9. The Material Between the Stars

    • Very massive, luminous, and hot stars ionize the surrounding interstellar material, which radiates emission lines. These regions are called emission nebulae.

    • An emission nebula is also known as a region of ionized hydrogen, denoted H II region. The ionized hydrogen emits radio waves when the free electrons recombine with the free protons, in recombination radiation, or when the electrons are slowed in passing the protons, emitting bremsstrahlung, German for braking radiation.

    • The Strömgren radius is the radius of an emission nebula or H II region. A hotter star and more tenuous surrounding material will have a larger Strömgren radius.

    • Interstellar dust reddens the light of distant stars.

    • High-speed electrons spiral around the interstellar magnetic field in the Milky Way and emit radio waves by synchrotron radiation. It is a form of nonthermal radiation whose intensity increases at longer wavelengths.

    • The 21-cm radio line is emitted from interstellar hydrogen atoms in cold places known as H I regions.

    • Interstellar dust protects interstellar molecules from destructive stellar ultraviolet radiation. Molecules can form on that dust.

    • Abundant interstellar molecules include hydrogen molecules, carbon monoxide, ammonia, water, hydrogen cyanide, and even formaldehyde.

Copyright 2010, Professor Kenneth R. Lang, Tufts University