Observations of supershells in the interstellar medium of nearby galaxies

The formation of massive stars in a gas-rich spiral or dwarf galaxy has a dramatic effect on the surrounding Interstellar Medium (ISM). Newly formed massive stars (M > 8 M[o]) will have a major impact, first of all through their ionizing flux and stellar winds and, when they eventually have exhausted their fuel supply, as supernovae (SNe). Because massive stars usually form in clusters or associations a large amount of energy is dumped into the ISM within a small volume and within a short time span, creating large scale structures known as supergiant shells and superbubbles. These structures are filled with metal-enriched, coronal gas from SNe which, through overpressure, powers their expansion. Material swept up by these expanding shells can go "critical" and form the sites of subsequent (secondary or propagating) star formation. Shells that grow larger than the thickness of the gas layer will blow out of the disk, spilling enriched material into the halo (or in the case of violent starbursts, the Intergalactic Medium). In this paper I will critically review the observational evidence which has been accumulating and which is underpinning the picture painted above, I will highlight the importance of imaging surveys of the neutral, atomic gas (via its 21-cm hydrogen line emission), and mention some of the exciting projects which are currently underway that link the large scale structure of the ISM with "Triggered Star Formation in a Turbulent ISM"