Interstellar Medium (ISM)

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If you are sitting in an ordinary chair, lean over and look at the ground directly below your head. A cylinder the diameter of your eye drawn from your eye to the ground would enclose about as many particles of air as there are interstellar particles in a cylinder of the same diameter but extending between our solar system and the center of our galaxy 27,000 light years away. Though the space between the stars is emptier than the best vacuums created on the Earth (those are enclosed spaces devoid of matter, not the household cleaning appliances), there is some material between the stars composed of gas and dust. This material is called the interstellar medium. The interstellar medium makes up between 10 to 15% of the visible mass of the Milky Way. About 99% of the material is gas and the rest is "dust". Although the dust makes up only about 1% of the interstellar medium, it has a much greater effect on the starlight in the visible band---we can out see only roughly 6000 light years in the plane of the Galaxy because of the dust. Without the dust, we would be able to see through the entire 100,000 light year disk of the Galaxy. Observations of other galaxies are done by looking up or down out of the plane of the Galaxy. Dust provides a place for molecules to form. Finally, probably the most of important of all is that stars and planets form from dust-filled clouds. Therefore, let us look at the dust first and then go on to gas. The structure of the Galaxy is mapped from measurements of the gas.

Look out of the planet of the Galaxy to see beyond the Milky Way


The dust is made of thin, highly flattened flakes or needles of graphite (carbon) and silicates (rock-like minerals) coated with water ice. Each dust flake is roughly the size of the wavelength of blue light or smaller. The dust is probably formed in the cool outer layers of red giant stars and dispersed in the red giant winds and planetary nebulae.

a typical tiny piece of dust


Starlight passing through a dust cloud can be affected in a couple of ways. The light can be totally blocked if the dust is thick enough or it can be partially scattered by an amount that depends on the color of the light and the thickness of the dust cloud. All wavelengths of light passing through a dust cloud will be dimmed somewhat. This effect is called extinction.

Bok globules in IC 2948
Dark dusk clouds like these "Bok globules" in IC 2948 were once thought to be holes in the sky. The dark clouds block the light from the emission nebula behind.
Barnard 86 silhouetted against the stars
A dark cloud, Barnard 86, is silhouetted against a starry background. Stars form in the dark clouds. A young cluster, NGC 6520, probably associated with Barnard 86 is seen just to the left of it.

The discovery of the dust is relatively recent. In 1930 R.J. Trumpler (lived 1886--1956) plotted the angular diameter of star clusters vs. the distance to the clusters. He derived the distances from inverse square law of brightness: clusters farther away should appear dimmer. IF clusters all have roughly the same linear diameter L, then the angular diameter q should equal a (constant L) / distance. But he found a systematic increase of the linear size of the clusters with distance.

This seemed unreasonable! It would mean that nature had put the Sun at a special place where the size of the clusters was the smallest. A more reasonable explanation uses the Copernican principle: the Sun is in a typical spot in the Galaxy. It is simply that more distant clusters have more stuff between us and cluster so that they appear fainter (farther away) than they really are. Trumpler had shown that there is dust material between the stars! The extinction of starlight is caused by the scattering of the light out of the line of sight, so less light reaches us.

extinction and reddening of starlight


star with and without dust in frontNot all wavelengths are scattered equally. Just as our air scatters the bluer colors in sunlight more efficiently than the redder colors, the amount of extinction by the interstellar dust depends on the wavelength. The amount of extinction is proportional to 1/(wavelength of the light). Bluer wavelengths are scattered more than redder wavelengths.

The 1/l behavior of the scattering indicates that the dust size must be about the wavelength of light (on the order of 10-5 centimeters). Less blue light reaches us, so the object appears redder than it should. This effect is called reddening, though perhaps it should be called "de-blueing". If the dust particles were much larger (say, the size of grains of sand), reddening would not be observed. If the dust particles were much smaller (say, the size of molecules), the scattering would behave as 1/l4. Trumpler showed that a given spectral type of star becomes increasingly redder with distance. This discovery was further evidence for dust material between the stars. If the Sun is in a typical spot in the Galaxy, then Trumpler's observation means that more distant stars have more dust between us and them.

You see the same effect when you observe the orange-red Sun close to the horizon. Objects close to the horizon are seen through more atmosphere than when they are close to zenith. At sunset the blues, greens, and some yellow are scattered out of your line of sight to the Sun and only the long waves of the orange and red light are able to move around the air and dust particles to reach your eyes.

preferential scattering of short wavelengths

At near-infrared (slightly longer than visible light) the dust is transparent. At longer wavelengths you can see the dust itself glowing and you can probe the structure of the dust clouds themselves as well as the stars forming in them (young stars that are hidden from us in the visible band). The Spitzer Space Telescope observes in the infrared and has opened up a new universe to us. Although it is not the first infrared space telescope, it is the largest infrared space telescope ever launched so it has the greatest light-gathering power and resolution of any infrared telescope. A couple of examples of the power of infrared I use in my class are the movie of the dark globulae IC 1396 and the movie of Herbig-Haro 46/47 on the Spitzer images site that transitions from the visible light image to the near infrared to the mid-infrared. Other movies contrasting the views at visible with infrared of other places in the galaxy as well as other galaxies are available there too.


dust extinction interstellar medium

Review Questions

  1. What is the interstellar medium composed of?
  2. How do astronomers know that the non-gaseous part of the ISM cannot be made of rocks but, rather, of small "dust" particles?
  3. How does dust make stars appear redder than they actually are?
  4. How does dust cause the extinction of starlight?
  5. Where is the dust thought to come from?

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last updated: June 8, 2010

Is this page a copy of Strobel's Astronomy Notes?

Author of original content: Nick Strobel