Abstract
To introduce our subject, we provide an overview of the interstellar medium (ISM) in our Galaxy and its relation to the stellar component. We then discuss the phase structure of the ISM, with a special emphasis on why the hydrogen takes the different forms it does. We then outline the observational signatures of the Hot Ionized, Warm Ionized, Warm Neutral, and Cold Neutral media, followed by a brief introduction to the molecular component of the Galaxy. A description of Photo-Dissociation Regions is presented with emphasis on the transition region from atomic to diffuse molecular gas. The role of dust and cosmic rays is also briefly described. Finally, we discuss the concept of a molecular cloud, a central theme of this book.
It is clear to everyone that astronomy at all events compels the soul to look upwards, and draws it from the things of this world to the other. — Plato
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Notes
- 1.
We will follow the standard astronomy convention that, when referring to the Milky Way, the word Galaxy be capitalized.
- 2.
HI is the traditional astronomical way to refer to neutral hydrogen; ionized hydrogen is not typically referenced to as H+, but, rather, HII. This convention continues with other neutral and ionized atomic species.
- 3.
The continuous range of wavelengths less than the Lyman limit (912 Å).
- 4.
In the millimeter-wave regime there are transparent windows, but also very opaque ones.
- 5.
- 6.
Although C+ is present, this gas is overwhelmingly neutral.
- 7.
In their classic 2003 paper, Heiles and Troland (2003) refer to the WNM as “…the key to the Universe…”.
- 8.
The relationship between velocity dispersion of an ensemble of Galactic objects and their scale height is discussed in Sect. 10.3
- 9.
A few of the people who tried to bridge the gap in this heroic age were Bob Dickman, Steve Federman, Göran Sandell, Robert Willson, and Ken Lang.
- 10.
First discovered by electrostatic measurements of the upper atmosphere (the ionosphere) in balloon ascents a century ago by Hess (1912) they were recognized as particles and, in that sense, penetration of new regimes by probes has been characteristic of studies of the lower energy range of such particles.
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Magnani, L., Shore, S.N. (2017). A Quick Look at the Diffuse Interstellar Medium. In: A Dirty Window. Astrophysics and Space Science Library, vol 442. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-662-54350-4_1
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