Physics 321
Astrophysics II:  Lecture #11
Prof. Dale E. Gary

Interstellar Medium

Interstellar Medium (ISM)

Solar wind and IPM (Interplanetary Medium)
There are three aspects of the Interstellar Medium in this photograph.  The blue nebula at left surrounding the stars of the Pleiades is a reflection nebula, which scatters blue light from the stars.  The red nebula at right (California nebula) is an emission nebula, shining in the red light of hydrogen-alpha.  In the upper left and slanting diagonally across the middle of the photograph, the background stars appear to be less numerous due to interstellar absorption from dust, which obscures the more distant stars.
Gas vs. Dust: Interstellar Dust

Dark Nebulae and Extinction

Extinction (= Absorption + Scattering) means reduction of overall brightness of objects seen through dust.  Distance modulus equation is modified when we take dust into account:
m - M = 5 log d - 5 + A

where A is amount of absorption in magnitudes.  For A = 1, star is reduced in brightness (flux) by a factor of 2.5.  If A were uniform throughout galaxy, then A = kd (proportional to distance) but A is not uniform.
Dust in a galaxy

Interstellar Reddening Interstellar Polarization Reflection Nebulae
  • When you see dust from the side, it appears BLUE and polarized.
  • This polarization does not necessarily depend on grains being elongated.

  • Pleiades
    Nature of Interstellar Grains Interstellar Gas
    Again, 99% of ISM by mass is gas, and there are 1012gas "particles" for every dust grain.

    Interstellar Absorption Lines

    Emission Nebulae--H II Regions Supernove Remnants and Planetary Nebulae
    Image, Latest Image (shock wave slams into ring)

    Planetary Nebulas
    Image 1
    Image 2

    Interstellar Radio Lines

    21-cm line Recombination lines Molecular Lines
    Inside a molecular cloud -- OMC-1:  Image
    Star formation in M16: link
    Egg Nebula:  picture