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 ionized gas with a density of approximately 10-4 molecules 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:
(That density, by the way, is equivalent to having a few dust motes floating around in Ohio Stadium.)
There are two types of cloud present in the interstellar medium:
1. Molecular clouds (cold!)
2. HI clouds (cool)
There are two types of intercloud gas:
1. Intercloud medium (warm)
2. Coronal gas (hot!)
(Caveat: the above temperatures and densities are approximate, to give you a feel for what the interstellar medium is like. By terrestrial standards, it is all very, very low in density.)
In a previous lecture we learned that hot, low-density gas produces an emission spectrum.
An emission nebula consists of a region of hot, low-density gas surrounding a hot star, which heats and excites the gas. The gas in an emission nebula produces an emission spectrum.
Let me also remind you that a hot dense object (such as a grain of dust, for example) produces a black body spectrum. Dust in the interstellar medium is at a temperature T = 100 Kelvin, so it produces radiation with Lambdamax = 30,000 nanometers (in the infrared). Pictures taken in the infrared (using an orbiting telescope) reveal that our galaxy is full of infrared cirrus, wispy clouds laden with dust.
Let me further remind you that cool, low-density gas produces an absorption spectrum. Clouds of interstellar gas produce absorption lines in the spectra of distant stars. These interstellar absorption lines are narrow, which implies the clouds are cold.
(Heating a gas produces more random motions of its atoms; greater random motion along the line of sight produces a larger range of Doppler shifts; a larger range of Doppler shifts produces a broader absorption line).
A reflection nebula consists of a region of dusty gas surrounding a star. The dust reflects starlight, and is thus visible from here on earth.
Individual dust grains are more effective at scattering blue light than at scattering red light. Some consequences: