Interstellar medium (noun, “IN-ter-STEH-ler MEE-dee-um”)
The interstellar medium is the gas, dust and radiation filling the space between stars within a galaxy.
Interstellar space refers to the regions between stars. And the interstellar medium — or ISM — fills this space. At a galaxy’s outer bounds, the ISM merges into intergalactic medium. That’s the matter and energy between galaxies.
The ISM is mostly gas, with a little dust mixed in. Exploding stars and other celestial objects eject some of this material. But much of it is left over from the galaxy’s birth. The ISM also contains radiation. This energy may be left over from the early universe. But some comes from stars and other objects.
Hydrogen and helium gas make up about 98 percent of the ISM’s mass. The other 2 percent is heavier elements, such as carbon and oxygen. Some of those atoms drift solo. Others bond to form small molecules. Most carry an electrical charge.
The gas molecules in interstellar space are sparse. That means there aren’t many of them, and they’re very spread out. They are a lot more sparse than gas molecules in the air on Earth.
Let’s do a comparison. On Earth, a refrigerator-sized volume of air contains about 100 sextillion gas molecules. That’s 10 followed by 23 zeros. In contrast, interstellar space might contain only about 1,000 molecules in that same volume. That very low density of molecules never occurs on Earth. Not even in the lab. Scientists’ best vacuum chambers come nowhere near the low particle density of interstellar space.
Like the ISM’s gas molecules, its dust grains are very sparse. But across vast stretches of space, that dust adds up. It’s enough to block the visible light from distant stars. Infrared light, though, can pass through this dust. That’s why our farthest-seeing telescopes — such as the James Webb Space Telescope — do not look for visible light. Instead, they look for infrared light.
In a sentence
Astronomers predict that the Voyager 1 probe will become the first craft to enter interstellar medium.
