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Name:__SOLUTIONS______ _________ Section:___ NatSci102 In-Class Exercise
This tutorial will give you a better understanding of the size of the Milky Way Galaxy by
investigating the distances to objects within the Galaxy and to other objects in the Universe.
Below is a picture of a spiral galaxy. This is a picture of NGC 3184, a spiral galaxy similar to the
Milky Way. Because we are located within the Milky Way, we are unable to take a picture of our
entire galaxy from the outside. Let’s assume that this picture represents our Milky Way Galaxy
and has the dimensions labeled below. Note that in this picture, 1 centimeter (cm)
represents 10,000 light-years (ly); equivalently you can use 1 millimeter (mm) to
represent 1,000 light-years (ly).
50,000 ly
center to edge
Sun
A
B
C
E
D
100,000 ly
edge to edge
1) The Sun’s position in the Milky Way is shown in the picture above. What is the
approximate distance from the Sun to the center of the Milky Way? Recall that 1.0 cm
represents 10,000 ly.
The distance is about 2.5 cm or 2.5cm x 10,000 light years/cm = 25,000 light years
2) The table below lists four bright stars in the night sky. Write the letter of the dot (A – E)
from the picture above that best represents the location of each star. You can use
letters more than once. Recall that 1 mm represents 1,000 ly.
Star
Vega
Spica
Rigel
Deneb
Distance from Sun
(in light years)
Letter
Explanation
A
A
A
A
Vega is 26ly/1000ly/mm = 0.026 mm from the Sun
Spica is 260ly/1000ly/mm = 0.26 mm from the Sun
Rigel is 810ly/1000ly/mm = 0.81 mm from the Sun
Deneb is 1400ly/1000ly/mm = 1.4 mm from the Sun
26
260
810
1,400
This illustrates how big the Milky Way is!
3) We normally consider Deneb to be a bright but distant star at 1,400 ly away. Compared
to the size of our galaxy, is Deneb truly distant? Explain your reasoning.
Deneb is not truly distant – dot A is closest to representing is location relative to the
Sun, and it is barely a separate dot from the Sun. Deneb is only 1.4% of the diameter
of the Milky Way distant from the Sun!
4) Are the stars from question 2 inside or outside the Milky Way Galaxy? Explain your
reasoning.
They are inside the Milky Way – they are so close to the Sun that they must be part of the
Milky Way since the Sun is.
5) The table below lists two Messier objects and their distances from the Sun. Write the
letter of the dot (A – E) from the picture on the previous page that best represents the
location of each object. You can use letters more than once.
Messier object
M1 (Crab
Nebula)
M71 Globular
Cluster
Distance from Sun
(in light years)
6300
12,700
Letter
B
C
Explanation
M1 is 6300ly/1000ly/mm=6.3mm
from the Sun
M71 is 12700 ly/ 1000ly/mm=
12.7mm from the Sun
6) Are these Messier objects part of the Milky Way? Explain your reasoning.
They are also part of the Milky Way because in any direction from the Sun the Milky Way
extends more than 6.3 or 12.7mm from the Sun.
7) The Crab Nebula has a width of about 11 light years. If you wanted to accurately draw
the Crab Nebula on your diagram, would you use a small blob or a tiny dot at the
location you indicated? Explain your reasoning.
Note: the dots marking the locations on the picture are about 1 mm across.
A dot should be used as 11 light years = .011 mm, much smaller even than the size of the
dots.
8) The Sun is much smaller than a nebula. We used a dot to represent the Sun’s location
in the picture. Is this dot too small, too large or just the right size to represent the size of
the Sun on the picture? Explain your reasoning.
At the scale of the picture, the Sun should be much, much smaller than a dot so a dot is
actually too large to represent the size of the Sun. In fact, the Sun’s diameter in light
years is 1.4x10-8 light years = 1.4x10-11 mm so you could not see it as disk at the scale of
the picture using even the best microscope known!
9) The Milky Way Galaxy is one of the largest galaxies in the Local Group. The following
table lists the distances to the centers of t Local Group galaxies. Draw a dot on your
picture (if possible) to represent the center of each galaxy. Don’t worry about the
direction (left, right, up, or down) for each galaxy; just place a dot an appropriate
distance from the Sun and label it with the galaxy’s name.
Galaxy
Distance from Sun
Distance in cm
(in light years)
80000ly/10000ly/cm = 8cm
Sagittarius Dwarf Elliptical Galaxy
80,000
(SagDEG) – closest galaxy to Milky Way
2500000ly/10000ly/cm = 250cm
Andromeda Galaxy (M31)
2,500,000
Do any of these galaxies fit on the page? Which one(s)?
The Sagittarius Dwarf would fit.