Number 231 (Story #1), June 23, 1995 by Phillip F. Schewe and Ben Stein|
PRIMORDIAL HELIUM HAS BEEN DETECTED by an ultraviolet telescope on board the Astro-2 observatory. According to the big bang model, the nuclei of primarily two elements--- hydrogen and helium---would have been created in the aftermath of the big bang. All heavier elements had to be incubated in the interiors of stars that formed in a later epoch. Arthur Davidsen of Johns Hopkins reported at last week's meeting of the American Astronomical Society in Pittsburgh on how the helium detection was made. The telescope looked at the ultraviolet rays issuing from the quasar HS1700+64. Coming from so far away (10 billion light years) and so far back in time (when the universe was but a fraction of its current age) the UV rays had to traverse some of the primordial helium, which left its mark on the UV spectrum by subtracting energy at a characteristic wavelength. The detection of primordial helium, but not primordial hydrogen, is explained in this way: the hydrogen was entirely ionized by the intense radiation of later stars. Without its lone electron, the hydrogen nucleus was no longer an atom, and no longer capable of absorbing the radiation. Helium atoms, with two electrons, were at times able to retain at least one electron, making it possible for them to absorb some of the quasar's UV photons.