Great Balls of Fire

After learning about gravity and taking the midmorning break,  the Peregrine 3-4 graders and I worked on understanding nuclear fusion in the core of the Sun and where elements come from.

I started by setting the context.  The students had studied atoms and molecules the previous year so I started by drawing a molecule of water (two hydrogen atoms and one oxygen atom) and reminding them of the evidence for atoms and molecules.  Then we zoomed in to one hydrogen atom and discussed the Rutherford experiment showing that atoms are very fluffy; most of their volume is nearly empty while nearly all their mass is concentrated in a tiny volume in the center (nucleus).  Then we zoomed in further by a factor of 10,000 to the nucleus.  For a hydrogen atom, the nucleus is a single positively charged particle called a proton.  I held up a ping-pong ball as a proton and said that if protons really were that size, the atom would have to be the size of South Davis.

To reinforce the sense of scale, I showed the movie Powers of Ten.  This classic ten-minute movie should be seen by anyone wanting to understand the universe.  I also took the time to answer questions about it.

The basic rules of nuclear physics are actually understandable by anyone. Last year we investigated the effects of electrical charge, and concluded that like charges repel while opposite charges attract. Atoms beyond hydrogen in the periodic table have more protons.  But why do the protons stick together if they repel each other? There must be some form of glue.  I demonstrated two magnets  which repelled each other.  They were "donut" magnets threaded onto a rod so they didn't flop around and the repulsion was clear.  But when I turned the rod vertically and one magnet fell with enough speed onto the other one, they touched briefly.  That was enough for the velcro on their surfaces to attach and keep them together.  The velcro is a short-range force, like the strong nuclear force which keeps a nucleus together.

But protons alone can't generate sufficient strong nuclear force to keep nuclei together.  Another type of particle, with similar mass but no charge and called a neutron, provides the glue.  Nuclei need roughly equal amounts of protons and neutrons to be stable.  I modeled this with a bunch of ping-pong balls I had wrapped with velcro.  The "protons" had velcro hooks and the "neutrons" had velcro loops, so that you needed roughly equal numbers of each to build up a large nucleus.  (The different types were also different colors to make the idea plainly visible.)  Adding a neutron to a nucleus adds mass, but doesn't otherwise change the properties of the atom.  For example, a proton plus a neutron is still hydrogen, but we call it a different isotope of hydrogen.  Similarly, carbon-12 (usually written with a superscript 12 on the left) and carbon-14 are different isotopes of carbon which differ by two neutrons.

With that in mind, we can start building up more complicated elements from hydrogen.  Element number 2 (two protons) is helium, and we need two neutrons to provide the glue so the most common isotope of helium is helium-4.   The protons have to be smashed together at very high speed if they are to ever get close enough for the "velcro" of the strong nuclear force to make them stick, so we need very high temperatures to make this fusion process happen. (High temperature means the individual microscopic particles are wiggling or bounding around at high speed.)  We find it difficult to make these high temperatures on Earth, but the core of the Sun is 15 million degrees (Celsius; tens of millions of degrees if you think in Fahrenheit) and this happens quite routinely.  In fact, most stars turn hydrogen into helium in their cores.

Fusing helium into even heavier elements is harder, but most stars will do that as well by the ends of their lives.   It turns out that crashing two heliums together results in an unstable isotope of element 4 (beryllium), which quickly decays back into two helium-4 nuclei. But if you manage to crash a third helium into the two heliums before the two-helium complex has a chance to decay, you make carbon-12 (the most common isotope of element 6, carbon; again, equal amounts of protons and neutrons).  Then, if you crash another helium into that, you get element number 8: oxygen. Another helium into that produces element 10, argon.  These helium capture reactions are common in massive stars (substantially more massive than the Sun), and they create more of the even-numbered elements than the odd-numbered elements (nitrogen, fluorine, etc).  They go all the way up to iron (element 26).  I modeled all this with the velcro-covered ping-pong balls.

Have you noticed what we've done here?  We've explained the origin of the elements using basic, well-understood physical processes. That's pretty cool! Here's a graph of the observed abundances:

You can see that hydrogen is the most abundant, followed by helium, then the even-numbered elements carbon, oxygen....through iron.  But why are there elements beyond iron if stars only make up to iron? Well, stars make up to iron when they are in equilibrium.  But when they explode (a supernova), so much energy is released that even more complicated nuclei can be made.  I won't explain the details here, but the abundances of all the elements beyond iron are well understood as consequences of supernovae.  That we can understand all the features of the above plot is, to me, one of the most amazing things in all of science.

The supernova explosions are also what throw the newly-manufactured elements back into space, where they can mix into gas clouds that eventually collapse to form new stars. That means that the atoms in your body were once inside another star.  (Not from the Sun, because new atoms made in the Sun won't escape until the end of its life.)

