Voyagers of Discovery

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Science  23 Dec 2011:
Vol. 334, Issue 6063, pp. 1647-1648
DOI: 10.1126/science.1200166

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City lights reflected by gas and dust in Earth's atmosphere create a bright sky foreground that prevents us from seeing all but the very brightest stars at night. An analogous effect occurs when astronomers try to observe emission from our galaxy, the Milky Way, in the brightest spectral line emitted by the most abundant element, the Lyman-alpha (Lyα) line of atomic hydrogen. This ultraviolet emission line at 121.6 nm is a key diagnostic of the young star-formation rate in the Milky Way, the ionization environment in which the atmospheres of young planets evolve, and the amount of shocked gas in the interstellar medium. However, this critically important emission line in our galaxy has never been observed. On page 1665 of this week's issue, Lallement et al. (1) report observations of the galactic Lyα emission by the Voyager spacecraft that have traveled to distant regions of the solar system where the solar wind meets the interstellar medium.