Talking
Points Memo: Using
NASA's
Fermi Gamma-Ray Space Telescope
(Fermi), astronomers have measured the total
extragalactic background light (EBL), the "cosmic fog" created
by all the visible and UV photons emitted by all the stars that
have ever existed. Launched in 2008,
Fermi uses its primary instrument, the Large Area
Telescope, to scan the entire sky for high-energy gamma rays
emitted by blazarsâmdash;distant galaxies with
supermassive black holes. Because some of the gamma rays will
collide with photons in the EBL and become electrons and
positrons, the number of gamma rays emitted by a given source
decreases over distance; hence, the more distant the blazar,
the fewer high-energy gamma rays are detected by
Fermi. By calculating the average gamma-ray
attenuation across several distance ranges, astronomers were
able to estimate the EBL's thickness. They found the average
stellar density to be about 1.4 stars per 100 billion cubic
light-years, which means the average distance between stars is
about 4150 light-years. Their
paper
was published online yesterday in
Science Express.
