10 Interstellar Medium

1. Interstellar Matter

Milky Way Mosaic

The Milky Way Galaxy photographed almost from horizon to horizon, thus spanning nearly 180°. This band contains high concentrations of stars, as well as interstellar gas and dust.

  • The dark regions are dust clouds, blocking light from the stars beyond.
  • This region is called InterStellar Medium (ISM) and it is not homogeneous.
  • The ISM consists of gas and dust.
    • Gas is individual atoms mostly hydrogen and helium (10-10 m = 0.1 nm) and small molecules (< 10-9 m)
      • They are transparent to EM radiation
    • Dust is more like soot or smoke; larger clumps of particles (~ 10-7 m = 0.1 µm)
      • EM cannot penetrate.
      • Dust absorbs light, and reddens light that gets through.
      • Reddening can interfere with blackbody temperature measurement, but spectral lines do not shift.

Extinction and Reddening

Scattering depends on the diameter (ie. size) of the particle and the wavelength of the radiation.

(a) Starlight passing through a dusty region of space is both dimmed and reddened, but spectral lines are still recognizable in the light that reaches Earth.

  • By recognizing stellar spectral features and inferring a star’s intrinsic properties, astronomers can estimate the amount of obscuring dust along the line of sight.
  • Note that this reddening has nothing to do with the Doppler effect—the frequencies of the lines are unchanged, although their intensities are reduced.

(b) This dusty interstellar cloud, called Barnard 68, is opaque to visible light, except near its edges, where some light from background stars can be seen.

  • Because blue light is more easily scattered or absorbed by dust than is red light, stars seen through the cloud appear red.

(c) It illustrates how infrared radiation can penetrate Barnard 68, although it too is preferentially stripped of its shorter wavelengths.

  • DUST is transparent to long wavelength (radio, IR)
  • DIST is opaque to short wavelength (optical, UV, XR)

Extinction: Dimming of starlight by ISM (apparent brightness diminished). So blue component is rubbed out and they appear redder.

Reddening: Stars tend to appear redder than they really are.


  • ISM is cold: at 273 K water freezes and at 0 K all molecular motion stops
  • Temperature of ISM ranges from at few K - to - a few hundred K; on average ~ 100 K


  • Density is extremely low (Lab vacuum: 1010 molecules / m3 )
    • GAS: 106 atoms / m3 ➤ 1 atom / cm3
    • DUST: 10-6 particles / m3 ➤ 103 particles / km3


  • How a sparse ISM can effectively diminish light: Size! (space is vast)
  • SPACE IS DIRTY because of DUST
    • Earth's atmosphere is about million times cleaner
    • [mass] = [density:] 10 H atoms / cm3 x [volume:] 1 pc3 = 1 M
  • Across the Galaxy (15 000 x 300 pc) the same density corresponds to
    • [mass] = 20 x 109 M


  • GAS: Analysis of specific absorption profiles
    • 90% atomic or molecular H
    • 9% He
    • 1% heavier elements [C, O, Si, Mg, Fe] - lower than solar system
  • DUST: It is not very well known. Analysis of IR observation
    • Silicates, Graphite, Fe
    • Dirty Ice : water + ammonia, methane (frozen cometary nuclei)


  • Atoms in GAS are spherical
  • DUST particles are elongated

(a) A diagram of a typical interstellar dust particle, as inferred from polarization studies.

  • The average size is only one ten-thousandth of a millimeter

b) The results of a computer simulation of how grains may grow as dust particles collide, stick, and fragment in interstellar space.

  • The resulting grains are linear, or rodlike, on small scales, but may become tangled and twisted in complex ways on larger scales.

2. Emission Nebula

A wide-angle photograph in the direction of the center of our Galaxy.

  • It shows regions of brightness (vast fields of stars)
  • It also shows regions of darkness (where ISM obscures the light from more distant stars).

Nebula is a general term used for fuzzy objects in the sky:

  • Dark nebula: Dust cloud
    • The dust lanes visible are part of the nebula and are not due to intervening clouds
  • Emission nebula: Glows, due to hot stars
    • They generally glow red — this is the Hα line of hydrogen.

Nebular Structure

When UV radiation from one or more hot stars ionizes part of an interstellar cloud Emission Nebula is observed.

