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.


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