galaxy螺旋星系.ppt

* Astronomy:the science of the stars The universe is made up of stars, planets, and other matter scattered throughout space. The Milky Way is a large spiral galaxy（螺旋星系） measuring about 100,000 light-years across. It is about 14 billion years old and takes 225 million years or so to rotate（旋转） once. Like all spirals, it contains plenty of gas and dust, from which new stars are formed. The dense nucleus（核心） is the oldest part and has no gas left for new star. The Solar system 1.Mercury(水星) 4.Mars（火星） 7.Uranus（天王星） 2.Venus(金星) 5.Jupiter（木星） 8.Neptune（海王星） 3.Earth(地球) 6.Saturn（土星） 9.Pluto（冥王星） Our solar system is made up of the sun and the objects that orbit it. These include the nine planets (and probably more than nine) and their moons, as well as asteroids（小行星）, comets, and meteors（流星）. The sun is the nearest star to the earth. It is a globe of hot gas, mostly hydrogen（.氢）, and lies at the center of our solar system. It contains no solid material. Stars are large balls of hot gas that produce light and heat through nuclear reactions. Our Sun is an average yellow star, but seems bright because it is so close. Mercury The Roman god of merchants and travellers Venus The Roman goddess of love Mars The Roman god of war Jupiter The king of the Roman gods Saturn The Roman god of seeds and sowing Uranus The Greek god of the sky Neptune The Roman god of the sea Pluto The Roman god of the dead Stars are large balls of hot gas that produce light and heat through nuclear reactions. Our Sun is an average yellow star, but seems bright because it is so close. Planets are the largest objects that circle around the stars. They may be rocky, like the earth, or made mostly of gas and liquid, like Jupiter. The word planet is Greek for wanderer. The name comes from the way planets appear to move against the stars over time. It is thought that the planets formed at about the s