NASA's twin Voyager spacecraft have given astronomers their first direct look at emissions important for estimating the rate at which the Milky Way is making stars.
The emissions are known as Lyman-alpha emissions. Their direct detection opens a new avenue for studying star-forming regions in the galaxy, according to a study published in the Dec. 2 issue of the journal Science.
Until now, astronomers have had to rely on another type of emission as a stand-in for gauging star-forming rates in the Milky Way and in other galaxies.
But the theoretical underpinnings that gave researchers the OK to use this surrogate haven't been tested against the real cosmos.
With future measurements from new spacecraft, researchers finally will be able to subject the surrogate to reality check.
"This is a pioneering observation," says Jeffrey Linsky, a researcher at the University of Colorado's Joint Institute for Laboratory Astrophysics in Boulder, Colo., who was not part of the research project.
Star-formation rates represent a critical element in the story of how galaxies form and evolve. Of particular interest, Dr. Linsky says, is the formation of so-called first-generation stars and how that leads to the birth of second-generation stars like the sun.
As the name implies, first-generation stars are the first to form as regions within vast, cold clouds of dust and hydrogen collapse under their own gravity and heat up. At a critical point, pressures and temperatures at the center of each ball of gas become so great that the hydrogen atoms fuse together to form heavier elements. This process releases enormous amounts of energy, igniting the stars.
As these stars burn, they heat and compress the gas around them, triggering the formation of "Star Birth: Next Generation."
As the stars burn, their spectra show the characteristic fingerprint of hydrogen. In ultraviolet light, where the first-generation stars shine brightest, the standout in this spectral fingerprint is dubbed the Lyman-alpha line. In visible light, the fingerprint shows up as so-called Balmer lines.
Being a star, the sun is a strong source of the Lyman-alpha signature – so strong that it forms a kind of diffuse glow within the solar system that swamps the weaker signatures astronomers want to see coming from outside the solar system.
This has led astronomers to use Balmer lines as surrogates for Lyman-alpha lines in gauging star-formation rates in the Milky Way and beyond.
Enter a team led by Rosine Lallement, with the Paris Observatory. The team was using ultraviolet spectrometers aboard the two Voyager craft – launched in 1977 and now at the boundary between the solar system and interstellar space – to study hydrogen features at the boundary.
The researchers noticed that the sun's Lyman-alpha glow was accompanied by a broader, more diffuse signature. When the team checked to see where the diffuse signature was strongest, they found that it came from the galactic plane, defined by the edge-on view of the galaxy's spiral arms. The arms contain numerous star-forming regions.
The Voyager observations represent a proof-of-concept that Lyman-alpha signatures from outside the solar system can be detected, Linsky says.
Additional observations, plus a test of the theory underpinning the use of Balmer lines as a surrogate for Lyman-alpha lines, potentially could come from the ultraviolet spectrometer aboard NASA's New Horizons flyby mission, which is bound for Pluto and beyond, he says.