Announcements. The relative size of objects in the Solar System

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1 Announcements 1. Today s topics: It s astronomical: sizes and ages in the universe It s boring but oh so useful: math review for working with astronomical numbers Astronomy 101: The Solar System 1st homework assignment STUDENT CONTRACT The OWL web page is now OPEN. Use your student ID # for the login and your last name for the password. 2. The first OWL homework is a tutorial on how to use the web page. This counts as a full-credit homework. Due September 25 before midnight. 3. Please register your PRS clicker in OWL before next lecture (Sept. 9) 4. Also, if you have not yet done so, print the student contract, read it, sign it, and turn it in. Our first goal: acquiring some astronomical perspective The relative size of objects in the Solar System 1. Print the contract from the web page 2. Print first three initials of last name 3. Print full name 4. Sign and date Due by Sept Radius(Sun) = 100x Radius(Earth) Radius(Jupiter) = 10x Radius(Earth) approximately Radius(Sun) = 10x Radius(Jupiter) Therefore: Radius(Sun) = 100x Radius(Earth) (This is an example of the simple types of deductions that are sometimes required in homework/exam problems in this class) In astronomy, it is often much easier to determine the relative size, mass, or distance of some interesting object. Determination of absolute or true properties is often extremely challenging in this field. Course theme: how can we learn from relative measurements? 1

2 Planet distances calculated by Copernicus ( ) Astronomical unit (AU): average distance between Earth and Sun Planet Mercury Venus Earth Mars Jupiter Saturn (Copernicus) 0.38 AU 0.72 AU 1.00 AU 1.52 AU 5.22 AU 9.17 AU (today) 0.39 AU 0.72 AU 1.00 AU 1.52 AU 5.20 AU 9.54 AU The relative size of objects in the Solar System The massive outer planets are giant balls of hydrogen gas. Jovian gas giants. The planets closest to the Sun are relatively small and made of rocky/ metallic material. Terrestrial planets. Between Mars and Jupiter, there is a large number of rocky objects similar to Terrestrial planets but smaller. The asteroid belt. Probably about a million asteroids Ceres, the largest asteroid Oh please. Everyone knows that Pluto is a planet! Ceres, the largest asteroid Pluto is a mega comet. Beyond Neptune, there is a huge number of icy objects with Rocky cores. Comets. Astronomers estimate that there are Approximately a trillion comets in the outer solar system. Pluto is a mega comet. Problem 1: If Pluto is a planet, then Ceres should be a planet too. Problem 2: There are many things In the outer solar system similar to Pluto. Ceres, the largest asteroid Pluto is a mega comet. Wouldn t you be stressed out if astronomers suddenly told you that there are 23 planets in the solar system?? In 2003, an object bigger than Pluto was found The new object, 2003 UB 313, is bigger than Pluto. Problem: Pluto should never have been a planet in the first place. There are many Pluto-like objects in the outer Solar System; this is where the comets are located most of the time. Discovered by Brown, Trujillo, and Rabinowitz (2005) 2

3 Astronomer Mike Brown believes that both Pluto and 2003 UB313 should be considered to be full-blown planets He also wants to name The new planet Xena Discovered by Brown, Trujillo, and Rabinowitz (2005) The relative size of objects in the Solar System In this figure, relative sizes are o.k., but the relative s are terribly wrong The universe is an empty place. The distances between objects are vast. For perspective, consider the following model in which the Sun is scaled down to the size of a large tomato (this is a 1-to-10-billion scale). The Solar System, scaled to the size of the Massachusetts Turnpike If the Earth is a 12 globe at Boston University, the Sun would be a 10-story building at South Station in Boston. If the speed of light were reduced to 16 mph, after a ray of light leaves the sun, it would take: 3 minutes to get to Mercury 6 minutes to get to Venus (which would be near Fenway Park) 8 minutes to get to Earth at BU 3 seconds past BU, the Moon 12.5 minutes to arrive at Mars 43 minutes to reach Jupiter 1 hour & 17 minutes to reach Saturn (Framingham exit) 1 hr & 38 min: Uranus 2 hours & 9 minutes to make it out to Neptune (Palmer exit) 5 hr & 28 min: last stop in the Solar System, Pluto Nearest star to the Sun (alpha Centauri): 4.4 years! Nearest big galaxy (the Andromeda galaxy): 2.2 million years!! Object Real Diameter Real from Sun Model Diameter Model from Sun Mercury 4880 km 57.9 million km 0.5 mm 6m Earth km million km 1.3 mm 15 m Jupiter 143,000 km million km 14.3 mm 78 m Pluto 2260 km 5900 million km 0.2 mm 590 m Galaxy: island of 100,000,000 to 1,000,000,000,000 stars Zooming out a bit Local group: 40 galaxies in the nearby universe. Two largest local group galaxies: Milky Way & Andromeda Solar system: the Sun, nine planets, asteroids, comets eight 3

