Issue 5.01

The Astrophysics Spectator

January 16, 2008

The vacuum of space is something of a misnomer, because gas and dust pervade the space between the stars.  In many places, this gas is visible through the light it radiates or by the light it blocks, but more often it is as invisible to our eyes as the air we breath.  It plays the nuisance role of blocking starlight, hiding much of the Galaxy from view.  It provides a medium for the propagation of sound waves, particularly supernova shock waves.  It can also look pretty, like neon lights, when bright stars shine their light onto it.

The interstellar medium  is quite substantial, for it accounts for almost one-third of the mass in the Galactic plane, and it is the material from which new stars are made.  It is symbiotic with the stars.  Stars heat the interstellar gas and dust, governing the rate at which new stars are born, and they alter the composition of the interstellar gas, modifying the chemical composition of the next generation of stars.

This first issue for the year of this web site introduces two pages on the interstellar medium.  The first page is a brief introduction to the characteristics of the interstellar medium.  The second page describes why one sees cool, dense clouds floating in a warm, tenuous gas in the Galactic disk.  Pages to be added in the coming weeks on the interstellar medium will lay the foundation for discussing star formation.

Next Issue:  The next issue of The Astrophysics Spectator is scheduled for January 30.

Milky Way Galaxy

The Interstellar Medium. The space between the stars is filled with gas, dust, magnetic fields, and cosmic rays.  Together they form within the Galactic disk a tenuous, pressure-balanced atmosphere of surprising variety.  The cool, dense clouds we see in the distance against the stellar background are the most obvious manifestation of this material.  Surrounding these clouds is a warm, tenuous gas that is interspersed with bubbles of hot, thin gas.  (continue)

Cold Clouds in the Warm Gas. The gas in the interstellar medium is segregated into cool, dense regions surrounded by warm, tenuous regions. Despite the differences in density and temperature, each of these regions is thermally stable and in pressure equilibrium with the other.  The immediate reason these regions exist simultaneously is tied to how different chemicals within the interstellar gas emit radiation, but the deeper reason is that the temperature and density of the interstellar gas depend on the rate at which young, massive stars are born.  (continue)