What is the Interstellar Medium?
Outer space is not empty space. The interstellar medium (ISM) is the name for the stuff that is in space between stars in our Milky Way Galaxy. The ISM is mostly made of clouds of hydrogen and helium. The rest of the ISM mostly consists of heavier elements like carbon. About one percent of the ISM is in the form of dust. In some places in space the ISM is not dense at all, but it is much more dense in other regions. However, even the densest parts of the ISM are 1014 (100,000,000,000,000 or 100 trillion) times less dense than the Earth’s atmosphere. The density of the ISM ranges from 0.003 molecules per cubic centimeter in regions of hot ionized gases, or plasma, to more than 100,000 molecules per cubic centimeter in regions where stars form. On average, there are only 1,000 grains of dust in each cubic kilometer of space! > Click here for more information on the composition of the ISM. Stars form in regions of the ISM that are dense enough for gravity to pull the gas and dust together
The interstellar medium is the name scientists give to the highly dispersed gas and dust found between the stars in a galaxy. Of the mix, 99% is gas and 1% dust. Of the gas, 90% is hydrogen and 10% helium. The density of the interstellar medium varies depending where you are and in what galaxy, but it varies between a few thousand to a few hundred million particles per cubic meter, with an average in the Milky Way of approximately a million particles per cubic meter. Contrast this with the intergalactic medium, which has only 100-1000 particles per cubic meter, or the average density of the universe, which contains numerous massive voids, bringing it all the way down to only a single particle per cubic meter. Astrophysicists are very interested in the precise qualities of the interstellar medium because it strongly influences the process of star formation. Denser regions are more likely to condense into stars. Very old galaxies, such as those which power quasars, are thought to have a
At first glance, the term “Interstellar Medium” may seem a little intimidating. It does not have to be, however. As with any difficult term, it helps to break the word up to see where it comes from: First, let’s look at the word “Interstellar.” It has two main parts: the prefix “inter-” and the ending “-stellar.” The prefix inter- is defined as among, between, or amid. The word stellar means of or pertaining to stars or full of stars. If the two are put together, we come up with the definition of interstellar as being “between or among the stars.” Second, we look at the word medium. A medium is something that lies in the middle, or between other things. A medium is also defined as a substance through which an effect is transmitted from one thing to another: when you speak, the waves of sound are carried by the air, so the air is the medium though which sound travels. Putting the definitions for both words together, we can get a definition for the term “Interstellar Medium.” Quite simpl
Simply put, the interstellar medium is the material which fills the space between the stars. Many people imagine outer space to be a complete vacuum, devoid of any material. Although the interstellar regions are more devoid of matter than any vacuum artificially created on earth, there is matter in space. These regions have very low densities and consist mainly of gas (99%) and dust. In total, approximately 15% of the visible matter in the Milky Way is composed of interstellar gas and dust.
The interstellar medium, which makes up ~1/3 of the visible mass of the galaxy, is almost entirely comprised of hydrogen gas. Depending on the proximity to nearby stars, it can be either molecular (H2), atomic (HI), or ionized (HII). The next most abundant element is helium. That’s pretty much it. In fact, many astronomers refer to elements in space as hydrogen, helium, and “metals”. Those metals, which make up ~1% of the mass of the ISM are primarily composed of small iron particles and silicate and carbon soot. This soot, or “dust” as it is generally called, while a tiny fraction of the galaxy, plays an extremely important role both in how we observe the heavens, and on our life on this planet.
