In the beginning there was light—the brilliant light of the big bang shining through a sea of protons, neutrons and electrons. But as the universe expanded and cooled, the electrons joined the protons, making neutral hydrogen atoms, and as the universe cooled further, the light went dark. Eventually, however, something tore the electrons from the protons, thereby re-ionizing the universe. Space has remained ionized—a plasma of positive ions and electrons—ever since. Now an upcoming paper proposes the surprising cause: runaway stars flung from their galactic birthplaces.

"Re-ionization was one of the major cataclysmic events in the history of the universe," says Charlie Conroy, an astronomer at the University of California, Santa Cruz. Yet because re-ionization occurred so long ago, even the greatest telescopes struggle to probe this distant epoch. Observations of the farthest quasars as well as the big bang's afterglow indicate that re-ionization took place sometime between a redshift of 6 and 12, a measure of astronomical distance that corresponds to the time when the universe was only 380 million to 960 million years old. But exactly what caused this great transformation is a mystery.

The culprit must have been something hot, because only hot objects emit extreme ultraviolet light, whose wavelength is less than 912 angstroms, or 91.2 nanometers (a nanometer is a billionth of a meter), and whose photons are so energetic they rip electrons from protons. Quasars once seemed a logical bet, because they are hot and bright and existed long ago. But astronomers have failed to find enough quasars at great distances to do the job.

So the focus has turned instead to hot stars. Trouble is, these stars presumably lived in galaxies filled with neutral hydrogen gas—which absorbs extreme ultraviolet light.

"Of all the ionizing photons that hot stars are producing in today's galaxies, only about a percent of the photons actually leave the galaxy," Conroy says. "Those ionizing photons are being wasted in the galaxy."

Now, in work to appear in the August 20 issue of The Astrophysical Journal, Conroy and Kaitlin Kratter of the Harvard-Smithsonian Center for Astrophysics propose a resolution to this dilemma. They say that hot stellar runaways—stars shot out of their nurseries—may have reionized the early universe.

Stars born with the most mass—more than 16 times that of the sun—are so hot that astronomers classify them as spectral type O. These O-type stars glow blue and emit most of their radiation at ultraviolet wavelengths, then explode just a few million years after birth. In our own galaxy, the Milky Way, about 30 percent of them are runaway stars. One famous example, visible to the naked eye, is the star Mu Columbae, which is fleeing its birthplace in the constellation Orion and now resides in another constellation altogether.

A runaway star can make its escape when one star orbiting another explodes, freeing its partner at high speed. Runaways also emerge from star clusters, where gravitational encounters among stars fling some of the members away.

Our galaxy is so huge, with a disk more than 100,000 light-years across, that runaway O-type stars don't escape it during their short lives. In the early universe, however, galaxies were much smaller. During the epoch of re-ionization, they may have been just a few hundred light-years across. "If a star is traveling at a high velocity, it can actually leave the parent galaxy," Kratter says. Then the star's extreme ultraviolet light shines into the space beyond the galaxy. As a result, Conroy says, "These stars could be a very important player in the re-ionization of the universe."

5 Comments

1. 1. rloldershaw 10:54 AM 7/31/12

   
   Another excellent class of candidates for the reionizers is "primordial" stellar-mass black holes, which also fit the bill for the dark matter.
   
   Discrete Scale Relativity definitively predicts the exact mass spectrum for these fundamental black holes, which are every bit as fundamental as protons of electrons.
   
   In fact, according to Leonard Susskind the only difference between fundamental particles and fundamental black holes is their mass/space/time scale.
   
   Robert L. Oldershaw
   http://www3.amherst.edu/~rloldershaw
   Discrete Scale Relativity
   Fractal Cosmology

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2. 2. scientific earthling 08:31 PM 7/31/12

   Light as far as humans are concerned is photons in a very small bandwidth. When the big bang took place, if it ever was so, you got elementary particles and some of them quickly became protons & neutrons, but no atoms till much later. So light, as we see it, could not have existed. Photons are generated when electrons move between different energy levels in an atom.

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3. 3. kenkoskinen in reply to scientific earthling 12:33 PM 8/3/12

   scientific earthing, you have it wrong. It is the excess photons of the early universe that were firstly able to free flow. This is so as after atoms formed they were neutral and no longer interacted with free flowing photons. The neutral atoms create virtual photons during energy changes within levels. It is the same today, even within your body and mine. If this were not so we would not exist and further nor would any stable matter.

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4. 4. kenkoskinen 12:41 PM 8/3/12

   I have to further add that real photons do interact with matter and this is the reason, for example, why things heat up. However it takes a lot of energy to ionize molecules/atoms. It takes a lot of energy to do this but we now things melt and or become gases. The next step is to become plasma and this is ionized gas.
   
   However the in the early universe when there was sufficient space/time expansion real photons did free flow and many missed the atomic targets. It is then we can say there was "light."

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5. 5. vinodkumarsehgal 08:37 AM 8/4/12

   Could any reader or author of this article indicate if astronomers have observed thru empirical means any runway star leaving MW galaxy or any other galaxy. Can't the hot stars ionize the gases while staying within galaxy?
   
   When a runway star leaves a star cluster or a binary pair within a galaxy, as the author has indicated, can't it be trapped again by the gravitation force of a black hole or a star cluster?

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