Astronomers measure cosmic 'fog,' estimate space between stars

Stars may burn out and die, but their light goes on forever. All the light ever produced by stars is still circulating through the universe, a phenomenon known as extragalactic background light or EBL. This light is a kind of cosmic "fog" that dims light from distant stars passing through it, much like the beams from a lighthouse are dimmed by real fog. Now, for the first time, astronomers have been able to measure the sum total of EBL and to calculate the spacing of stars in the cosmos. They reported Thursday in the journal Science that the average density of stars in the universe is about 1.4 per 100 billion cubic light-years. That means that the average distance between stars is about 4,150 light-years.

To measure EBL, astronomers needed a kind of galactic signpost to allow them to determine how much light from distant stars is being lost. As their measuring marks, a team headed by Marco Ajello of Stanford University chose blazars -- galaxies emitting large amounts of gamma-radiation. Blazars are characterized by massive black holes at the center of the galaxies. As matter falls into the black hole, some of it is accelerated outward at nearly the speed of light. In the gamma-ray spectrum, these beams of light are especially bright and their sources are called blazars. The highest-energy gamma rays tend to pass through the EBL more efficiently that light in the visible spectrum.

Over the last four years, NASA's Fermi Gamma-ray Space Telescope has been examining the entire universe looking for, among other things, blazars. The satellite has so far identified more than 1,000 of the brilliant objects. Ajello and his team examined 150 blazars that emit gamma-rays of very high energy -- greater than 3 billion electron volts, or more than a billion times the energy of visible light. The team calculated the amount of gamma-radiation emanating from blazars ranging in age from 4 billion years to 11.2 billion years.

Some of the gamma-rays traveling through space strike photons in the EBL. These collisions form an electron-positron pair, destroying the gamma-radiation. By measuring the attenuation of gamma radiation, the team was able to produce the best estimate of total EBL obtained so far, allowing them to calculate the average star density.

The universe is known to be 13.7 billion years old. During the first 400 million years or so, the universe was composed primarily of pure hydrogen and helium gas and was essentially dark, according to team member Volker Bromm of the University of Texas at Austin. The universe then underwent a very rapid transition to star formation, with those first stars having masses ranging from 10 to 100 times that of our sun. That is when most of the heavy elements in the universe were created, he said. The peak of star formation occurred when the universe was about 3 billion years old, and star formation has been declining ever since.

By measuring EBL, Bromm said, Fermi is providing a shadow image of the first stars. Astronomers hope to see them directly when the powerful James Webb Space Telescope is launched in 2018.