Page 46 - Curriculum Visions Dynamic Book

The Sun, which has been shining for more than four billion years, is important to the solar system because it is the prime source of energy of all kinds, including heat, gravity, light, radio waves, and magnetism. A tiny fraction of this energy reaches the Earth and provides the heat and light that allow life to survive on the planet.
The Solar System is made up of a part of space within the galaxy called the Milky Way.
The vast majority of the Solar System is “empty space.” Within it, and arranged reasonably evenly, lie small masses of gas and even smaller masses of rock.
The largest mass of gas is a star we call the Sun. Smaller by far are its dependent planets, of which the four gas planets—Jupiter, Saturn, Uranus, and Neptune—are the dominant features. Smaller still are the rocky planets—Mercury, Venus, Earth, Mars, and Pluto. Many of these planets have captive bodies big enough to be called moons. There are 60 known moons.
Smaller again are the millions of bodies we call asteroids and comets. There are several thousand asteroids, mostly orbiting between Mars and Jupiter. Beyond Pluto there are several billion small ice and rock bodies that sometimes change direction and move toward Earth. When they do so, they are called comets.
The space between all of these bodies is not quite empty. It consists of occasional particles of ionized (charged) gas called plasma, with occasional particles of dust. They are mainly particles streaming out from the Sun in the solar wind, as described on page 38.
For more on the planets see Volume 3: Earth and Moon, Volume 4: Rocky planets, and Volume 5: Gas giants.
asteroid Any of the many small objects within the Solar System.
axis (pl. axes) The line around which a body spins.
comet A small object, often described as being like a dirty snowball, that appears to be very bright in the night sky and has a long tail when it approaches the Sun.
galaxy A system of stars and interstellar matter within the universe.
gravity The force of attraction between bodies.
magnetism An invisible force that has the property of attracting iron and similar metals.
milky way The spiral galaxy in which our star and Solar System are situated.
moon The name generally given to any large natural satellite of a planet.
orbit The path followed by one object as it tracks around another.
plane A flat surface.
planet Any of the large bodies that orbit
the Sun.
plasma A collection of charged particles that behaves something like a gas. It can conduct an electric charge and be affected by magnetic fields.
radio waves A form of electromagnetic radiation, like light and heat. Radio waves have a longer wavelength than light waves.
solar system The Sun and the bodies orbiting around it.
solar wind The flow of tiny charged particles (called plasma) outward from the Sun.
star A large ball of gases that radiates light. The star nearest the Earth is the Sun.
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