A new discovery from old probes

[Revised & corrected: 3 December 2011]

Voyager I and II were launched in 1977, and after 33 years they are both still doing well and sending information back home. Voyager I is currently 119 AU from Earth, while Voyager II has travelled a distance of 97 AU. This means that the probes are now leaving the heliosphere.

The two Voyager spacecraft leave the solar system

Their current positions allow them to detect Lyman-alpha emission from star forming regions in the Milky Way for the first time.  Lyman-alpha is an emission line of hydrogen at 121.57 nm – in the far-ultraviolet part of the electromagnetic spectrum – and is ubiquitous in astrophysical objects. Lyman-alpha emission results from UV light hitting neutral hydrogen atoms and splitting them into protons and electrons.  The Lyman-alpha is emitted when particles recombine to form an excited hydrogen atom.  Since young stars emit a lot of UV radiation and star forming regions are filled with hydrogen, we can expect a lot of Lyman-alpha near young stars.  Lyman-alpha is used by astronomers to investigate star forming galaxies at redshifts between 0.3 and 6 where the Lyman-alpha line is shifted to longer wavelengths (between the near-UV and visible).

Until now Lyman-alpha has not been detected in the Milky way because the radiation at this wavelength is “obscured” by our Sun. The small amount of hydrogen from the interstellar medium that finds it s way into our solar system scatters the Sun’s Lyman-alpha emissions in random directions, which effectively produced a “fog” of Lyman-alpha, obscuring more distant Lyman-alpha radiation from within our Galaxy. The Voyager spacecrafts are now far enough from the Sun that the Lyman-alpha emission from the Milky Way galaxy is finally revealed.

Lyman-alpha is easily scattered by the interstellar medium (ISM) and the intergalactic medium (IGM), and hence can be used to provide information about the column density of neutral hydrogen in the IGM.  Within galaxies Lyman-alpha is even more easily scattered by dust, which provides information about their dust content. The problem is that we know almost nothing about the  spatial distribution of dust and gas in high redshifts galaxies, and so by analysing the Milky Way Lyman-alpha we can extrapolate to infer the amount of dust and gas in distant galaxies.

In this week’s edition of Science, Rosine Lallement and collaborators report on detections made by the Ultraviolet Spectrographs on board of the Voyager spacecrafts of diffuse Lyman-alpha emission from our Galaxy. Now that the Voyager spacecrafts are outside the heliosphere, the Lyman-alpha from the Sun does not interfere with measurements of galactic Lyman-alpha, allowing astronomers to detect Lyman-alpha from star forming regions in our own Galaxy for the first time.  Lallement et al. use these new observations to refine models of Lyman-alpha emission for the Milky Way, which can then be applied to high redshift galaxies to constrain the amount of gas and dust that they contain, and their star formation. For more information, see

[Posting by Guido Moyano Loyola, Catarina Ubach, Francesco Pignatale and Sarah Maddison, with assistance from Michael Murphy]

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One Response to A new discovery from old probes

  1. Jaal says:

    119 AUs! That’s one heck of a distance

    … in 33 years. That’s a bit depressing.
    The true scale of the universe and our expectations will always clash.

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