E8 Interstellar Medium and Nebulae

Topic

The interstellar medium contains gas—predominantly hydrogen and helium—along with dust; both are distributed across phases that differ markedly from one another within that medium: cold molecular clouds, regions of hot ionized gas, and a diffuse gas that is even hotter than the previous two phases.

The dust within the interstellar medium attenuates and reddens the light from background objects viewed through it—a phenomenon known as interstellar extinction. This attenuation increases exponentially with the amount of dust along the line of sight; consequently, doubling the thickness of the dust traversed does not merely double the light loss but multiplies it far more significantly. The resulting color excess quantifies precisely the extent to which the light has been reddened during its journey.

Regions of ionized gas are bubbles that form around young, hot stars within the interstellar medium itself. The size of these bubbles is determined by an equilibrium: a bubble expands until the rate of ion-electron recombinations occurring within it exactly matches the rate of ionizing photons produced by the star. It expands no further, thereby reaching a saturation limit in its ionized volume.

Molecular clouds within the interstellar medium are, in turn, the birthplaces of new stars.

The complete gas cycle forms a closed loop: a molecular cloud gives rise to a star, that star ends its life as a supernova, and the supernova re-enriches the interstellar medium with elements that will later serve to form the next generation of molecular clouds and stars. Thus, from the various phases of the interstellar medium with which this document begins to the enrichment of that same medium with which it concludes—encompassing interstellar extinction and the equilibrium that determines the size of ionized regions—the interstellar medium ensures that a galaxy's matter never ceases to recycle, passing repeatedly from cloud to star and back from star to cloud.

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