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Dr. Shauna Sallmen

Study of Interstellar Shells in our Galaxy

Last updated August, 2006

Dr. Sallmen and undergraduate students at UW-La Crosse are studying interstellar shells within our own Milky Way Galaxy.

Attention undergraduates! Information about doing research with Dr. Sallmen may be found here


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. 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.


The major goal of research supported by the National Science Foundation (RUI: Study of Interstellar Shells in our Galaxy) and Research Corporation 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 imporve the number of shells identified for study, the number of shells with detailed optical narrow-band imaging, and the number of shells in which absorption-line spectroscopy has been combined with multi-wavelength emission measurements.
  • A search for neutral hydrogen shells using the SETHI database of radio data (originally taken for the SETI@home project) will help identify a previously neglected shells that are suitable for followup observations.
  • Imaging shells in the Interstellar Medium using the WIYN 0.9-m telescope at Kitt Peak National Observatory will provide high angular resolution H-alpha maps of the ionized hydrogen in these shells.
  • Detailed comparison of the cool neutral hydrogen (using SETHI data) warm ionized hydrogen (using H-alpha images), and hot material (using archival X-ray images) allows us to use multi-wavelength analysis to determine the properties of the interacting gas.
  • Ground-based spectroscopic absorption-line studies of the shells allows us to determine accurate distances to the absorption components of the shells. This will allow us to study the velocity structure of the approaching and receding shell walls for both neutral and ionized gas. These data will also allow us to investigate the physical state of the gas in these expanding shells. The improved distances will allow us to improve our estimate the shell sizes, which are important input parameters in the energetic considerations of all theoretical models of bubble formation and evolution.
This research complements other studies of the ISM and interstellar shells undertaking by Dr. Sallmen. These include:
  • Investigations of hot gas in the ISM using NASA's Far Ultraviolet Spectroscopic Explorer satellite.
  • Data reduction and analysis of archival radio pulsar data (from NRAO's 85-foot telescope in Green Bank, WV) on the Vela pulsar to investigate the properties of the ISM and the Vela Supernova Remnant.

Dr. Sallmen's Publications: A list of publications by Dr. Sallmen may be found by searching the ADS (NASA Astrophysics Data System) database.

Last updated August, 2006

University of Wisconsin-La Crosse 1725 State Street La Crosse, Wisconsin 54601 (608) 785-8000
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Comments To: Shauna Sallmen