  • The nebula’s reddish color is produced as electrons and protons recombine to form hydrogen atoms.
  • Dust lanes may be seen if part of the parent cloud happens to obscure the emitting region.
  • If some starlight happens to encounter another dusty cloud, some of the radiation, particularly at the shorter wavelength blue end of the spectrum, may be scattered back toward Earth, forming a reflection nebula.

The Galactic Plane. The picture show stars, gas, and dust, as well as several distinct fuzzy patches of light known as emission nebulae.

The plane of the Milky Way is marked with a white diagonal line.

M20–M8 Region A true-color enlargement of the bottom of Figure 18.6, showing M20 (top) and M8 (bottom) more clearly. The two nebulae are only a few degrees apart on the sky.

Trifid Nebula (a) The picture shows only M20 and its interstellar environment.

  • The nebula itself (in red) is about 6 pc in diameter.
  • It is often called the Trifid Nebula because of the dust lanes (in black) that trisect its midsection.
  • The blue reflection nebula is unrelated to the red emission nebula; it is caused by starlight reflected from intervening dust particles.

(b) A false-color infrared image reveals bright regions of star-forming activity mostly in those lanes of dust.

(a) M16, the Eagle Nebula.

(b) A Hubble image of huge pillars of cold gas and dust inside M16.

  • It shows delicate sculptures created by the action of stellar ultraviolet radiation on the original cloud.

(c) M8, the Lagoon Nebula.

(d) A high-resolution view of the core of M8, a region known as the Hourglass.

  • Notice the irregular shape of the emitting regions:
    • the characteristic red color of the light in the left frames
    • the bright stars within the gas, and the patches of obscuring dust.
  • The insets at right are not shown in true color, rather the various colors accentuate observations at different wavelengths:
    • Green represents emission from hydrogen atoms.
    • Red emission from singly ionized sulfur.
    • Blue emission from doubly ionized oxygen.

Emission Nebula Spectrum

Emission nebulae are made of hot, thin gas, which exhibits distinct emission lines. It consist of hydrogen, helium, and trace components.

(a) Nebula NGC 2346 is a glowing patch of gas about 0.2 pc across and residing some 700 pc away from our solar system.

(b) The emission spectrum of NGC 2346, showing light intensity over the entire visible portion of the EM spectrum, from red to deep violet.

  • Some of the spectral lines, such as those of nitrogen, are from contaminants in Earth’s atmosphere.

3. Dark Dust Clouds

Obscuration and Emission

(a) At optical wavelengths, this dark dust cloud (known as L977) can be seen only by its obscuration of background stars.

(b) At radio wavelengths, it emits strongly in the CO molecular line, with the most intense radiation coming from the densest part of the cloud

The Ophiuchus dark dust cloud

It resides only 170 pc away, surrounded by colorful stars and nebulae that are actually small illuminated parts of a much bigger, and invisible, molecular cloud engulfing much of the region shown.

  • The dark cloud itself is “visible” only because it blocks light coming from stars behind it.
  • Notice the cloud’s irregular shape, and especially its long “streamers” at upper left.
  • The bright, giant star Antares, the star cluster M4, and a nearby blue reflection nebula are also noted.

Horsehead Nebula

(a) Located in the constellation Orion, not far from the Orion Nebula, this Horsehead Nebula is a striking example of a dark dust cloud, silhouetted against the bright background of an emission nebula.

(b) The “neck” of the horse is about 0.25 pc across. This nebular region is roughly 1500 pc from Earth.

Absorption by Interstellar Clouds

(a) Optical observations might show an absorption spectrum like that traced in (b).

  • The wide, intense lines are formed in the star’s hot atmosphere; narrower, weaker lines arise from the cold interstellar clouds.
  • The smaller the cloud, the weaker are the lines.
  • The redshifts or blueshifts of the narrow absorption lines provide information on cloud velocities.

4. Hydrogen 21 cm Radiation

A ground-state hydrogen atom changing ...

  • from a higher-energy state (electron and proton spinning in the same direction)
  • to a lower-energy state (spinning in opposite directions).

The emitted photon carries away an energy equal to the energy difference between the two spin states and it has a wavelength of 21 centimeters (corresponding to 1420 MHz) which is in the radio portion of the electromagnetic spectrum.

Typical 21-cm radio spectral lines observed from several different regions of interstellar space.