Supernovae make some unstable elements, like uranium.  The most common type of decay for a heavy element is to violently eject a "bullet"  made of two protons and two neutrons, in other words a helium nucleus (again I modeled this with the ping-pong balls).  This is why there is helium on Earth; our gravity is too weak to hold on to helium gas, but helium produced by radioactive decays is trapped in rocks underground.  When we drill for natural gas, we can capture some of this helium and eventually use it to fill balloons.  When it escapes from the balloon, it eventually escapes into space.

Big Bang 

I left out one detail in the story above: most of the helium in the plot was actually made in the Big Bang.  Some of the kids had expressed interest in the Big Bang previously, so I used the remaining time to talk about that.  I used the usual balloon-with-stickers demo, and I also showed this interactive tool made by an undergraduate student of mine. The point of the tool is to show that although we see all galaxies moving away from us, observers in all other galaxies also see all galaxies moving away from them. So we are not at the center of anything. If we think back in time, all galaxies were closer to each other, so the universe was denser (and hotter).  Far enough back in time, the universe was so hot (everywhere) that a fair amount of hydrogen fused into helium.  This is called Big Bang nucleosynthesis (BBN). We can look at the abundance of various  BBN byproducts, like hydrogen-2 (aka deuterium) and confirm that this really happened.

Wrapping up

Most of this trimester we worked on understanding the immense size of space.  If this makes you feel insignificant, remember that you are made of atoms from another star.  You are a part of the universe which can actually understand itself. 

The Gravity of the Situation

Friday was my last day doing astronomy with the 3-4 graders at Peregrine School. The one standard I hadn't yet covered was gravity, so we did gravity before the break (this post) and after the break we discussed nuclear fusion in the Sun's core (next post).

I reviewed some ideas about motion we had discussed last year.  If you roll a marble, you expect it to go in a straight line unless something (another kid, perhaps, or a wall) interferes by pushing (exerting a force) on the marble. That's Newton's first law of motion. I then put a donut on a string and spun the donut in a circle over my head.   What will happen if the string is cut? Will the donut continue in a circle, fly off in a straight line, or fly off in a curve? We took a vote. I always clarify that the question is about what happens immediately, not about what happens eventually, like the donut falling due to the gravity in the room. This means that when we do the experiment, they have to really pay attention!

In reality I don't cut the string, but the string pulls through the soft donut, and it flies off in a straight line---Newton's first law again.  This is a pretty vivid demonstration that the Moon wouldn't keep going  around the Earth, nor the planets around the Sun, unless there was a force keeping them from flying off in a straight line.  Kids this age already know that we call that force gravity, but gravity is also the force that makes things fall when I drop them.  Why do we call these two forces by the same name?

I also have a tennis ball on a string so I can demonstrate circular motion as much as needed.  I do this and ask the kids what direction the force must be in.  It must be towards the center of the circle, where my fist is holding the string.  That's clear because the only direction a string can exert a force is pulling along the string! So whatever force is pulling on the Moon, it must be pointed toward the center of the Earth.  And that's exactly what we observe about gravity on Earth! (It helps to draw an Earth and how the arrow of gravity points in your location vs in, say, Australia.)  So it's quite plausible that these two forces are really the same force.

To bolster the argument that these are the same force, we should look not just at the direction, but also the strength.  I had the kids whirl the tennis ball on a string at various speeds, and feel whether the higher speed requires more force, less force, or the same force (the answer is more).  So let's look at the planets' speeds around the Sun and see if we can relate that to the force of gravity.  I asked the kids for suggestions as to what would affect the planet speed.  The two main suggestions were planet size, and planet distance from the Sun.  It would have been great to investigate both of these possibilities, but we were running short on time so we just did planet distance from the Sun.  I had the kids make graphs of planet speed vs planet distance from the Sun.  We took our time doing this right, figuring out how to draw the axes with reasonable scales, and adding planets one by one, starting with the most familiar ones.

A pattern did emerge: more distant planets are slower, as the graph below shows.

By our tennis ball experiment, slower circular motion implies a weaker pull (less acceleration). Therefore this graph implies that more distant planets feel a weaker pull, and planets closer to the Sun feel a stronger pull.  Does this make sense if the Sun's gravity is what keeps the planets from flying off in straight-line paths?  The kids agreed that it did.

[If we had also made the graph of speed vs planet size, we would not have seen such a clear pattern.  It happens that the outer planets tend to be bigger, so that there would be a tendency for bigger planets to be slower, but it would only be a tendency, not a law, because the biggest planet happens to be the nearest (fastest) of the outer four.  And the pattern would really be broken if we also included Pluto, which is a very distant (hence very slow), small object, providing a counterexample to the fast inner planets which happen to be small and which therefore might give someone the false impression that small means fast.]

I liked this 40-minute activity and I think it worked well.  I did simplify some details to avoid getting bogged down (eg the distinction between force and acceleration), but I think it was appropriate for 3-4 graders who wanted to focus on astronomy rather than physics. We also got in some more practice with graphs, which is important.  And we learned something which in Newton's time was revolutionary: the same laws of physics which we can deduce here on Earth also apply to objects in the sky.   This was one of the most wonderful discoveries in the history of science, and it's what allows us to understand the universe.