Density may seem like a simple idea - maybe you already have an idea of what this term means. For gasses, we define density as the average number of gas particles in a certain amount of space, or volume. For example, the air we breathe has about 30 quintillion (that's a 3 followed by 19 zeros!!) molecules in a cubic centimeter (a cube that has sides 1 centimeter long.)
The cube drawn above is about one cubic centimeter - hard to believe a cube this big contains 30,000,000,000,000,000,000 molecules. This number is so huge, it's almost impossible to imagine. Get this, if you wanted to fill two football stadiums with sand, it would take about 30 quintillion grains of sand. This gives you an idea of how very tiny molecules really are. It also might make you think twice about saying something is empty!
Think about this: if there were 1000 of these cubes out in the Local Bubble, only 1 would contain a molecule! That's how diffuse the ISM is in the Local Bubble compared to the air in our atmosphere. If the same cube were out in space, somewhere outside the Local Bubble, it would contain 500 molecules! So the ISM in the Local Bubble is unusual stuff indeed - which is part of the reason the CHIPS mission is studying it.
This is just the beginning, though. Even though you now have a definition for the density of a gas - there is a lot more to understand. Here are some questions for you to think about and discuss with your class. By the time you finish this lesson, you might have a different way of thinking about these things and your answers to these questions might change.
Is the density of a gas constant, or can you change it? If it is not constant, how might you change it? How about the density of a liquid, or a solid - is it constant?
You might have heard the expression "Hot air rises?" Is this true? Why? Does this have anything to do with density?
The air on top of a mountain feels much cooler than the air at the bottom. Why is this? What about that saying, "Hot air rises?" - does this mean that the air on top of mountain should be warmer?
We'll come back to these questions later - but for now, make a note of what you think. Later, when we've explored a few more things, we'll ask these questions again and you can see if your thinking has changed.
The CHIPS mission is studying the ISM - which is a very, VERY low density gas (with a little dust mixed in) in the vast spaces between the stars. Understanding density is the first step towards solving the puzzle that CHIPS is studying.
Before going on to the next section, try this activity.