We’ve long known that the spaces between the stars are not empty, but are pervaded by a highly dilute mix of gas and dust. Now we’re getting maps that show the presence of large cavities in this interstellar medium, created by supernova events as well as outflowing solar winds from clusters of hot, young stars. The Sun resides in the so-called Local Cavity, a low-density area of neutral gas that is about 80 parsecs in radius. The Local Cavity is, in turn, surrounded by a ‘wall’ of dense, neutral gas, with gaps in the wall — ‘interstellar tunnels’ — that are low-density pathways to surrounding cavities.
We study the interstellar medium by looking at the light produced by stars and using absorption line spectroscopy to see how that light is affected by gases between us and the stars in question. Johannes Hartmann’s classic study of the spectrum of Delta Orionis in 1904 was a huge advance, looking at absorption from the ‘K’ line of calcium and concluding that the gas was not present in the atmosphere of the star but within the matter in space along the line of sight to the star. Interstellar sodium was detected fifteen years later and the study of the interstellar medium went into higher gear, especially in the sightline toward Orion.
This Wikipedia article on the interstellar medium quotes Norwegian explorer and physicist Kristian Birkeland, who described the medium as understood in 1913:
“It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or nebulae, but in ’empty’ space.”
Have a look at the image below, which draws this into perspective. It’s based on new data, gathered primarily at the European Southern Observatory in Chile, that has been folded into previously published results. A French-American team is behind the work, offering up a catalog of absorption measurements toward 1857 stars within 800 parsecs of the Sun. The image shows cold and neutral gas density within a distance of about 300 parsecs.
Image: Map of partially ionized interstellar gas within 300 parsecs around the Sun, as viewed in the Galactic plane. Triangles represent the sight-line positions of the stars used to produce the map. White to dark shading represents the low to high values of the gas density, and orange shading is for areas with no reliable measurement. The Local Cavity is shown as the white area of low density gas that surrounds the Sun at about 80 parsecs. Credit: B. Welsh/R. Lallement/S. Raimond/J.-L. Vergely.
We’re still early in the quest to understand the local interstellar medium, even though many surveys at various wavelengths have been completed. Knowing the chemical and physical characteristics of the medium will help us understand the evolution of stars as they exchange matter with the space around them. From a spaceflight perspective, probing beyond our own Solar System with future technologies will require understanding the spatial distribution and dynamics of the material we’re pushing into, much as early ocean voyagers had to acquire a working knowledge of wind patterns and ocean currents.
The Local Cavity within which our Sun resides is thought to have been created about 15 million years ago by supernova activity, but its history remains highly speculative. The paper is Welsh et al., “New 3D gas density maps of NaI and CaII interstellar absorption within 300 pc,” to be published in Astronomy & Astrophysics 510 (2010), A54 (abstract).