4 Ch.1: Our Place in the Universe 15,000,000,000,000,000,000 miles!
5 Brief review of some math... (see Appendix C in Cosmic Perspective)1. Powers of 10: number of times to multiply 10 by itself.e.g., 102 = 10 x 10 = 100e.g., 104 = 10 x 10 x 10 x 10 = 10,0002. Negative powers of 10: reciprocal of positive powere.g., 10-2 = 1/102 = 1/100 = 0.01Rules 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 thedecimal point.
7 Multiplying & Dividing Powers of 10 104 x 102 = 10,000 x 100 = 1,000,000 = 106105 x 10-1 = 100,000 x 0.1 = 10,000 = 104Rule of thumb:Multiplication: 10n x 10m = 10n+mDivision: 10n 10m = 10n-mTHERE ARE NO SIMPLE RULES FOR ADDINGOR SUBTRACTING POWERS OF 10.
8 Scientific Notation: expressing large numbers with powers of 10 EVERY NUMBER CAN BE EXPRESSED AS A NUMBERBETWEEN 1 AND 10 x POWER OF 106,000,000,000.0 = 6.0 x 109237,112 = x 105= 4.0 x 10-8Two steps: 1. Move the decimal to appear after the first non-zero digit2. Count the number of places that the decimal point moved;this is the power of 10. The power is negative if the decimalmoved to the right; positive if moved to the left.Shortcuts:Multiplication: (6 x 103) x (2 x 104) = (6 x 2) x (103 x 104) = 12 x 107Division: (6 x 103) (2 x 102) = (6 2) x (103 102) = 3 x 101
10 Do not be a perfectionist! Approximations can be a good thing. Consider Jupiter vs. Sun size comparison:Jupiter radius = 71,492 kmSun radius = 695,000 km, i. e., 9.7x largerJupiter radius ≈ 70,000 kmSun radius ≈ 700,000 kmTherefore, the Sun is about 10x larger
11 Units of MeasurementDistance 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.
12 A light year is a unit of distance, not time! “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!A light year is a unit of distance, not time!
13 Converting Units: a worked example Question: How many cm are there in 3 km?
14 Start by writing down what you know. There are 100 cm in a meter: so 100cm = 1m100cm1m= 1and= 11m100cmThere are 1000 m in a kilometer: so 1000m = 1km1000m1km= 1and= 11km1000mNow we multiply by 1 until we get our desired units.1000m100cm3km = 3kmxx= 3 x 1000 x 100 cm1km1m= 3 x 105cm
15 What about light years? Sounds like a unit of time... is??
16 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 miles60 seconds60 minutessecondminutehour= 6.7 x 108 mph!Speed = DistanceTimeDistance = Speed x Time
17 One light year is the distance that light travels in one year. Distance = Speed x Time1 light year = (speed of light) x (1 year)1 year = 3.15 x 107 secondsSpeed of light (in metric units) = 3.0 x 105 km/s1 light year = (3.0 x 105 km/s) x (3.15 x 107 seconds)1 light year = 9 x 1012 km
18 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 Sun8 years to reach us from Sirius (8 light-years away)1,500 years to reach us from the Orion NebulaThe farther we look out in the Universe,the farther back in time we see.
20 Your ancestors may surprise you In the early history of the Universe, there was nothing but hydrogen and helium.All other elements were manufactured deep in the cores of stars by nuclear fusion reactions, or in nuclear reactions that occur when a massive star ends its life in a violent explosion called a supernova.Seek to be one with the cosmos? You already are!
21 The size of things in astronomy Terrestrial planetsGas giantsGas giants
22 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 Mercury6 minutes to get to Venus (which would be near Fenway Park)8 minutes to get to Earth at BU3 seconds past BU, the Moon12.5 minutes to arrive at Mars43 minutes to reach Jupiter1 hour & 17 minutes to reach Saturn (Framingham exit)1 hr & 38 min: Uranus2 hours & 9 minutes to make it out to Neptune (Palmer exit)5 hr & 28 min: last stop in the Solar System, PlutoNearest star to the Sun (alpha Centauri): 4.4 years!Nearest big galaxy (the Andromeda galaxy): 2.2 million years!!
23 The Age of the UniverseIf the entire age of the Universe were 1 calendar year, then 1 month would be equivalent to roughly 1 billion years
24 The Age of the UniverseIf the entire age of the Universe were 1 calendar year, then 1 month would be equivalent to roughly 1 billion years(Figure 1.12)
25 The funkiest bit of all: 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.
26 The funkiest bit of all: the expansion of the Universe! Raisin Number Distance before baking Distance after baking Speedcm cm 2 cm/hrcm cm 4 cm/hrcm cm cm/hrThe funkiest bit of all: 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.
27 How can we sleep when the earth is turning How can we sleep when the earth is turning? The many ways the Earth moves...Contrary to our perception, we are not “sitting still.”We are moving with the Earth.and not just in one directionThe Earth rotates around it’s axis once every day
28 The Earth orbits around the Sun once every year The Earth’s axis is tilted by 23.5º
29 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 yearsReading assignment for next lecture: Chapter 2