GS 107 Star Count Project
NAME ________________________
DUE _________
How many stars are visible at night? The answer varies with the darkness of the night
sky; under ideal conditions up to about 3,000 stars are visible to the naked eye from a
single location. In this project you will determine the number of visible stars by first
constructing a simple observing instrument (a tube), then you will use your tube to survey
a small fraction of the sky and finally you will use the results of your sky survey to
calculate the total number of visible stars.
BASIC IDEA: Looking through a tube you will examine a small section of the night sky
and count the number of stars visible within the tube’s field of view. You will repeat this
procedure several times to determine the average number of stars visible in the tube’s
field of view. Then by comparing your tube’s field of view to the size of the entire sky
you will calculate the total number of visible stars. (e.g. If your tube images 1/100 of the
night sky you would multiply the average number of stars visible in your tube by 100 to
determine the total number of stars visible in the sky.)
Finding/Creating Your Tube
Any tube will do; any thing from paper towel rolls or toilet paper rolls to lead pipes to
drinking straws will work fine. If you want to create your own tube just take a sheet of
paper, roll it into a tube and tape it so it won’t unroll, viola your own tube!
Making Your Observations
If possible make your observations at a dark site away from city lights. If you have to
make your observation from a bright city site no worries, we will be comparing results
from dark and not so bright sites. Choose a site with an unobstructed (as possible) view
of the sky and as far away from lights as you can get. Choose a night with few clouds and
a small moon. When you get to your site give your eyes 10 to 20 minutes to adapt top the
darkness before making your observations. I suggest working with a partner who records
the data for you; this will make this project easier and more fun. If you decide to work
with a partner or partners from this class make sure that each student makes their own set
of observations. Choose an observing partner you like and bundle up for the cold nights.
After arriving at your site the first thing to do is find the big dipper and use the pointer
stars to locate Polaris which will indicate to you which direction is north. Then use the
little dipper and the star chart on the last page of this lab to determine the limiting
magnitude which is the dimmest magnitude visible to the naked eye near the zenith. For
your observations examine ten random locations of the sky with your tube. For each sky
location you will record its altitude and azimuth and count the number of stars visible
through your tube. Try to remain as still as possible while counting your stars!
Observations
Observing Location:
Description of Observing Location:
Weather Conditions:
Cloud Cover (to nearest 1/8th) ____________________
Moon: below horizon above horizon
size if above horizon: _____________
Number of stars of little dipper visible:
Limiting Magnitude:
Observation
1
Altitude
Azimuth
Number of Stars
2
3
4
5
6
7
8
9
10
TOTAL
Calculate the average number of stars visible in your tube:
average number of stars =
average number of stars =
__total number of stars _
number of observations
Calculating the Total Number of Visible Stars
One of the tricks of the science trade is to use more than one method to solve a problem.
If all the different methods give the same answer this increases the scientists’ confidence
in their answer. If the different methods give different results then the scientists know
they have more work to do! In this lab you will use two different methods to calculate the
total number of visible stars. Your results from these methods should agree, if they don’t
go back and check your work because you have more work to do!
Method One
In this approach you will calculate the size of the angular field of view of your tube and
then compare this to the total angular size of the sky.
1) Calculate the angular field of view of your tube:
Length of tube (give units!) _______________
Width of tube (give units!) _______________
Angular field of view in degrees =
width x
length
57.3
Angular field of view in degrees =
Review Question:
Which will have a smaller angular field of view?
a short fat tube
a long skinny tube
2) Calculate the number of square degrees in the field of view of your tube:
Your tube has a circular field of view, so we will have to determine the area of this circle.
The area of a circle is equal to pi x radius2 where pi is equal to 3.14.
The radius of your tube is one half the diameter of your tube. For this calculation you
want to measure the radius of your tube in degrees.
Area of field of view in degrees =
Area of field of view in degrees =
3.14 x (radius)2 = 3.14 x radius x radius
3) Determine the fraction of the night sky sampled by your tube:
The night sky can be thought of as a hemisphere (half a sphere). The total surface area of
a hemisphere is 2,0627 square degrees. Your tube images only a small fraction of this
hemisphere.
Fraction of hemisphere sampled = __tube’s area of view in square degrees__
total area of hemisphere in square degrees
Fraction of hemisphere sampled =
4) Calculate the total number of stars:
Now that you know the fraction of the sky sampled by your tube you need to only divide
the average number of stars visible in your tube by the fraction of the sky visible through
your tube to determine the total number of visible stars.
Total number of visible stars = average number of stars visible through tube
fraction of sky visible through tube
Total number of visible stars =
Method Two
Examine the sketch above. To left is a sketch of a tube of length L and radius r. Think of
the tube as the radius of a hemisphere, this is illustrated in the sketch to the right. It is
possible to calculate the surface area of this hemisphere from the length of the tube. The
area of the hemisphere viewed by the tube can also be calculated knowing its radius.
Knowing the average number of stars visible in you tube and the fraction of the
hemisphere viewed through the tube you can calculate the total number of stars in the
hemisphere.
1) Calculate the surface area of the hemisphere:
Surface area of hemisphere = 2 x pi x (length)2 = 2 x 3.14 x length x length
Surface area of hemisphere =
2) Calculate the area viewed through the tube:
Area viewed through tube =
pi x (radius)2 = 3.14 x radius x radius
Area viewed through tube =
3) Determine the fraction of the hemisphere viewed through the tube:
Fraction of hemisphere viewed = area of viewed through tube
area of hemisphere
Fraction of hemisphere viewed =
4) Calculate the total number of stars:
Now that you know the fraction of the hemisphere viewed by your tube you need to only
divide the average number of stars visible in the tube by the fraction of the hemisphere
viewed through the tube to determine the total number of visible stars.
Total number of visible stars = average number of stars visible through tube
fraction of sky visible through tube
Total number of visible stars =
 The result from these two different methods should agree, if they don’t go back
and check your work!
Source:
http://www.phys-astro.sonoma.edu/observatory/observers/limiting_magnitude.html