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Mapping the Interstellar Medium

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).


Comments on this entry are closed.

  • Kenneth February 10, 2010, 12:19

    One of the big issues for any potential Interstellar Travel over the next couple of Centuries is what does the Interstellar Medium look like between Sol/Terra and Alpha Centuari as well as other candidate stars out to about 20 Light Years. This is probably our maximum travel radius for the next couple of hundred years anyway at least given that it is hard to see how we are going to get much above 70% the speed of light in velocity under even the more optimistic technical scenario’s. This is due to the fact that there seems to be basic technical and engieering limitations for even the most advanced “non warp drive” propulsion methods that render velocities above .7C hard to imagine right now, at least for the next couple of hundred years.

    The very preliminary date suggests that the the Interstellar Medium between Sol/Terra and Alpha Centuari is managable given the potential very limited exposure time of any future ship traveling such a relatively short distance, and the fact that the Interstellar Medium between Sol/Terra and Alpha Centuari is not very dense as our two systems seem to be riding in something of a local bubble together. This may be the result of our two protective Heliosphere’s almost touching each other given our relatively close proximity or for some other currently unknown reason. However, beyond such a local “Interstellar hop” things start to get far more complicated and potentially difficult very quickly. Short of finding a way to somehow easily push or repel the Interstellar Medium as we pass through Interstellar Space on longer duration missions we may find that traversing the Interstellar Medium in any trip other then to Alpha Centauri is a significat limitation for Human Crewed Interstellar Travel unless “new physics” is discovered.

    The bottom line situation is that given the emerging potential travel obstacles posed by portions of the Interstellar Medium , and at least for the next few Centuries, there may be a trip to Alpha Centauri which in someways is almost an extended Interplanetary journey, and everything else. Hopefully, this is not the case, but Human crewed Interstellar Travel beyond about a 5 Light year radius from Sol/Terra is starting to look much harder then we originally thought given currently conceivable technologies, and advanced propulsion and basic shielding challenges may be the least of our problems.


  • Erik Anderson February 10, 2010, 13:25

    Interesting study, Paul. It looks like there’s at least one door left open to see clearly out of — along 0, 45. (But if the plot was in three dimensions, we’d see “windows,” e.g., Baade’s Window)

  • James M. Essig February 10, 2010, 19:37

    Hi Folks;

    One should never go anywhere without a road map and that includes travel to the stars. I enjoy thinking about magneto-hydrodynamic-plasma drives, electro-hydrodynamic-plasma drives, and electro-dynamic-hydrodynamic-plasma-drive space craft in general. Perhaps the concept of the ISR can be ressurected in some usefull form, or some form of as yet unspecified ZPF energy extraction units can be developed that power electrodynamic field generators that reacts against the interstellar medium.

    EDHPD space craft might also be powered by real mass-energy streams such as by laser, microwave, or Rf radiation beams, or by beamed fuel such as beamed matter-antimatter, or antimatter fuel.

    But to utilize such EDHPD systems, we will need a good road map.

  • Stevo Darkly February 11, 2010, 19:44

    Would a systematic effort to locate and map low-density regions be called a “cavity search”?

  • Pat Galea February 13, 2010, 4:30

    @Stevo “Would a systematic effort to locate and map low-density regions be called a “cavity search”?”

    Only if the program is being run by Ben Dover.

  • James M. Essig February 13, 2010, 20:58

    Hi Folks;

    I just bought a new book from the Scientific American Book Club. The subject is on the magnetic field distributions within the universe. Allthough it is written for the layperson as a popularizing book, I cannot wait to start reading it this evening.

    The point I would like to make is that the magnetic energy of free interstellar and intergalactic space although of a very low density, might offer use as a reactive field for space craft with electromagnets on steriods or perhaps even space craft with nano-scale continuously adjusted superstrong permanent magnets. Some sort of application of the fractal theory of chaotic phenomenon might permit such fine tuned adjustment of permanent magnets and so permit energy extraction from the universe.s magnetic field without the need for continuously powered superconducting electromagnets.

    The second point is that anyway we can characterize and utilize the interstellar medium is potentially of great use in doing manned interstellar space travel.