Written by Bogdan Cristescu on October 30, 2012
A reader wrote in with an interesting fact — that the core of planet Earth is as hot as the surface of the sun. We talk about the heat inside Earth — and at the sun’s surface. Is it true that the temperature in the center of our planet equals the surface temperature of our sun!
That is right. The interior of the sun is vastly hotter than Earth’s interior — but the sun’s surface temperature is about 5,500 Celsius — probably about as hot as it is at the center of the Earth.
Earth’s core and the sun’s surface are hot for different reasons, though. Earth’s core is mostly iron, and the sun’s surface is mostly hydrogen gas. The sun’s surface temperature depends how much energy the sun generates in its interior — which in turn depends on how much mass the sun has. It’s the outward flow of energy through the sun that heats up the surface.
Meanwhile, Earth’s core is hot because it formed hot. As the materials fell together to make our world, they picked up speed. The energy released when they slammed together was converted mostly to heat. Also, throughout Earth’s history, energy has been released by radioactive decay. Some of Earth’s heat is lost to space.
But Earth is big and has lots of insulation around its core — so a lot of heat stays trapped inside our planet.
The sun’s basic source of energy is thermonuclear fusion reactions that convert hydrogen to helium.
The earth’s core is liquid. It’s solid only at the very center.
Stars support their weight by generating energy. If a star has a lot of mass, it has to make a lot of energy to generate enough internal pressure to prevent its own self-gravity from crushing it. Stars with less mass don’t have to make as much energy — if they did make as much as the higher-mass stars, their internal pressure would cause them to expand — and ultimately they would blow themselves to pieces.
The opacity of the gasses — that’s how well they transfer heat — determine how easily energy leaks out to the surface. In the case of the sun, that opacity depends in part on a peculiar property of the hydrogen atom in which it gloms on to an extra electron.
The visible surface of sun is called the photosphere — meaning “sphere of light”.
Tim Brysky also asks “…does our moon have a molten center with the same being true?” Dr. Seeds responds:
“The best I can learn is that the moon has no molten core, and observations support that conclusion. Its observed density is very low, about that of the earth’s crust, so it can’t contain much of an iron core. Also, the Apollo astronauts left seismometers on the lunar surface that could pinpoint the location of moonquakes. The only ones they detected came from deep inside the moon near what we might expect to be the outer boundary of a core. But these quakes were about the energy of a large firecracker, and they probably represent creaking more than quaking.
“Theoretically we would not expect the moon to have a molten core. The moon is quite small, 1/4 the diameter of the earth, so its volume is 1/64th that of the earth, and that means it never contained as much heat as the earth. Also, the core of the moon is only 1/4th as far from the lunar surface as the core of the earth is from the earth’s surface, and that means that heat from the lunar core could flow outward into space much more easily than heat from the earth. Small worlds cool much faster than large worlds, so we would not expect the moon to have a hot interior.
“I’ve had one planetary astronomer sneer and say, ‘Of course the moon doesn’t have a molten core.’ She may have been a bit more emphatic than the evidence warrants, but if someone found a molten core in the moon, I, and most other astronomers, would be astounded. There is no heat source, and the heat leaks away too fast. If there is a molten core in the moon, it had better have a note from its Mama or it is in big trouble.”
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