Study Notes on Inte'rstellar Dust

Interstellar Dust

Overview of Interstellar Dust

  • Interstellar dust constitutes only a small percentage of interstellar matter.

  • Despite its small proportion, it has noticeable effects on light.

How Interstellar Dust Influences Light

1. Reflection Nebulae

  • Dust in reflection nebulae scatters light, particularly short (blue) wavelengths.

  • The average size of dust grains is approximately 100 nanometers (nm).

  • The scattered reflected light is polarized, indicating that:

    • Dust grains are elongated and aligned.

2. Dark Nebulae

  • Dark nebulae have a high density of dust that blocks the background light, rendering the area blacked out.

  • The composition of dust within dark nebulae is difficult to determine but may include:

    • Ice-coated grains of silicon

    • Carbon compounds

    • Various other minerals

3. Interstellar Reddening

  • Dust causes scattering primarily at short wavelengths ($ ext{short-}oldsymbol{ ext{λ}}$).

  • A thin haze of dust scatters most of the blue end of the spectrum from a background star.

  • The light that makes it through is from the redder portion of the spectrum.

  • As a result, a red star observed may not actually be cool, but instead, may be a hotter star that is filtered by invisible dust.

Visual Representation (Color Scattering)

  • The expected color of a star is tied to its temperature.

  • Excess reddening provides information about the amount of dust present.

  • It is critical to correct for dimming caused by dust before using stellar faintness to determine distance.

4. Infrared Emission by Dust

  • Interstellar dust not only scatters starlight, it also absorbs a portion of that light.

  • This absorbed energy is then re-radiated back into space at infrared wavelengths.

Summary of Dust Observations

  • Dark Nebulae: High-density dust that obscures background light.

  • Reflection Nebulae: Dust that scatters blue light and imparts a polarized reflection.

  • Interstellar Reddening: Scattering process leading to a red shift in stellar light as dust filters the shorter wavelengths.

  • Infrared Emission: Dust re-emitting absorbed starlight in the infrared spectrum.