Interstellar Medium and Cosmic Dust

Matter and radiation in the space between the star systems in a galaxy.

The universe is a vast expanse of space, filled with galaxies, stars, and planets. But what fills the space between these celestial bodies? This is where the interstellar medium and cosmic dust come into play.

What is the Interstellar Medium?

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.

The ISM plays a crucial role in astrophysics precisely because of its intermediate role between stellar and galactic scales. Stars form within the densest regions of the ISM, molecular clouds, and replenish the ISM with matter and energy through planetary nebulae, stellar winds, and supernovae. This interplay between stars and the ISM helps to regulate the rate of star formation in a galaxy.

Composition of the Interstellar Medium

The ISM is composed primarily of hydrogen, followed by helium with trace amounts of other heavier elements. The three-phase model of the ISM explains how the interstellar medium can coexist in three distinct phases at the same time:

1. Cold Neutral Medium (CNM): This phase consists of relatively cold, neutral hydrogen atoms. The temperature in this phase is about 100 K, and it has a density of about 1 atom per cubic centimeter.

2. Warm Neutral Medium (WNM): This phase consists of neutral hydrogen atoms with temperatures around 8000 K and a density of about 0.1 atoms per cubic centimeter.

3. Hot Ionized Medium (HIM): This phase is composed of ionized hydrogen atoms—that is, hydrogen atoms that have lost their electrons. The temperature in this phase is about 1 million K, and it has a very low density.

Cosmic Dust

Cosmic dust, also called extraterrestrial dust or space dust, is dust which exists in outer space, or has fallen on Earth. Most cosmic dust particles are between a few molecules to 0.1 µm in size. A smaller fraction of all dust in space consists of larger refractory minerals that condensed as matter left by stars.

Cosmic dust can be further distinguished by its astronomical location: intergalactic dust, interstellar dust, interplanetary dust (such as in the zodiacal cloud) and circumplanetary dust (such as in a planetary ring). In the Solar System, interplanetary dust causes the zodiacal light.

Cosmic dust contains some complex organic compounds (amorphous organic solids with a mixed aromatic-aliphatic structure) that could be created naturally, and rapidly, by stars. A smaller fraction of dust in space consists of larger refractory minerals that condensed as matter left by stars.

Conclusion

The interstellar medium and cosmic dust are more than just the "empty space" between stars and galaxies. They are active and dynamic, contributing to the birth of stars and the evolution of galaxies. Understanding these components of our universe is crucial to our overall understanding of astrophysics and cosmology.