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Running Lights: Did Expelled Stars Reionize the Ancient Universe?

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.

The culprit must have been something hot, because only hot objects emit extreme ultraviolet light and whose photons are so energetic they rip electrons from protons. Quasars once seemed a logical bet. 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.

Conroy and Kaitlin Kratter of the Harvard-Smithsonian Center for Astrophysics propose a resolution to this dilemma. They say that hot stellar runaways may have reionized the early universe.

Stars born with the most mass 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.

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.