goddard Space Flight Center

Sometimes a gamma ray is lost as it hits the starlight, which is important to Ajello's study. His team determined that blazars that are further away are missing most high-energy gamma rays because of these collisions.

"In effect, the process dampens the gamma ray signal in much the same way as fog dims a distant lighthouse," NASA stated in a press release about the finding.

Scientists used the gamma ray disappearance, across three different distances, to figure out the average thickness of the fog.

Their calculations show the average distance between stars is about 4,150 light years. The density of stars is just 1.4 stars per 100 billion cubic light years.

A paper describing the findings was published in Science Express.