Download A Sense of Where We Are

A Sense of Where We Are
Outline
1. Sizes of Some Astronomical Things
A.
B.
C.
D.
E.
F.
G.
H.
I.
J.
K.
L.
Earth Diameter: 7900 mi
Earth-Moon Distance: 240,000 mi
Solar Diameter: 865,000 mi
Jupiter Diameter: 89,000 mi; Pluto diameter: 1440 mi
Earth’s Orbit Radius: 93 million miles = 150 million km  1 AU
Pluto’s Orbit Diameter: 100 AU
Oort Cloud Radius: 100,000 AU
Distance to Proxima Centauri: 4.2 light years
Milky Way Galaxy Diameter: 100,000 ly
Local Group Diameter: 3 million ly
Local Supercluster
The Cosmic Address: Earth, Solar System, etc . . .
2. Models
A. Modeler’s Equation
Real Size/Dista nce of Object 1
Model Size/Dista nce of Object 1

Real Size/Dista nce of Object 2
Model Size/Dista nce of Object 2
B.
C.
D.
E.
Earth/Moon Model
Earth/Sun Model
Sun/Pluto Model
Sun/Proxima Centauri Model
3. Events in the Life of the Universe
A.
B.
C.
D.
E.
Big Bang
Galaxy Formation
Star Formation
Planet Formation
Appearance of Life
4. Carl Sagan’s Cosmic Calendar
Review Questions
1. Which is larger in size: the Solar Neighborhood or the Local Group?
2. What is the astronomical unit (AU)? How is it related to the light year? (i.e., much larger, much
smaller, about the same) How do you know?
3. Match a description on the left with a distance/size on the right:
____ possible distance of a planet orbiting the sun
a) 10 light years
____ distance to stars nearest to the sun
b) 10 AU
____ distance to galaxies nearest to the Milky Way
c) 10 billion light years
____ distance to very distant galaxies
d) 1 million miles
____ diameter of the sun
e) 100,000 light years
____ diameter of the galaxy we inhabit
f) 1 million light years
4. Where is the true boundary of the solar system?
5. You board a spaceship and zoom off to a point 10,000 AU from the Sun, in a direction toward
Proxima Centauri. Relative to these two stars, where are you?
a) Much closer to the Sun than to Prox Cen.
b) Much closer to Prox Cen than to the Sun.
c) About midway between the Sun and Prox Cen.
6. Ques. #2, pg. 25.
7. Ques. #5, pg. 25
8. What is the ratio of the diameter of Jupiter to the diameter of Earth? (See pg A-11 in your text for
planetary data). Using this ratio, find the diameter of a model. Earth if Jupiter is modeled as a large
beach ball (diameter = 2.0 ft).
9. Given the mean distance between the sun and Jupiter (pg. A-11), what is the scale distance between
the sun and the beach ball Jupiter?
10. Using a smaller beach ball to represent Jupiter would have what effect on your answer to #9?
11. Ques. #10, pg. 26.
Answers
1. The Local Group is (much) larger. Local group is the small cluster of nearby galaxies which
includes the Milky Way as a member. It’s about 3 million light years across. The Solar Neighborhood
is the collection of stars near the Sun, comprising a patch of space ~15 light years across.
2. The Astronomical Unit (AU) is the average distance between Earth and the Sun., and is much
smaller than the light year. How do you know? Well, you could look it up! Or, remember the model
we discussed in class: When the Sun is reduced to a nerf ball 10 cm across, Pluto, at 40 AU, lies about
0.25 mile from the ball, and the next nearest star (at 4 light years from the Sun) is found about 1800
miles from the ball. So, in the model, an AU is evidently scaled to 0.25/40 mi = 0.0063 mi, and a light
year is evidently scaled to 1800/4 = 450 miles. Now, 450 miles >>> 0.0063 mi, so in reality a light
year is evidently much bigger than an AU!
3. Match a description on the left with a distance/size on the right:
b) possible distance of a planet orbiting the sun
a) 10 light years
a) distance to stars nearest to the sun
b) 10 AU
f) distance to galaxies nearest to the Milky Way
c) 10 billion light years
c) distance to very distant galaxies
d) 1 million miles
d) diameter of the sun
e) 100,000 light years
e) diameter of the galaxy we inhabit
f) 1 million light years
4. In the view of your instructor, the outer edge of the solar system is coincident with the outer edge of
the Oort comet cloud – about 100,000 AU from the Sun. Oort cloud comets are the most distant
objects orbiting the Sun, so they must belong to the solar system.
5. a) Much closer to the Sun than to Prox Cen: Prox Cen lies about 267,000 AU from the Sun.
10,000 AU << 267,000 AU, so you’ve got to be much less than ½ the distance from the Sun to Prox
Cen.
6. Earth, through Solar System, to the stars, Milky Way Galaxy, other galaxies, Local Group, other
galaxy clusters, Local Supercluster, and finally to the Universe as a whole, in which galaxies and
galaxy clusters appear to be arrayed along filaments.
7. Light carries information about the Universe. Light travels at a finite speed, so information about
distant objects arrives at Earth only after a lapse of time. Thus, we see objects out there only as they
were in the past, not as they are now. Light can have traveled from distant objects for a time interval
no greater than the age of the Universe, which we believe to be about 13 billion years. So, the most
distant objects we can see (at the edge of the observable Universe) must lie no more than about 13
billion light years from Earth.
8. You need to solve the second equation below to solve this problem:
Diameter of Real Jupiter
Diameter of Model Jupiter

Diameter of Real Earth
Diameter of Model Earth
143,000 km
2.0 ft

12,800 km
X
In this case:
 12,800 km 
X  
  2.0 ft  0.18 ft  2.2 inches
 143,000 km 
9. You need to solve the second equation below to solve this problem:
Real AU
Model AU

Diameter of Real Jupiter
Model Jupiter Diameter
1.50  10 8 km
X

143,000 km
2.0 ft
Solve for X: X = 2100 ft. So, if Jupiter is reduced to a 2.0-ft beach ball, then the AU is
reduced to 2100 ft. Now, Jupiter lies 5.2 AU from the Sun, so in the model the distance
between the Sun and Jupiter is:
5.2 AU  2100 ft/AU  11,000 ft  2.1 miles (!)
10. A small Jupiter would of course result in a smaller AU. For example, a 1-ft Jupiter in the model
would result in a model AU about 1 mile in length. (The sizes decline proportionately.)
11. Well, the lifetime of the human race on Earth is pretty small compared to the 4.5 billion-year
history of the Earth (and Solar System). Each day on the Cosmic Calendar represents 4.5 billion
years/365 days = 12 million years (approximately). Human ancestors arose only a few million years
ago (according to the fossil record), so these creatures have existed for less than a day on the Calendar!