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1. Our Place in the Universe © 2004 Pearson Education Inc., publishing as Addison-Wesley 1.1 A Modern View of the Universe Our goals for learning: • What is our physical place in the Universe? • Describe our cosmic origins and why we say that we are “star stuff.” • Why does looking into space mean looking back in time? © 2004 Pearson Education Inc., publishing as Addison-Wesley Address: Earth • How would the Universal post office find us? © 2004 Pearson Education Inc., publishing as Addison-Wesley Star A large, glowing ball of gas that generates heat and light through nuclear fusion © 2004 Pearson Education Inc., publishing as Addison-Wesley Planet A moderately large object which orbits a star; it shines by reflected light. Planets may be rocky, icy, or gaseous in composition. © 2004 Pearson Education Inc., publishing as Addison-Wesley Moon An object which orbits a planet. © 2004 Pearson Education Inc., publishing as Addison-Wesley Asteroid A relatively small and rocky object which orbits a star. © 2004 Pearson Education Inc., publishing as Addison-Wesley Comet A relatively small and icy object which orbits a star. © 2004 Pearson Education Inc., publishing as Addison-Wesley Solar (Star) System A star and all the material which orbits it, including its planets and moons © 2004 Pearson Education Inc., publishing as Addison-Wesley Nebula An interstellar cloud of gas and/or dust © 2004 Pearson Education Inc., publishing as Addison-Wesley Galaxy A great island of stars in space, all held together by gravity and orbiting a common center © 2004 Pearson Education Inc., publishing as Addison-Wesley Universe The sum total of all matter and energy; that is, everything within and between all galaxies © 2004 Pearson Education Inc., publishing as Addison-Wesley Atom Microscopic “building blocks” of all chemical elements © 2004 Pearson Education Inc., publishing as Addison-Wesley Where do we come from? • The first (and simplest) atoms were created during the Big Bang. • More complex atoms were created in stars. • When the star dies, they are expelled into space…. to form new stars and planets! Most of the atoms in our bodies were created in the core of a star! © 2004 Pearson Education Inc., publishing as Addison-Wesley Announcement • (Short) quiz next Weds on material covered thro’ next Mondays lecture. • Closed book -please sit spaced apart and no conferring • Quiz will be graded and we will look at solution sheet shortly afterwards but you can pick up graded quizzes the Tues or Thurs of the week after a given quiz - from Yu Zhou in Physics 422, available Tues & Thurs noon1pm. • Reminder ~ 1 week for grade appeals (ie must appeal within 2 weeks of sitting the quiz) © 2004 Pearson Education Inc., publishing as Addison-Wesley Looking back in time • Light, although fast, travels at a finite speed. • It takes: – 8 minutes to reach us from the Sun – 8 years to reach us from Sirius (8 light-years away) – 1,500 years to reach us from the Orion Nebula • The farther out we look into the Universe, the farther back in time we see! © 2004 Pearson Education Inc., publishing as Addison-Wesley Timeout • Scientific Notation • Scientific Notation is a compact and convenient way of expressing very large and very small numbers using powers of 10. You've all probably encountered scientific notation before. I hope the examples below are reminders for those who haven't used it in a while. • Examples of Scientific Notation: • • The Mass of the Sun: 1,989,000,000,000,000,000,000,000,000,000 kilograms = 1.989x1030 kilograms • • The Size of a Hydrogen Atom: 0.0000000000106 meters = 1.06x10-11 meters © 2004 Pearson Education Inc., publishing as Addison-Wesley Timeout A quick word on powers of 10 • • • • • • • • • Factor Prefix 103 kilo106 mega109 giga1012 tera10-2 centi10-3 milli10-6 micro10-9 nano- Examples kilogram, kilometer, kilobyte megawatt, megayear, megabyte, megaton gigayear, gigaton, gigabyte terawatt, terabyte centimeter millimeter, millisecond, milliliter microsecond, micron nanosecond, nanometer © 2004 Pearson Education Inc., publishing as Addison-Wesley Timeout A quick word on powers of 10 • Common Examples • • • • • Length: 1 kilometer = 103 meters (1000 meters) 1 centimeter = 10-2 meters (1/100th of a meter) 1 millimeter = 10-3 meters (1/1000th of a meter) 1 micron = 10-6 meters (short for "micrometer") • • • • Time: 1 nanosecond = 10-9 s (1 billionth of a second) 1 Gigayear = 109 years (1 Billion years) 1 Megayear = 106 years (1 Million years) © 2004 Pearson Education Inc., publishing as Addison-Wesley Timeout • Astronomical Units of Length • Meters and kilograms are fine for most terrestrial applications, but when we start talking about the enormous distances between the planets, or between stars and galaxies, we need to define special units to keep the numbers from getting too big. The most important of these for our purposes in this course are: • The Astronomical Unit (AU): • 1 AU is the Mean Distance from the Earth to the Sun: • • 1 AU = 1.496x108 kilometers The AU is used for expressing the distances between planets. • In round numbers, you can use "1 AU = 150 Million km" for the purposes of this class. • The Light Year (ly): • 1 Light Year (ly) is the Distance Traveled by Light