From our vantage point inside the Galaxy, studying the structure and evolution of stars and gas in the Milky Way requires a unique set of tools and resources. We gain an opportunity with this perspective to trace the organization and processes of a galactic machine from very large to tiny scales. However, it also complicates efforts to accurately measure and separate structures to establish the true global structure of the Milky Way.
Establishing patterns in the distribution, composition, and physical conditions of stars and gas traces the history of our evolving home galaxy. The stellar component retains its most ancient history as star distributions and orbits reveal the large-scale organization dictated by gravity. Anomalies in these patterns highlight past interactions and mergers with neighbors. On the other hand, the gas bears the imprint of more recent activity, particularly that associated with the violent events of star formation and death. This interstellar medium (ISM) catches some of the energy output from stars, responds with changes in temperature and density by several orders of magnitude, and then provides some support for the Galaxy against the inevitable crush of gravity. The coupling of the stars and gas creates a fascinating feedback cycle that regulates star formation and galactic evolution in massive galaxies like the Milky Way.
Wisconsin has a rich history of strong ISM programs. Current researchers contribute to Galactic Astronomy through absorption and emission spectroscopy from the radio through ultraviolet regions of the spectrum as well as modeling large-scale processes that shape the ISM. The observational studies collect information over gigantic distances, yet ultimately explore details of the fundamental processes of atomic and molecular physics in the unique environment of space. As instrumentation has become more sensitive and specialized, our groups have been collecting large survey datasets. These are beginning to provide the next round of breakthroughs in our understanding of Galactic structure.
New insights into the Milky Way's stellar structure now complements our traditional focus on the gas component. A major impetuous for this new interest comes from the GLIMPSE (Galactic Legacy Infrared Mid-Plane Survey Extraordinaire) project, a Spitzer Space Telescope Legacy program anchored by researchers at UW-Madison and UW-Whitewater. Its ability to isolate specific types of stars and trace them as a function of distance has helped sharpen our picture of the inner Galaxy, including the bar and the major spiral arms.