Physics 321
Astrophysics I:  Lecture #8
Prof. Dale E. Gary

Interstellar Medium

A. Interstellar Medium (ISM)

Solar wind and IPM (Interplanetary Medium)


There are three aspects of the Interstellar Medium in this photograph.  The blue nebula at upper right is a reflection nebula from dust, which scatters blue light from the stars.  The red nebulae seen in several places (e.g., Lagoon nebula below center) are emission nebulae from gas, shining in the red light of hydrogen-alpha.  In several places, especially middle left, the background stars appear to be less numerous due to interstellar absorption from dust, which obscures the more distant stars. Photo by Steve Mazlin and Jim Misti.
Gas vs. Dust: B. 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.
Turbulent Cauldron of Starbirth in Galaxy Centaurus A, With Ground View (inset)

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.

  • 1. Pleiades 2. More Pleiades 3. Wider yet

    Here is another beautiful example of the three aspects of the Interstellar Medium.  Reflection nebulae from dust are seen in the vicinity of hot, luminous stars on both left and right.  The red nebulae at left, including the cone nebula, are emission nebulae from gas, shining in the red light of hydrogen-alpha.  The absorption by dust is obvious throughout the region. The gas and dust is obviously sculpted by light and energy from the luminous stars on the left, forming bow-shaped features. An open cluster of stars appears near the center of the photo. Photo by Adam Block and Tim Pucket.

    Nature of Interstellar Grains

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

    Interstellar Absorption Lines

    Emission Nebulae--H II Regions Supernove Remnants and Planetary Nebulae

    Mysterious Ring Structure around Supernova 1987A

    Video (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