4 Notice a difficulty presented by the numbers 15,000,000,000,000,000,000 miles! A small problem: numbers in astronomy are, well astronomical from the earth to the Sun: 93,000,000 miles from the earth to the Sun: 1 Astronomical Unit (AU) Diameter of our Galaxy: roughly 4,000,000,000 AU We can define units that are more convenient, e.g., the astronomical unit is easier to work with than 93,000,000 miles. However, sooner or later we will run into large (and small!) numbers in astronomy. Brief review of some math (see Unit 3 in Pathways to Astronomy) Examples: 1. Powers of 10: number of times to multiply 10 by itself. e.g., 10 2 = 10 x 10 = 100 e.g., 10 4 = 10 x 10 x 10 x 10 = 10, Negative powers of 10: reciprocal of positive power e.g., 10-2 = 1/10 2 = 1/100 = 0.01 Rules of thumb: A positive power indicates the number of zeros that follow the 1. A negative power indicates the number of places to the right of the decimal point. Multiplying & Dividing Powers of x 10 2 = 10,000 x 100 = 1,000,000 = x 10-1 = 100,000 x 0.1 = 10,000 = 10 4 Rule of thumb: Multiplication: 10 n x 10 m = 10 n+m Division: 10 n 10 m = 10 n-m THERE ARE NO SIMPLE RULES FOR ADDING OR SUBTRACTING POWERS OF 10. Scientific Notation: expressing large numbers with powers of 10 EVERY NUMBER CAN BE EXPRESSED AS A NUMBER BETWEEN 1 AND 10 x POWER OF 10 6,000,000,000.0 = 6.0 x ,112 = x = 4.0 x 10-8 Two steps: 1. Move the decimal to appear after the first non-zero digit 2. Count the number of places that the decimal point moved; this is the power of 10. The power is negative if the decimal moved to the right; positive if moved to the left. Shortcuts: Multiplication: (6 x 10 3 ) x (2 x 10 4 ) = (6 x 2) x (10 3 x 10 4 ) = 12 x 10 7 Division: (6 x 10 3 ) (2 x 10 2 ) = (6 2) x ( ) = 3 x

5 Scientific notation rules of thumb ( x!10 a )! ( y!10b ) = ( x " y )!10 a +b (x% ( x "10 a ) ( y "10b ) = && ## "10 a!b ' y$ Don t be a perfectionist! Approximations can be a good thing. Consider Jupiter vs. Sun size comparison: Jupiter radius = 71,492 km Sun radius = 695,000 km, i. e., 9.7x larger Jupiter radius 70,000 km Sun radius 700,000 km Therefore, the Sun is about 10x larger Units of Measurement can be measured in km, cm, miles, yards, furlongs, etc. Units are extremely helpful, but you must get them right!! In this course, we will encounter a variety of units, and we ll have to make conversions. Question: How many cm are there in 3 km? Start by writing down what you know. There are 100 cm in a meter: so 100cm = 1m 100cm 1m = 1 You ve never heard of the Millennium Falcon? It s the ship that made the Kessel Run in less than 12 parsecs. She s fast enough for you, old man. Bzzzzt. A parsec is a unit of distance, not time! and 1m 100cm = 1 There are 1000 m in a kilometer: so 1000m = 1km 1000m =1 1km and 1km =1 1000m Now we multiply by 1 until we get our desired units. A light year is a unit of distance, not time! 1000m x 100cm 3km = 3km x 1km 1m = 3 x 1000 x 100 cm = 3 x 105cm 5

6 What about light years? Sounds like a unit of time is?? What about light years? Sounds like a unit of time Nothing can travel at infinite speed. Light is the fastest thing in the universe, but even light travels at a finite speed. Speed of light = 186,000 miles 60 seconds 60 minutes second minute hour = 6.7 x 10 8 mph! Speed = Time = Speed x Time One light year is the distance that light travels in one year. = Speed x Time 1 light year = (speed of light) x (1 year) 1 year = 3.15 x 10 7 seconds Speed of light (in metric units) = 3.0 x 10 5 km/s 1 light year = (3.0 x 10 5 km/s) x (3.15 x 10 7 seconds) 1 light year = 9 x km Looking back in time: astronomers have a time machine! Sort of. Light, although fast, travels at a finite speed. It turns out that 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 we look out in the Universe, the farther back in time we see. The Age of the Universe The Age of the Universe If the entire age of the Universe were 1 calendar year, then 1 month would be equivalent to roughly 1 billion years Jan. 1: The Big Bang Feb: Our galaxy formed Sept. 3: Earth formed Sept. 22: Earliest life on Earth Dec. 26: Rise of dinosaurs Dec. 30: Dinosaurs disappear 50 seconds before midnight: pyramids built 6