ASP: The topsy-turveying of planets, stars, and lava lamps

The topsy-turveying of planets, stars, and lava lamps

by George Musser, Astronomical Society of the Pacific

Figure 1
Convection in a pot. The flames under the pot heat the water on the bottom. As the water gets hotter, it expands and rises. The colder water above it sinks down to the bottom, where it too starts to get hot. The process continues until you shut the stove off. The convection cycle distributes the heat of the stove evenly throughout the water in the pot.

A primer on convection

Stars do it, icy moons do it, even pots of coffee do it. From the small to the large, things undergo convection. Convection -- the movement of heat by the movement of matter -- is a recurring theme in science, and a vivid reminder that astronomical objects aren't really so different from everyday objects. They may be bigger and hotter and farther away, but the same laws of physics determine how they behave.

Convection is a consequence of the most basic drive in nature: Hot things want to cool down. If the thing is a fluid -- air, water, anything that can flow -- it can cool down by moving around. Hot air rises, cold air sinks: It's common sense. It's convection. And from this basic idea can come some funky phenomena.

The next time you boil water, fill a jar with hot water and notice how much lighter it is than a jar of cold water. (If you don't trust yourself, put the jars on a balance.) This difference in weight keeps convection going. If you put water in a pot on the stove, the water on the bottom gets hot first. The colder fluid above it is heavier, so it sinks. This forces the hot fluid on the bottom to go up on top. The stove heats up the cold water on the bottom; the air cools down the hot water on the top. So they switch places again. And so on. It sets up a cycle, as shown in Figure 1. This cycle transports heat from the stove up to the air.

Cool It Down

It turns out that convection is an excellent way to move heat from one place to another. Stars and planets use this kind of cycle to get heat from the inside out. Heat loss is the biggest problem that stars and planets face. Stars generate heat from nuclear reactions; planets generate heat from radioactivity. Inside stars and planets, it's hot. If you went down into a gold mine 2 miles deep, the thermometer would rise above 170 degrees F. The heat has to escape somehow.

There are three ways how. First, heat can move by conduction: molecules bang into their neighbors and pass heat along in a domino effect. When you put your hand against a cold window on a winter's day, the window sucks the heat from your hand by conduction.

Second, heat can move by radiation. Usually, when people think radiation, they think nukes, mutants, Chernobyl. But to scientists, radiation just means any kind of light ray, either visible or invisible (such as infrared or ultraviolet). Hot things glow, giving off light that carries the heat away. That's why the Sun shines and warms the Earth. It's also why you feel hot when sitting in front of a fireplace or electric heater. The desert is so cold at night because you and the air around you are glowing in the infrared, losing heat to outer space. Survival kits contain "space blankets," basically big sheets of aluminum foil, and these keep you warm by reflecting the infrared radiation from your body back to your body.

Convection is the third way that heat can move. Instead of molecular domino-effects or streaming heat rays, convection relies on movement of fluid. When hot fluid moves, it carries the heat along with it. You can think of the three ways heat moves in terms of passing a love note to your sweetie across a classroom. You could give it to person sitting next to you and ask them to pass it along (like conduction), you could signal using a flashlight (like radiation), or you could get up, walk across the room, and give the note to the person directly (like convection).

Convection is a last-ditch way to get rid of heat. It takes effort to start convection, so heat usually prefers to escape by conduction or radiation. But conduction is slow, and radiation can't work where it's opaque: inside a planet and certain parts of a star. In those cases, only convection can do the job.

The mechanism of heat loss determines what a planet looks like. The inner planets of the solar system are composed of rock. Normally, we think of rock as a solid, but it can act as a liquid if you wait long enough; say, millions of years. Rock can transport heat either by conduction or by convection; rocks are opaque, so they block radiation. Small, cool planets, like the Moon and Mars, lose their heat by conduction. The Earth and Venus prefer convection (see Figure 2). Convection is much more exciting. It powers the plate tectonics and other fancy styles of geology that the Earth and Venus have.

Figure 2
The interior of the Earth or Venus. We build our buildings and eat our Eggos on top of a thin crust of solid rock, like scum on a pond. Underneath is a vast sea of fluid rock called the mantle, continually churned by convection. The mantle itself sits on a core composed of molten iron.

Inside a star, conduction doesn't work because the molecules are too far from one another, so heat moves either by radiation or by convection. Radiation operates where the gas of the star is transparent, as it is when it is especially hot. In medium-sized stars like the Sun, radiation transports heat deep within the star, where it's hottest (see Figure 3), and convection operates toward the outside, where it's cooler. In small, cool stars, convection is the main mechanism; in large, hot stars, radiation dominates.

Figure 3
The interior of the Sun. Only a small part of the Sun, its core, actually generates energy. The rest of the gas just gets in the way as the heat tries to get out. Most of the Sun's interior is filled with transparent gas, and the energy passes through it in the form of radiation. But toward the surface, the gas is cooler and opaque, so radiation can't get through. Convection takes over.

Convection gives the Sun a mottled appearance, as astronomers see when they look at the Sun with specially designed telescopes (see Figure 4). Each of the little granules in Figure 4 marks a place where convection is gurgling up from below. Convection dredges up atoms manufactured in the core of the star. Astronomers like this because it gives them some idea what's happening deep within the star, where they can't see.

Figure 4
Fire burn and cauldron bubble. This highly magnified picture of the surface of the Sun shows a sunspot (dark splotches in center) and granules (light-colored grains). Each granule, small as it looks, is about the size of Texas. The granules flicker rapidly, making the Sun look like a burbling witch's brew. Granules are the top of convection cycles that bring material from deep within the Sun to the surface. Photo courtesy of Sacramento Peak Observatory.

No. 28 - Fall 1994

The topsy-turveying of planets, stars, and lava lamps

Cool It Down