Study of Interstellar Shells in our Galaxy


The interstellar medium (ISM) is low-density material (mostly gas) that lies between the stars. Stars forms through the mixing of this gas. When a star dies it replenishes the gas with new elements that are heavier than the original hydrogen and helium. New stars and planetary systems form from this enriched material, so "We are made of Star Stuff", as Carl Sagan says. I've also heard some stay "We're made of thermonuclear waste". Stellar winds and supernovae (exploding stars) send material out into the surrounding non-uniform ISM, often blowing a bubble. The ambient gas is disturbed and forms irregular shells. Young shells are typically filled with hot (about 1 million K) X-ray emitting gas, surrounded by a shell of cooler neutral material, with an interface layer of ionized material at intermediate temperatures.

Neutral Hydrogen (HI) image of an interstellar shell.
(SETHI Shell Catalog)

Research Projects

Dr. Sallmen and undergraduate students at UW-La Crosse are studying interstellar shells within our own Milky Way Galaxy. Overall, the main goal is to increase our sample of neutral hydrogen shells that have been studied in detail, in order to enhance our understanding of the processes whereby these objects interact with the ambient ISM. An adequate sample of well-studied neutral shells is essential for testing competing models of supershell formation, as well as for understanding the processes whereby energy from supernovae and stellar winds affect the structure of the ISM as a whole. This research will improve the number of shells identified for study, the number of shells with detailed optical narrow-band imaging, and the number of shells with multi-wavelength emission comparisons.

In particular, the following projects meet these goals:

This research complements other studies of the ISM and interstellar shells previously undertaken by Dr. Sallmen. These include:

Online ISM Introductions

Funding Acknowledgements

This research has been funded in part by:

Back to Website for Dr. Shauna Sallmen

Last updated August, 2014