Observations

by the Voyager spacecraft are providing a clear view of the spectral emissions of the Milky Way. 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. In this week’s Science Express, Lallement et al. (1) repor