

Lectures

Table of Contents

Astro 100 
Course Overview and the Scale of the Universe
Outline
 Course Format, Philosophy and Requirements
1. Course Format, Philosophy, and Requirements
 Motivation and Goals
 Lectures and Reading
 WWW Pages
 Homework
 Exams and Grades
 What is Astronomy?
Powers of 10
Outline
 The Scale of the Universe
 Powers of 10 and Scientific Notation
 The Speed of Light
Terms to know
order of magnitude
exponent
scientific notation
1. The Scale of the Universe: How Big is Astronomy?
Your address in the Universe
Person: 
2 meters (~ 6 feet) 
Earth radius: 
6378 km (~4000 miles) 
Average Earth <> Sun Distance: 
1 Astronomical Unit (AU)~ 150,000,000 km 
Average Pluto <> Sun Distance: 
39.44 AU ~ 5,900,000,000 km (size of Solar System) 
Sun <> Proxima Centauri (nearest star): 
4.2 light year (l.y.) ~ 265,000 AU ~ 42,000,000,000,000
km 
Sun <> Center of Milky Way Galaxy: 
25,000 l.y. ~ 1,500,000,000 AU ~ 240,000,000,000,000,000
km 
Milky Way <> Andromeda Galaxy: 
2 million l.y. 
Size of Observable Universe: 
14 billion l.y. 




...And that's ONLY THE BEGINNING!
Two ways to express very large (or very small)
numbers:
 Define NEW UNITS (AU, l.y., etc.); or
 Use Powers of 10 and Scientific Notation
2. Powers of 10 and Scientific Notation
The range of sizes, time, temperature, pressure, density,
radiation, etc. etc. in astronomy and physics is so huge that some
special techniques are required to keep track of it all.
one 
1 = 10^{0} 
1 = 10^{0} 
one 
ten 
10 = 10^{1} 
0.1 = 10^{1} 
tenth 
hundred 
100 = 10^{2} 
0.01 = 10^{2} 
hundredth 
thousand 
1000 = 10^{3} 
0.001 = 10^{3} 
thousandth 
million 
1,000,000 = 10^{6} 
0.000001 = 10^{6} 
millionth 
billion 
1,000,000,000 = 10^{9} 
0.000000001 = 10^{9} 
billionth 
etc. ...
bigger

etc. ...
smaller

Names for scales:
Trillion 
Tera 
(T) 
10^{12} 
Billion 
Giga 
(G) 
10^{9} 
Million 
Mega 
(M) 
10^{6} 
Thousand 
Kilo 
(K) 
10^{3} 
... 
thousandth 
milli 
(m) 
10^{3} 
millionth 
micro 
(mu) 
10^{6} 
billionth 
nano 
(n) 
10^{9} 
trillionth 
pico 
(p) 
10^{12} 
 Examples:
 thousand = 1,000 10^{3} = 10 x 10
x 10 = 10^{1} x 10^{1} x 10^{1} = 10^{1+1+1}

 million = 1,000,000 10^{6} = 10^{3+3}
= 10^{3} x 10^{3} "a million" = "a thousand thousand"
Lengths Expressed as Powers of 10 (VERY round numbers):
atomic nucleus 
10^{16} m 
Hydrogen atom 
10^{10} m 
your height 
10^{0} m 
earth radius 
10^{7} m 
earthsun distance 
10^{11} m 
solar system 
10^{13} m 
nearest star 
10^{16} m
~ one light year (lyr)

sun to galactic center 
10^{20} m 
nearest galaxy 
10^{22} m
10^{6} lyr, or 1,000,000 lyr, or "million" lyr

Scientific Notation is just a way of expressing numbers in two parts:
 A power of 10 (gives the approximate scale, or "Order
of Magnitude"), multiplied by...
 A number somewhere between 1 and 10 (gives the
precise value)
 For Example: 15 = 1.5 x 10 = 1.5 x 10^{1}
100 = 1.0 x 10^{2} 1500 = 1.5 x 10^{3} 39466.98 = 3.946698
x 10^{4} 1 = 1.0 x 10^{0} 0.1 = 1. x 10^{1} 0.035
= 3.5 x 10^{2}
Can you see that the power of ten tells you how many digits
the decimal place should be moved over?
 Examples of some astronomical scales in scientific
notation: 1 AU = 150 million km = 150 x 10^{6} km = 1.5
x 10^{8} km 1 yr = 3.15 x 10^{7} s 1 l.y. = (3 x 10^{5}
km/s) x 3.15 x 10^{7} s = 9.45 x 10^{12} km (~ 1.0 x 10^{13}
km) 1 Gyr = 10^{9} yr = 3.15 x 10^{16} s Your Age (roughly)
= 20 yr = (20) x (3.15 x 10^{7} s) = 6.3 x 10^{8} s Age
of Universe = 20 Gyr = 20 x 10^{9} yr x (3.15 x 10^{7} s/yr)
= 6.3 x 10^{17} s
3. The Speed of Light and Units
speed = distance / time = meters / seconds = m / s
c = speed of light = 3 x 10^{8} m/s
 c is constant finite fastest
speed there is
Consequences:
It takes time for light to travel over
distance
time = distance / speed
Light travel times:
Distance 
Light Travel Times 
earth to sun 
1.5x10^{11} m 
5x10^{2}s 
8.3 minutes 
nearest star 
~10^{16} m 
3x10^{7}s 
1 year 
nearest galaxy 
~10^{22} m 
3x10^{13}s 
1 million, or 10^{6} years 
"Event Horizon" 
2x10^{26} m 
6x10^{17}s 
20 billion, 2x10^{10} years, or 20 Gyr 
Lectures

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Astro 100 
