THE SUN

INTERIOR Unlike Earth, the sun does not have a solid surface. The sun is a ball of glowing gas through and through. About 3/4 of the sun's mass is hydrogen and 1/4 is helium. Like Earth, the sun has an interior and an atmosphere. The sun's interior consists of the core, the radiation zone, and the convection zone.

CORE The sun produces an enormous amount of energy in its core, or central region. The sun's energy comes from nuclear fusion. During nuclear fusion, hydrogen atoms join together to form helium. Nuclear fusion only can take place under conditions of extremely high pressure and temperature. The temperature inside the sun's core reaches 15 million C, high enough for nuclear fusion to take place.

The total mass of the helium produced by nuclear fusion is slightly less than the total mass of the hydrogen that goes into it. What happens to this mass? It is changed into energy that slowly moves outward from the core, eventually escaping into space.

RADIATION ZONE The energy produced in the sun's core moves outward through the middle layer of the sun's interior, the radiation zone. The radiation zone is a region of very tightly packed gas where energy is transferred mainly in the form of electromagnetic radiation. Because the radiation zone is so dense, energy can take more than 100,000 years to move through it.

CONVECTION ZONE The convection zone is the outermost layer of the sun's interior. Hot gases rise from the bottom of the convection zone and gradually cool as they approach the top. Cooler gases sink, forming loops of gas that move energy toward the sun's surface.

THE SUN’S ATMOSPHERE Photosphere: The inner layer of the sun's atmosphere. Chromosphere: The glow that comes from the middle layer of the sun's atmosphere. Corona: The outer layer that looks like a white halo around the sun. The corona extends into space for millions of kilometers and gradually thins into streams of electrically charged particles called solar wind.

CORONA SOLAR WIND

FEATURES ON THE SUN Features on or just above the sun's surface include sunspots, prominences, and solar flares.

Solar Flares: Gaseous eruptions on the sun's surface. Sunspots: Areas of gas on the sun's surface that are cooler that the gasses around them. Cooler gases don't give off as much light as hotter gases, which is why sunspots look darker than the rest of the sun's surface. Prominences: Huge reddish loops of gas that often link different parts of sunspot regions. Solar Flares: Gaseous eruptions on the sun's surface. SUNSPOTS

PROMINENCE SOLAR FLARE

THE MOON

SURFACE For thousands of years, people thought that the moon's surface was smooth. In 1609, the Italian scientist, Galileo Galilei, heard about a telescope, a device built to observe distant objects by making them appear closer. He built his own telescope and observed that the moon has an irregular surface with a variety of remarkable features.

Features on the moon's surface include maria, craters, and highlands.

The moon's surface has dark, flat areas, which Galileo called maria, the Latin word for "seas." Although Galileo thought the maria were oceans, they are actually hardened rock formed from huge lava flows that occurred between 3 and 4 million years ago.

Galileo saw that the moon's surface is marked by large round pits called craters. Some craters are hundreds of kilometers across. Although scientists thought that these craters had been made by volcanoes, they were actually caused by the impacts of meteoroids, chunks of rock or dust from space.

Galileo noticed that there were lights spots on the moon's surface Galileo noticed that there were lights spots on the moon's surface. He called these light spots highlands, or mountains. The peaks of the lunar highlands and rims of the craters cast dark shadows, which Galileo could see. The lunar highlands cover much of the moon's surface.

CHARACTERISTICS The moon is 3476 kilometers in diameter, a little less than the distance across the United States. This is about 1/4 Earth's diameter. However, the moon has only 1/80 as much mass as Earth.

On the moon's surface, temperatures range from 130 C in direct sunlight to -180 C at night. Temperatures on the moon vary so much because it has no atmosphere. The moon's surface gravity is so weak that gasses can easily escape into space.

The moon has no liquid water The moon has no liquid water. However, there is evidence that there may be large patches of ice near the moon's poles. Some areas are shielded from sunlight by crater walls. Temperatures in these regions are so low that ice there remains frozen. If a colony were built on the moon in the future, any such water would be very valuable. It would be very expensive to transport large amounts of water to the moon from Earth.