The Sun A star is a huge ball of hot, glowing gases. The Sun is a star. The width of the Sun is equal to the width of 100 Earths placed side by side. The Sun is extremely hot. The surface of the Sun has a temperature of about 5,500ºC. Temperatures at the center of the Sun are much hotter. The Sun s center is much hotter than the surface. The Sun is closer to Earth than any other star. This makes the Sun look larger and brighter than any other star. Other stars look smaller than the Sun because they are much farther away. Earth and other objects move around the Sun in patterns. The Sun causes these patterns. The movement of the Earth makes it hard to figure out these patterns. It took many years for scientists to figure them out. The Sun looks large and bright in the sky. Other stars look much smaller. Temperatures on the Sun s surface reach 5,500ºC. 2 3
Objects in the Solar System A planet is a large body of matter that travels around the Sun. Planets and other objects that travel around the Sun move in regular patterns that can be predicted. The path an object takes as it moves around the Sun is its orbit. Each planet s orbit has a slightly oval shape. The Sun, the planets and their moons, and other objects that orbit the Sun make up the solar system. At the center of the solar system is the Sun. The strong pull of the Sun s gravity keeps the planets orbiting. Planets of the Solar System The planets closest to the Sun are Mercury, Venus, Earth, and Mars. The planets farthest from the Sun are Jupiter, Saturn, Uranus, and Neptune. In 2006, scientists decided to call Pluto a dwarf planet. The closest planets orbit the Sun faster than the farthest planets. Between Mars and Jupiter are thousands of objects that are smaller than the planets. These are asteroids. An asteroid is a large mass of rock. Asteroids orbit the Sun. This diagram shows the solar system s features. The distances and sizes are not true to scale. Asteroid Belt Jupiter Uranus Neptune Venus Mercury Earth Mars Saturn 4 5
The Motion of the Moon The Moon makes one complete turn on its axis in 27 Earth days. It orbits Earth once in about the same amount of time. Because the Moon turns so slowly on its axis, the same side of the Moon always faces Earth. Whenever you look at the Moon, you always see the same side of it. People saw the far side of the Moon for the first time in 1959 when a spacecraft took pictures of it. The Moon is the closest object to Earth in space. Other than the Sun, the Moon is the brightest object in the sky. The Moon does not make its own light. It appears lit because sunlight shines on it. This light is reflected to Earth. The picture below shows how Earth and the Moon appear from space. Each night we see a different amount of the lit part of the Moon. Several planets move across the fixed pattern of stars. Like the Moon, these planets reflect light from the Sun. In the time that the Moon takes to orbit Earth, it also makes a turn on its axis. The moonlight we see from Earth is reflected sunlight. 6 7
Studying Objects In Space A telescope is a tool for making distant objects appear nearer and larger. Many telescopes use pieces of glass called lenses to bend light. Telescopes with large, curved mirrors can collect larger amounts of light. The light is directed to a lens to create a bright image. Binoculars are two telescopes joined together as a unit. The image you see through binoculars is sharp and clear. However, binoculars cannot magnify objects as much as telescopes can. Studying the Planets Mercury, Venus, Mars, Jupiter, and Saturn can all be seen without a telescope. But you need binoculars and telescopes to see things on the planets surfaces. For example, Mars has polar ice caps. With a telescope, you can see them clearly against the planet s red soil. The details of Uranus and Neptune are too far away to be seen even with a telescope. We have learned about the features of these planets from space probes. These probes have taken pictures and made measurements. Using a telescope or high-powered binoculars, you can see Saturn s rings. With a telescope you can see Jupiter s clouds. You can also see four of Jupiter s moons. Neptune has rings. At times, clouds appear on its surface. 8 9
Looking at Stars and Galaxies Suppose you re outside on a clear, dark night. You look up to see thousands of tiny stars. Through a telescope you can see even more stars. These stars are different in size, color, and brightness. Sometimes a galaxy will look like a star. A galaxy is a group of stars, dust, and gas that are in the same area. Comparing Stars You might think at first that the brightest stars are closer than the ones that are less bright. But stars differ in size and make different amounts of light. Sometimes smaller stars make more light than bigger ones. Stars that appear dim may be closer to Earth than stars that appear bright. If you view the Andromeda galaxy without a telescope, it looks like a small, blurry star. The stars in this group have different colors and sizes. 10 11