in 1 Year: • • 1 ly = 9.46x1012 kilometers The light year is used for expressing the distances between stars. © 2004 Pearson Education Inc., publishing as Addison-Wesley Timeout • Space is BIG • For example, what is the distance between the Earth and: • • • • • * The Moon: 384,000 kilometers * The Sun: 1 AU (149,600,000 km) * Alpha Centauri (nearest star): 4.2 light years (266,000 AU) * Center of the Milky Way Galaxy: 26,000 light years (1.65x109 AU) As you can see, if you only use kilometers or meters, the numbers would get out of hand very fast. © 2004 Pearson Education Inc., publishing as Addison-Wesley Our goals for learning: • What does our solar system look like when viewed to scale? • How far away are the stars? • How do human time scales compare to the age of the Universe? © 2004 Pearson Education Inc., publishing as Addison-Wesley How large is the Solar System? • Let’s view it to scale – say the Sun is the size of a large grapefruit (13.9 cm) – then: © 2004 Pearson Education Inc., publishing as Addison-Wesley How large is our Galaxy? The Sun ~ 26,000 lightyears from the center of the Milky Way Galaxy, which is about 80,000 to 120,000 light-years across (and less than 7,000 light-years thick). We are located on on one of its spiral arms, out towards the edge. © 2004 Pearson Education Inc., publishing as Addison-Wesley How far away is the nearest galaxy? © 2004 Pearson Education Inc., publishing as Addison-Wesley How large is the Universe? • Now let’s view the Universe in terms of meters – Powers of 10 or 10 ? © 2004 Pearson Education Inc., publishing as Addison-Wesley How old is the Universe? • The Cosmic Calendar – if the entire age of the Universe were one calendar year – one month would be approximately 1 billion real years © 2004 Pearson Education Inc., publishing as Addison-Wesley 1.3 Spaceship Earth Our goals for learning: • Describe the basic motions of “spaceship Earth.” • How do we know that the Universe is expanding? © 2004 Pearson Education Inc., publishing as Addison-Wesley A Universe in motion • Contrary to our perception, we are not “sitting still.” • We are moving with the Earth. – and not just in one direction The Earth rotates around it’s axis once every day! © 2004 Pearson Education Inc., publishing as Addison-Wesley The Earth orbits around the Sun once every year! The Earth’s axis is tilted by 23.5º! © 2004 Pearson Education Inc., publishing as Addison-Wesley Our Sun moves relative to the other stars in the local Solar neighborhood! Our Sun and the stars of the local Solar neighborhood orbit around the center of the Milky Way Galaxy every 230 million years! © 2004 Pearson Education Inc., publishing as Addison-Wesley The Milky Way moves with the expansion of the Universe! • Mostly all galaxies appear to be moving away from us. • The farther away they are, the faster they are moving. – Just like raisins in a raisin cake; they all move apart from each other as the dough (space itself) expands. © 2004 Pearson Education Inc., publishing as Addison-Wesley What have we learned? • What is our physical place in the Universe? • Earth is a planet in a solar system, which is one of some 100 billion star systems in the Milky Way Galaxy, which is one of about 40 galaxies in the Local Group, which is part of the Local Supercluster, which is part of the Universe. • Describe our cosmic origins and why we say that we are “star stuff.” • The Universe began in the Big Bang, which produced only two chemical elements: Hydrogen & Helium. The rest have been produced by stars, which is why we are “star stuff.” © 2004 Pearson Education Inc., publishing as Addison-Wesley What have we learned? • Why does looking into space mean looking back in time? • Light takes time to travel through space. Thus, when we look farther away, we see light that has taken a longer time to reach us. • What does our solar system look like when viewed to scale? • On a scale of 1-to-10 billion, the Sun is about the size of a grapefruit. The Earth s the size of a ball point and Jupiter the size of a marble on this scale. The distance between planets are huge compared to their sizes. © 2004 Pearson Education Inc., publishing as Addison-Wesley What have we learned? • How far away are the stars? • On the 1-to-10 billion scale, the nearest stars to the Sun would be thousands of kilometers away. The rest of the Milky Way must be viewed on a different scale. It would take thousands of years to count them all. • How do human time scales compare to the age of the Universe? • On a cosmic calendar that compresses the history of the Universe into one year, human civilization is just a few seconds old. © 2004 Pearson Education Inc., publishing as Addison-Wesley What have we learned? • Describe the basic motions of “spaceship Earth.” • Earth rotates on its axis once each day and orbits around the Sun once each year. Our Solar System orbits the center of the Milky Way Galaxy about every 230 million years. Galaxies in the Local Group move relative to one another, while all other galaxies are moving away from us with expansion of the Universe. • How do we know that the Universe is expanding? • We observe nearly all other galaxies to be moving away from us, with more distant ones moving faster. © 2004 Pearson Education Inc., publishing as Addison-Wesley