(updated for 2013)
- Gas: by mass, gas is 60% Hydrogen, 30% Helium. By number, gas is 88% H, 10% He, and 2% heavier elements
- Dust: The term “dust” applies roughly to any molecule too big to name. The size distribution is biased towards small (0.2 m) particles, with an approximate distribution . The density of dust in the galaxy is
- Cosmic Rays: Charged, high-energy (anti)protons, nuclei, electrons, and positrons. Cosmic rays have an energy density of . The equivalent mass density (using E = mc^2) is
- Magnetic Fields: Typical field strengths in the MW are . This is strong enough to confine cosmic rays.