An Overview of the ISM

What exactly is the interstellar medium (ISM)? In his 2011 textbook, Bruce Draine writes ‘The interstellar medium, or ISM, is, arguably . . . the most important component of galaxies, for it is the ISM that is responsible for forming the stars that are the dominant sources of energy.’  Essentially, when we talk about the ISM, we are talking about everything in our galaxy that is between the stars.  Draine divides all of this ‘stuff’ into seven components:

  1. Interstellar gas: Anything (ions, atoms, or molecules) that exists in the gas phase and has a nearly thermal velocity distribution.  This is primarily in the form of hydrogen or helium along with some additional heavy elements
  2. Interstellar dust: Dust in space! Or, in more technical terms, small solid particles mixed in with the interstellar gas
  3. Cosmic rays: Ions and electrons that are moving relativistically (not thermal!)
  4. Electromagnetic radiation: Photons from any source that might produce radiation (the cosmic microwave background, stars, thermal emission from dust, etc.)
  5. Interstellar magnetic field: Magnetic field produced by electric currents in the ISM
  6. Gravitational field: Fairly self-explanatory- force due to all of the matter in the galaxy (ISM, stars, dark matter, etc.)
  7. Dark matter particles: Particles that only exhibit weak interactions, but account for the majority of mass in the galaxy


To learn even more about the ISM and what different wavelength observations tell us about  its many component, refer to this Prezi presentation:

The Illustrated ISM

Or, look at the related tour and WordPress site written by Kate Alexander:

The Multiphase ISM


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