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.

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.

(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. 

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

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.






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

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

2. Emission Nebula

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

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

Nebular Structure

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

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.

(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.

(c) M8, the Lagoon Nebula.

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

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.

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.

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).

4. Hydrogen 21 cm Radiation

A ground-state hydrogen atom changing ...

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.