A First Discovery in the Milky Way

December 4, 2011

Jean-Loup Bertaux, a researcher at Boston University’s Center for Space Physics, is a member of an international team of astronomers who have detected for the first time ultraviolet (UV) emissions of neutral hydrogen within the Earth’s own galaxy, the Milky Way. Although hydrogen is the most abundant element in the universe, the so-called Lyman alpha emission of Hydrogen, lying at far UV wavelengths, had up to now escaped detection in the vicinity of the Solar System.


The findings were published in the December 2, 2011 issue of the journal Science in an article titled “Voyager Measurements of Hydrogen Lyman-α Diffuse Emission from the Milky Way.” ( The authors, lead by Rosine Lallement, research scientist at the GEPI-CNRS/Paris Observatory (GEPI–CNRS/Observatoire de Paris/Université Paris Diderot)) base their study on an analysis of data from NASA’s Voyager 1 and Voyager 2 space probes. As a result of these findings, it now will be possible to test locally (within the Milky Way galaxy) models designed to measure of the same types of emissions that are observed in distant galaxies.


Lyman alpha emissions, with a wavelength of 121.6 nanometers, are the principal signature of hydrogen atoms in the universe and are used as indicators of the formation of stars in galaxies shortly after the Big Bang (the primordial universe). However, because these emissions are at the ultraviolet end of the spectrum, they are completely obscured by the earth’s atmosphere and the proximity of the Sun, which is composed primarily of hydrogen and produces an intense flux of ultraviolet photons. As the two Voyager probes moved farther and farther from the Sun—from 1993 to 2003, they travelled between six and 13 billion kilometers (40 to 90 Astronomical Units) from the Sun—the glow of local Lyman alpha emissions in the vicinity of the probes has become easier to detect, growing 20 times more intense than if observed from Earth orbit.


Lallement’s team of researchers used the residual ultraviolet light detected by the Voyager probes to develop a theoretical model of the interplanetary glow observable in the sky. The model makes it possible to measure the slight excess radiance in the direction of the Milky Way, which correlates with the “red” radiance (H alpha with a wavelength of 656.3 nanometers) that is characteristic of the regions that surround young, hot stars. As a result, astronomers on Earth will be able to test models that have been developed to interpret observations of the Lyman alpha emission from very distant galaxies, where the emission is associated with the first bursts of forming stars.


Much of what is known about the distant universe is based on measurements of the Lyman alpha emission from distant galaxies, and some of these distant galaxies are detected only from this emission. This new model should help astronomers formulate a better understanding of the behavior of distant galaxies.


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