Now for something more interesting. A while back I wrote twelve pieces on the surprising ways helium has impacted our view of the universe. I thought I was finished, but recently, as I was researching a possible article on an astronomer named Henrietta Leavitt, I stumbled upon helium tale number thirteen.
Let’s start with something completely unrelated to helium, but which, I promise, will get us back to this amazing idea.
Imagine a pot of water boiling on a stove. Were you to insert a thermometer into that pot of water, you would discover something odd. The temperature of the water is (more or less) 100 degrees C (or 212 degrees F, if you prefer such things). Even though the stove is pumping lots of heat into this boiling water, the temperature does not change. It stays right at the boiling point of water until all the water is converted to vapor.
Now consider just the opposite. I’ll use the liquid-to-solid transition because it’s easier to deal with, but the vapor-to-liquid transition works, too. Imagine a pot of water in a freezer. Again, there’s a thermometer in the water. The temperature drops, drops, and drops until the thermometer reads 0 degrees C (32 degrees F). And then the thermometer stays there until every bit of the water is converted to ice. Even though the freezer is pulling heat away from the water, the temperature of the water does not change.
OK, familiar enough. This phenomenon (known as latent heat) is what drives thunderstorms and hurricanes, protects water-covered fruit during rare Floridian freezes, and makes steam burns so dangerous. It’s caused, by the way, by hydrogen bonds, amazing and important in the own right for many, many reasons. Maybe a future blog subject.
The important point for now is that the phase change provides a sort of reservoir for energy at a particular temperature. If you look at a graph of temperature change versus heat added for water, you get these plateaus where the phase change is occurring.
Notice that the water-vapor phase change plateau is enormous! It takes much, much more heat to boil a gallon of hot water into steam than it does to heat that same gallon from the freezing point to the boiling point. Hydrogen bonds are tough little boogers, and it’s hard work to get water molecules apart.
OK, enough with water. What about stars?
Stars are balancing acts, kinda like this:
Gravity is forever trying to squash the star down. Pressure, provided by the nuclear reactions in the star’s core, is trying to blow it apart. The balance between these two forces is the star we see. What if, for whatever reason, the star were a little too squashed? Were that ever to happen, increased pressure would drive the temperature up. Then the star would swell (and cool) until it was the right size (and right temperature) again. What if the star were too swollen? In that case, reduced pressure would cause the temperature to drop, and the star would contract back to where it belonged, all the while heating back up to its appropriate temperature. In fact, all stars go through these cycles a little bit all the time. Because most never go much beyond their equilibrium point, we never notice.
However, some stars, called Cepheid variables, get caught in large, days-long cycles that keep oscillating for many years. What causes this oscillation? Here it comes: helium!
Henrietta Leavitt was an astronomer in the early 1900s. She became an expert in recognizing and measuring Cepheid variables. Noticing that the Cepheids in the small magellanic cloud had a regular period-to-brightness relationship, Leavitt was able to build a crucial early piece of what would become our cosmic yardstick. Her story is both triumphant and tragic, and I’ll save it for my article if I ever write it. For now I’ll tell just the helium part of the story.
It turns out that the reason Cepheid variables are, well, variable, is that helium near the surface of these stars is undergoing a phase change. No, not from liquid to gas. Stars are much too hot for anything like liquid helium to exist. Instead, the phase change is from one type of ionized helium to another.
OK, so what is ionized helium?
Helium may be number one in our hearts, but on the periodic table it’s always number two. Every helium atom has two protons in its nucleus – otherwise it isn’t helium. Normally, helium also has two electrons buzzing about outside the nucleus. Normally.
But stars aren’t normal. As Laura Dern once said, they’re “hotter’n Georgia asphalt.”
Inside a hot star (stop it, this is a family blog!), there’s lots of energy. Deep in the interior, no atoms have electrons. As you move out toward the edge of the star, however, atoms start to recapture some or all of their lost electrons.
A Cepheid variable is a star nearing the end of its short life. For most of its life cycle, the star is no more variable than our own Sun is. But Cepheids are bigger than the Sun, and so they burn through their hydrogen fuel much more quickly than the Sun does, in just millions instead of billions of years. At some point, the mixture of chemicals in the star is just right to cause the star to start blinking brighter and dimmer. By the time such a star starts to blink, it has burned its hydrogen fuel and has started in on its helium. In the core, helium fuses into carbon, oxygen, and heavier elements. Away from the core, helium isn’t fusing, but it is hot enough to have lost one, but not both, of its electrons. We say that the helium is singly ionized. This ionized helium near the star’s surface is the key to its blinking behavior. How? Read on!
The funny thing about ions is that they interact strongly with light. This makes sense if you think about it; light is nothing more (and nothing less!) than waves of electromagnetism. Ions have an electric charge. When electromagnetic waves reach ions, they interact. This means the ions are somewhat opaque – they absorb the energy of the light and heat up.
When singly ionized helium ions heat up, they tend to lose their one remaining electron. This makes them – you guessed it – doubly ionized. Doubly ionized helium is even more opaque than singly ionized helium, so heating the helium results in the outer layer absorbing even more energy.
Now comes the key point. Think back to the water on the stove. Remember how it stayed at the boiling point as it boiled, not getting any hotter until all the liquid was changed to gas. A very similar thing happens to helium in a Cepheid variable star. Instead of driving the temperature up, much of that extra absorbed light drives the helium from its singly ionized state to its doubly ionized state. The temperature doesn’t go up (much) until essentially all the singly ionized helium has changed into doubly ionized helium. The star has found a helium plateau! In effect, the star is storing energy in this altered helium. So what happens next?
Once the helium is doubly ionized, there’s nothing to stop the star’s temperature from going up. The increasing temperature causes the star to swell, and this swelling causes the temperature to drop. But wait! Now there’s all this doubly ionized helium. As the temperature drops, we hit the helium plateau again, this time moving the other direction. The doubly ionized helium gains an electron and becomes singly ionized, releasing energy in the process and stopping the temperature drop, in the same way that steam turning to water or water turning to ice prevents temperature drops.
Why helium? With electrons in the very bottom shell, completely unshielded by the positive charge of the nucleus, and with two units of positive charge instead of just the single unit of hydrogen, helium is (like water’s hydrogen bonds, only on a much higher-energy scale) a tough booger. It takes a lot of energy to ionize helium, and this makes it perfect for a phase-change plateau inside stars. Once again, the deceptive simplicity of the helium atom makes the world a much more interesting place.
All this heating, cooling, ionizing, and deionizing causes the Cepheid to, again and again, overshoot its equilibrium. Each heating and swelling causes a cooling and collapse, and each collapse causes the next round of heating and swelling. That helium plateau stuck in the middle keeps the star from ever settling down (that is, until in its evolution it leaves the Cepheid variable stage). The star gets brighter, then dimmer, then brighter, then dimmer again, in a very regular and predictable pattern. And it gives us exactly the tool we need to discover just where we are in this vast and surprising universe. Once again, helium is the key.