Missing Starlight

Saturday, March 01, 1997
Since the 1920s, astronomers have known that when they look at many hot, bright stars, they don’t see all the light they should. Some wavelengths are apparently absorbed somewhere between the stars and Earth. But absorbed by what? Seventy years of speculation have led to suspects as varied as grains of dust to the current favorite among researchers, a type of carbon gunk known as polycyclic aromatic hydrocarbons. Yet no single explanation has been able to account for all the missing wavelengths of light. Now two laser experts at ibm’s T. J. Watson Research Center in New York have come up with a solution that relies on a complex interaction of light with the most common molecule in the universe: hydrogen.

Physicist Peter Sorokin began looking into the question three years ago. While most astronomers have been positing different absorbing materials to explain each missing wavelength of light, Sorokin’s background in lasers made him wonder whether a subtler interaction might be occurring instead. He knew that a molecule can absorb two photons of light simultaneously as long as the energies of the photons add up to an amount allowable under the rules of quantum mechanics.

Sorokin teamed up with colleague James Glownia and found that, indeed, molecular hydrogen could sop up many of the missing wavelengths if the visible light from stars somehow combined with stellar ultraviolet light, which hot stars produce in abundance.

This double absorption could occur, Sorokin says, if the ultraviolet light (which is of a wavelength that the hydrogen can’t absorb by itself) from a star bounced around inside large clouds of molecular hydrogen nearby. This would keep the uv light within the cloud, building it up until the cloud became saturated with uv photons. When a photon from another missing wavelength reached the hydrogen cloud, there’s a good chance that it could combine with a uv photon, thus creating an energy packet a hydrogen molecule in the cloud could absorb.

So far, Sorokin’s ideas have received a chilly reception from astronomers who don’t think hydrogen clouds can become saturated with trapped uv photons, and Sorokin admits his model can’t explain every missing stellar wavelength. But he says his theory may be supported by data from a uv telescope carried on the space shuttle Columbia last December. The data are still being analyzed, but Sorokin thinks they may show uv light leaking out of hydrogen clouds. If that ultraviolet emission is there, he says, then everyone will have to take this seriously.
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