What's in the Space Between Stars?

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It's Not all Just Empty Space Out There!

Stellar explosions like this one scatter elements such as carbon, oxygen, nitrogen, calcium, iron, and many others to the interstellar medium. Space Telescope Science Institute

Read about astronomy long enough and you'll hear the term "interstellar medium" used. It is just what it sounds like it is: the stuff that exists in the space between stars. The proper definition is "matter that exists in the space between star systems in a galaxy". 

We often think of space as being "empty", but in reality it is filled with material. What's there? Astronomers regularly detect gases and dust out there floating among the stars, and there are cosmic rays zipping through on their way from their sources (often in supernova explosions). Close in to stars, the interstellar medium is influenced by magnetic field and stellar winds, and of course, by the deaths of stars.

Let's take an up-close look at the "stuff" of space. 

The emptiest parts of the interstellar medium (or ISM) are cool and very tenuous. In some regions, elements exist only in molecular form and not as many molecules per square centimeter as you would find in thicker regions. The air you breathe has more molecules in it than these regions do.

The most abundant elements in the ISM are hydrogen and helium. They make up about 98 percent of the mass of the ISM; the rest of the "stuff" found there is made up of elements heavier than hydrogen and helium. This includes all materials such as calcium, oxygen, nitrogen, carbon, and the other "metals" (what astronomers call elements behind hydrogen and helium). 

Where does the material in the ISM come from?  The hydrogen and helium and some small amounts of  lithium were created in the Big Bang, the formative event of the universe and the stuff of stars (beginning with the very first ones). The rest of the elements were cooked up inside stars or created in supernova explosions. All of that material gets spread out to space, forming clouds of gas and dust called nebulae. Those clouds are variously heated by nearby stars, swept in shock waves by nearby stellar explosions, and torn apart or destroyed by newborn stars. They are threaded through with weak magnetic fields, and in certain places, the ISM can be quite turbulent. 

Stars are born in the clouds of gas and dust, and they "eat up" the material of their starbirth nests. They then live out their lives and when they die, they send the materials they "cooked up" out to space to further enrich the ISM.  So, stars are major contributors to the "stuff" of the ISM. 

Where does the ISM start? In our own solar system, the planets orbit in what's called the "interplanetary medium", which is itself defined by the extent of the solar wind (the stream of energetic and magnetized particles that flow out from the Sun). 

The "edge" where the solar wind peters out is called the "heliopause", and beyond that the ISM begins. Think of our Sun and planets living inside a "bubble" of protected space between the stars. 

Astronomers suspected that the ISM existed long before they actually got to study it with modern instruments. The serious study of the ISM began in the early 1900s, and as astronomers perfected their telescopes and instruments, they were able to learn more about the elements that exist there. Modern studies allow them to use distant stars as a way to probe the ISM by studying starlight as it passes through the interstellar clouds of gas and dust. This isn't too different from using light from distant quasars to probe the structure of other galaxies. In this way, they have figured out that our solar system is traveling through a region of space called the "Local Interstellar Cloud" that stretches across about 30 light-years of space. As they study this cloud using the light from stars outside the cloud, astronomers are learning more about the structures in the ISM both in our neighborhood and beyond.