Interstellar Medium

The interstellar medium is simply the matter concentrated between stars, which ranges from extremely tiny particles of dust to entire clouds of gas. Inside the medium are also other interstellar objects like comets, asteroids, and meteors.

The interstellar, like many other objects in space, are not static. They are constantly moving within their galaxies and taking up new locations in outer space. In this way, the interstellar medium is very similar to clouds on Earth. Clouds on Earth are always moving and deforming in response to changes in weather. Similarly, gas clouds and dust grains along with comets and other interstellar objects are always changing direction, speed, and location as they go throughout the universe.

Most astronomers estimate the mass of interstellar matter in the Milky Way Galaxy is about 10 billion times that of the Sun. For reference, the mass of the Sun is about 1.9885 x 10³⁰ kg.

The density of the interstellar medium is also noteworthy because of just how extremely low it is. To put it in perspective, imagine the following scenario. Say we could take all the interstellar matter in the Milky Way Galaxy and we spread it out evenly throughout interstellar space, like a carpet. There would then be only one atom of interstellar gas per cubic centimeter. This pales in comparison to air here on Earth, which is composed of approximately 10¹⁹ atoms of gas per cubic centimeter. You may be asking why the mass of the interstellar medium is so high if the density is so small. This is because the volume that interstellar matter takes up is enormously huge, far larger than most stars. To exemplify this, light takes about four seconds to travel the diameter of our Sun, but 4.4 years to travel through interstellar space to the nearest star. That alone shows just how much more massive the interstellar medium is to most stars.