Astronomy 162: Professor Barbara Ryden

# INTERSTELLAR MEDIUM

``They cannot scare me with their empty spaces
Between stars - on stars where no human race is.
I have it in me so much nearer home
To scare myself with my own desert places.''
- Robert Frost

## Key Concepts

• The interstellar medium consists of low-density gas and dust.
• Interstellar gas consists of cool clouds embedded in hot intercloud gas.
• The interstellar medium emits, absorbs, and reflects radiation.

#### (1) The interstellar medium (that's just a fancy name for the matter between stars) consists of low-density gas and dust.

The fact that we can look out far into the galaxy means that there must be very little matter between stars -- otherwise it would absorb all the starlight. Nevertheless, interstellar space is not entirely empty. 99% of the interstellar medium consists of very low density gas. How low in density is it? Let me tell you.

The air we breathe has a density of approximately 1019 molecules per cubic centimeter. (One cubic centimeter = 1 milliliter = 1/1000 liter).

By contrast, the lowest density regions of interstellar space contains approximately 0.1 atoms per cubic centimeter.

The remaining 1% of the interstellar medium consists of dust. That's right, dust -- like the stuff that accumulates on your bookshelves and under your bed.

The properties of interstellar dust:

• Composition: carbon, metals, silicates, and ice
• Size of grains: 500 nanometers or less in diameter (1 nanometer = 1 billionth of a meter)
• Number density of grains: 1 per million cubic meters
(That density, by the way, is equivalent to having a few dust motes floating around in Ohio Stadium.)

#### (2) Interstellar gas consists of cool clouds embedded in hot intercloud gas,

Half the interstellar gas is compressed into only 2 percent of our galaxy's volume. These relatively high density regions are called clouds or nebulae (``nebula'' is simply the Latin word for ``cloud'').

The densest nebulae can have densities of 10,000 molecules per cubic centimeter (or sometimes even more). The coolest nebulae can have temperatures of T = 10 Kelvin (or even less). A temperature of 10 Kelvin is colder than midnight on Pluto.

The other half of the interstellar gas is spread over the remaining 98 percent of the galaxy's volume. The lowest density gas has a density of 0.1 atoms per cubic centimeter (or less). The hottest interstellar gas has a temperature of 8000 Kelvin (or more). (The Solar System, by the way, seems to be located within a large, low-density bubble within the interstellar medium.)

#### (3) The interstellar medium emits, absorbs, and reflects radiation.

How do we know that the interstellar medium is there at all, given that it's so low in density, and so nearly transparent?

Sometimes we know the interstellar medium is there because it emits light. An emission nebula is a hot, ionized cloud, surrounding a hot, luminous star (of spectral type `O' or `B', thus possessing a surface temperature of tens of thousands of degrees). The gas in the emission nebula is heated by ultraviolet light from the star, and thus, like all hot, low-density gas, produces an emission line spectrum. [Example: the Orion Nebula, 450 parsecs away from us, in the constellation Orion, is an emission nebula.]

Sometimes we know the interstellar medium is there because it absorbs light. A dark nebula is a cold, dense cloud, containing a high concentration of dust. A dark nebula is dusty enough to be opaque at visible wavelengths. Thus, a nearby dark nebula blocks our view of more distant stars, making it look as if there were a ``hole in the heavens'' - a dark spot with no stars. The dust in a dark nebula, heated by starlight, re-radiates the light at infrared wavelengths. Thus, a `dark nebula', though dark at visible wavelengths, is luminous at infrared wavelengths. [Example: Barnard 86, 1700 parsecs away from us, in the constellation Sagittarius, is a dark nebula.]

Sometimes we know the interstellar medium is there because it scatters light. A reflection nebula is a dusty cloud surrounding a star. The dusty cloud is visible because the dust reflects starlight. The scattered starlight is always very blue, even if the star itself is red. Why is this? The individual dust grains, which are comparable in size to the wavelengths of visible light, are more efficient at scattering blue light than red light. A reflection nebula is blue because we are seeing scattered starlight. Stars seen through a dusty cloud are red because we are seeing the light left over after all the blue light has scattered away. [Example: the Pleiades, 117 parsecs away from us, in the constellation Taurus, are in the midst of a reflection nebula.]