Interstellar Medium

The physical space outside the bubbles of plasma (astrospheres) formed by stellar winds from individual stars — the space between stellar systems within a nebula or galaxy. For the Sun, the relevant boundary is the heliopause; beyond it lies interstellar space.

Composition

• ~70% lone hydrogen atoms
• Most of the remainder: helium atoms
• Trace amounts of heavier atoms from stellar-nucleosynthesis, ejected by stellar winds or during planetary nebula formation
• Supernovae propagate shock waves that distribute heavy elements throughout the medium

Density varies considerably:

• Average: ~10⁶ particles/m³
• Cold molecular clouds: 10⁸–10¹² particles/m³

Chemistry

Molecules form in interstellar space and can aggregate into dust particles as small as 0.1 μm. Radio astronomy discovers ~4 new molecular species per year. Molecular clouds enable organic polyatomic chemistry driven by collisions — cosmic rays ionize hydrogen and helium, triggering chains that can produce organic compounds.

Local Interstellar Medium

A region within ~100 parsecs of the Sun, nearly coinciding with the Local Bubble — a cavity in the Orion Arm of the Milky Way characterized by a lack of dense, cold clouds. Dense molecular clouds lie along its borders in Ophiuchus and Taurus.