Interstellar Space | Research & Encyclopedia Articles

Interstellar Space

Interstellar space (or the interstellar medium, or ISM), as it is sometimes more appropriately called) is the vast volume of space between the stars in our galaxy. Although so incredibly sparse that even its densest portions would constitute an excellent vacuum on Earth, it is a remarkably complex physical system. It is filled with an extremely dilute soup of matter (atoms, ions, electrons, molecules, and microscopic dust grains), radiation (in all bands of the electromagnetic spectrum, from long-wavelength radio waves to very short wavelength gamma rays), and high-energy particles (charged nuclei and electrons that have been accelerated to tremendously high energies). Moreover, as one immediately gathers by viewing the sky, the ISM is highly inhomogeneous, clumpier and denser in some regions than others, and hotter and more turbulent in others.

The properties of the interstellar medium can be deduced by several primary methods. The first is to study stars with known distances along different lines of sight. By carefully examining their color, it is possible to determine that those further away tend to be redder than a star with similar physical characteristics would have nearby. The reason is simple: gas and dust along the line of sight to the star absorb and scatter radiation from the star, with shorter (i.e., bluer) wavelengths more preferentially affected, leaving redder wavelengths in the beam along the line of sight. This same phenomenon is observed in terrestrial sunsets: the Sun appears redder because the blue light is more easily scattered out of the beam along the line of sight. By measuring the reddening properties of many stars along many lines of sight, one can infer the mean column density (i.e., mass per square area) along the lines of sight, and hence build up a map of interstellar space.

It is also possible to directly map the ISM by observing the gas in emission. Hotter regions of the sky are seen in shorter wavelengths; colder regions in longer wavelengths. For instance, in the visible band, we can detect the so-called forbidden radiation from hydrogen atoms, which has a characteristically orange reddish color. In longer wavelengths--the radio, microwave, and infrared bands--we can map out the emission of extremely cold molecules. At the opposite end of the spectrum, at shorter wavelengths--the ultraviolet, x ray, and gamma ray bands--we see gas heated to extremely high temperatures through stellar radiation, stellar winds, and novae and supernovae explosions.

Lastly, it is possible to see the sky through "diffuse" radiation, which includes radiation from many sources that have been scattered into our line of sight. For instance, when viewing nearby stars in star-forming regions, we often detect a bluish diffuse halo surrounding them, which is actually their radiation scattered into our line of sight. (Such scattered radiation appears blue for the opposite reason that light undergoing absorption appears red, as described above.)

Because of its extremely complex nature, the ISM has proven to be an extraordinary challenge to the many generations of astronomers and astrophysicists who have sought to understand it. The presence of both very hot gas, with temperatures exceeding 106 K, and blue, massive stars, tells us that the ISM within our galaxy is still a very active place. This is because such hot gas cools on timescales much shorter than the age of the galaxy, and similarly, such massive stars also live for a time much shorter than the age of the galaxy before ending their lives in supernovae explosions. Hence, there is a continual process by which gas cools from the ISM, condenses to form stars, with those stars driving winds and explosions into the ISM, from which other stars may be born. The most important details of that process--how stars form, and how the most massive stars explode--are themselves very fascinating subjects, and remain active topics of intense research by astronomers and astrophysicists today.