Direction: Please use the symbol key to annotate the text and write a summary of the text. Welcome to the Universe Scientists do not know exactly when the Universe began. There are many theories of the beginning of the Universe. Among them are the Big Bang theory, The Steady State Theory and the Oscillating Universe Theory. Of these the one most currently accepted in scientific circles is the Big Bang Theory. According to this theory, the Universe was formed somewhere between 10 - 20 billion years ago as a result of a huge expansion (not an explosion) called the Big Bang. Before the Big Bang, all the energy was concentrated into an infinitesimally small and incredibly hot volume much smaller than the period at the end of this sentence. As a result of the instant expansion during the Big Bang, the temperature dropped and some of the original energy was used to form matter in just a few minutes. The existence of cosmic background radiation and the visible expansion of parts of the Universe are considered solid support for this theory. Since 1609, when Galileo turned his new telescope toward the sky, scientists have studied its depths. Our Solar System, with its central Sun and nine planets and their satellites, exists in a galaxy called the Milky Way. A galaxy is a grouping of billions of stars held together by gravitational forces. The Milky Way galaxy is one of thousands that make up the Universe. Astronomer Edwin Hubble, for whom the Hubble Space Telescope is named, provided us with a classification system for galaxies based on their shape. Each galaxy has a particular shape due to gravitational forces. Our Milky Way Galaxy is classified as a spiral galaxy. It is 100,000 light years across and about 10,000 light years thick at its center. Our Sun is not in the center of the galaxy. It is 30,000 light years from the center. So our Solar System is just a part of the Milky Way Galaxy. Most of the energy on Earth can be related to the energy of the Sun, our closest star. All the stars studied in all the galaxies are made of the same elements as our Sun. All the galaxies we are able to study have chemical components that are very similar to our Solar System and galaxy. They may even contain solar systems that could support life. Our Solar System is named for the Sun (Sol) which contains 99% of the mass of the Solar System. Nine planets maintain an orbit around the Sun due to the gravitational force exerted by the Sun’s huge mass. As viewed from above, these planets orbit counterclockwise. The Sun is a glowing ball of fire made up mostly of the gaseous elements hydrogen and helium. Nuclear fusion is taking place in the Sun as it is in most stars. Light from the Sun provides almost all the energy needed for all the processes of Earth. Distances between the Sun and the planets are measured in astronomical units. An astronomical unit (AU)is the average distance between Earth and the Sun, which is about 150 million kilometers. The table below lists the planets in the order of their distances from the Sun in astronomical units. A useful memory device to remember this order is “My Very Energetic Mother Just Served Us Nine Pizzas.” It is also important to note that some of the planets are not solid like Earth. Jupiter, Saturn, Uranus, and Neptune have no solid surface. All the other planets have solid surfaces. Earth’s surface is 75% water, and Mars’ surface has the largest volcano and canyon. Pluto’s surface features are unknown. All the planets, except Mercury and Venus, have satellites called moons revolving around them. Pluto has one moon and Saturn has as many as twenty-two moons. Earth actually has three moons, but the one we see nightly in the sky is known as Earth’s Moon. Both the planets and their moons rotate on their axes as they revolve around the Sun. Scientists have been sending satellite probes into space orbit for many years. These probes have provided us with detailed information about the planets and their moons. Just as recently as the winter of 2003 - 2004, exploratory robots landed on Mars and provided pictures and information that are still being processed. The Moon is Earth’s primary satellite. It is the only place in our solar system, other than Earth, that humans have visited. As the moon orbits Earth, it looks as if it has different shapes. At any point during its orbit around Earth, the Sun lights up one half of the moon’s surface. If the half that is lit faces Earth, then we see a full moon. If only a small portion of the lit surface is visible and we mostly see its dark side, then we see a crescent. Between the full and the crescent moons only half of the lit side of the moon is visible. We call this phase the quarter moon. When the lit side is not visible at all, the phase is called the new moon. It takes about 29.5 days to complete a full lunar cycle or to go from one full moon to the next.