I spoke to a group of very young kids and their parents earlier today about the planets and stars. The kids were energized and eager to participate. When we watched the crashing asteroids video, they happily provided the sound of things blowing up. Then we discussed how you can’t hear anything in space because there’s no air. Their parents asked insightful questions. One question seemed simple on the surface: what’s the difference between a star like the sun, and a planet. The questioner posed it because he knew the Earth was hot inside like the sun.
A cutaway diagram shows the different layers of the Earth. The crust is the thinnest layer while the inner core and outer core combine to make up more than half the planet. Photo: Lawrence Livermore National Labs
This got me thinking. Yes, Earth is hot inside, and you don’t have to go all the way to the core to find that out. The Earth is divided into four main layers: crust, mantle, outer core and inner core. The rocky crust is about 18 miles thick, followed by the mantle, which starts below the crust and goes down some 1800 miles. Heat from the decay of radioactive elements like uranium makes the mantle rock soft and pliable. The lighter rocks that compose the continents float on top of the mantle, and get shoved around this way and that by the mantle’s plastic movement. That is the essence of plate tectonics.
Continuing our journey to the center of the Earth, we reach the outer core, which starts 1800 miles beneath our feet and extends 3,200 miles down. It’s composed of hot liquid iron and sulfur. Currents within the iron, coupled with Earth’s spin, create an invisible magnetic bubble around our planet called a magnetic field. Your compass points north because of the spinning iron in the outer core. The inner core is nearly pure iron and under such high pressure, it’s actually solid. Picture an 800 mile diameter ball of glowing, 11,000 degree metal. The tremendous pressure of all the matter above it keeps the inner core firm.
What we know about the Earth’s structure comes to us from studying how seismic waves from earthquakes are reflected from its interior. These waves travel where no human can.
Earth’s inner heat goes back to the beginning of its formation. As chunks of rock stuck together to form the growing planet 4 1/2 billion years ago, the energy of all the pieces falling together released an enormous amount of heat. Radioactive decay of uranium added even more, until the planet completely melted. Heavy stuff like iron sank to the core while the lighter rocks floated to the top. Long ago, the crust cooled enough for rocks to solidify and life to thrive, but the Earth’s interior still retains much of that early heat of gravitational collapse. The Earth radiates some of its heat energy into space but not enough to be felt by anything — not even the moon.
Temperatures of 40 million degrees in the sun’s core "cook" hydrogen into helium and release energy in the process. This energy slowly leaves the core and travels more than 400,000 miles before it reaches the surface as heat and light. The surface of the sun is called the photosphere; the corona is the sun’s atmosphere.
The sun is more than 100 times larger than the Earth and composed mostly of hot, flammable hydrogen gas. Checking its core temperature, we find the sun’s interior sizzles at 40 million degrees. At those temperatures and pressures, hydrogen atoms fuse together they way they do in manmade thermonuclear bombs. Every second, the sun fuses or converts 600 million tons of hydrogen atoms into 596 million tons of helium. The remaining four million tons is released as pure energy. The energy leaves the core and heads to the sun’s surface in a journey that lasts at least 10,000 years. From there it radiates out into space as light and heat. The sun has enough hydrogen to burn this way for billions of years. The next time the sun touches your cheek, think how much time it took that bit of warmth to travel from its origin in the sun’s center to the pleasant sensation on your face.
The sun and all the stars are literally burning spheres of gas that radiate prodigious amounts of heat and light across space. Lucky for us, we’re at the right distance from the sun for life to go wild. Stars are much too hot to provide a habitat for life — or anything solid for that matter. The cooler planets provide ideal nurseries for life the thrive. Photo: Halfdan
It’s interesting that the blazing solar surface, which we dare not stare at, is almost the same temperature as the center of the Earth. Our planet’s heat is modest and well-contained. We don’t share it with the rest of the planets. The sun and its kindred stars are flaming balls of hydrogen gas, bright enough to see across light years. They can’t help but share their bounty with the chilly, little planets. Remember too, that planets don’t shine by themselves, but only by the light they reflect from the sun. Switch off the sun, and Earth would orbit in eternal night.