Interstellar Medium
Introduction
The interstellar medium (ISM) is the matter and radiation that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field.
Composition
The interstellar medium is composed primarily of hydrogen followed by helium with trace amounts of carbon, oxygen, and nitrogen comparatively to terrestrial standards. The thermal pressures of these elements are in rough equilibrium with the surrounding cosmic rays. The interstellar medium is composed of multiple phases, distinguished by whether matter is ionic, atomic, or molecular, and the temperature and density of the matter. The interstellar medium is full of structures such as molecular clouds, Bok globules, H II regions, and high-velocity clouds.
Phases
The ISM is divided into distinct phases, each with different properties.
Ionized
The ionized material is the hottest and most tenuous phase of the ISM. The fully ionized gas in this phase is primarily composed of hydrogen and helium. This phase is often seen near hot, luminous stars that ionize the surrounding gas, and in the planetary nebulae that represent the final stages of stellar evolution for low-mass stars.
Atomic
The atomic hydrogen phase is a warm, partially ionized medium that is seen in the vicinity of cooler stars. This phase is often associated with the neutral hydrogen, or H I, spectral line.
Molecular
The molecular phase is the coolest and is primarily seen in molecular clouds and regions of high density. Molecular hydrogen is the primary component of the molecular ISM, and because of its stability, it is incredibly difficult to destroy.
Interactions with Galactic Objects
The interstellar medium plays a crucial role in the life cycle of stars. It provides the material from which stars and planetary systems form, and it is into the ISM that stars eject their outer envelopes at the end of their lives.
Star Formation
Stars form within the densest regions of the ISM, molecular clouds. These regions are dense enough to collapse under their own gravitational force, forming a protostar—a hot, dense, central object. With time, dust and gas falling onto the protostar become a planetary system.
Stellar Evolution
During the late stages of stellar evolution, stars return a significant portion of their mass to the ISM, enriching it with heavier elements generated by nuclear fusion in the star's interior.
See Also
