What are some other ways that a gas might
cool off? Well - you now know that "cooling off" really
means "molecules slowing down" or "losing
energy." The energy of motion is just one type of energy -
and you saw that when molecules bump into slower molecules they lose
some of their kinetic energy.
Light is another form of energy - does this give you any ideas about how the gas in the ISM might cool down?
You may already know that each element in the periodic table can emit light at certain frequencies. These frequencies form what is know as emission spectra.
Hydrogen Emission Spectra
But this is what you would see in a fairly cool cloud of hydrogen gas. What happens when the gas is very hot? For example, the interstellar gas in the Local Bubble is about one million degrees!
Again, think of the gas as being made up of tiny balls. One way to think about emission lines is to think about each molecule of gas as having a tiny lights in the colors of the emission lines. For example, we can think of each hydrogen molecule as having one red and one blue light. (There are actually more lines in hydrogen, but we will just focus on these bright lines for now.)
In a hot gas, the molecules are moving around very fast. In a large cloud of hydrogen, some of the molecules will be moving towards you, and some will be moving away from you. What effect does this have on the light waves from these molecules? (HINT: If you don't know about the Doppler Effect - click here (outside site).)