Download Star Counts Lab

Celestial Sphere
Name _______________________________ Pd ____
Lab : Star Counts
How many stars can you see? People say that some really big number is “more than the stars in the
sky,” but how many is that? In this lab you will make an estimate of how many stars you could see if you could
see the entire sky with your naked eye. It would not be possible to count all the stars visible in the sky during a
single lab meeting. For one reason, the number is very large; for another, only half the sky is visible at any
given time. So what you will do is to count the stars visible in a small area of the sky and then multiply your
count by the number of those areas it would take to cover the entire sky.
Knowing how many stars you can see will allow you to find the limiting magnitude visible to you. The
limiting magnitude is the magnitude of the dimmest stars visible. (Magnitude is the numerical value given to
the measure of a star’s brightness; you will hear more about this later in the course. Larger numbers mean
dimmer stars. From a very dark, clear site most people have a limiting magnitude of about 6.)
Here’s the basic procedure. You will use a toilet paper roll or paper towel roll of known length and
radius. You will look through this roll at several random patches of sky and count the number of stars you can
see. Then you will average those star counts to get an average number of stars visible through the roll. Then
you will multiply it by the number of those patches it would take to cover the sky. That will tell you the number
of stars visible to the naked eye. Finally, you will use these numbers to determine the limiting magnitude for
your naked eye.
For this to work one needs to know the number of patches of sky visible through your roll or telescope it
would take to cover the sky. Here’s how that is done. Imagine an area on the sky that is a square 1° by
1°. That area is one square degree or 1 sq deg. To cover the entire sky would take approximately 41,253 of
these squares, so the entire sky has an area of 41,253 sq deg. The view through your rolls has an area of 130 sq
deg, so the number of patches of roll it would take to cover the sky is 41,253/130 or 316. Call this number Mr,
to stand for the multiplying number for the roll. Multiplying Mr (or 316) by the average number of stars you
see through the roll will give you the number of stars visible to the naked eye
Materials
paper towel roll or toilet paper roll, calculator
Procedure
1. Read through this entire write up carefully before coming to lab.
2. Hold the roll against your eye and view a region of the sky over halfway up from the horizon.
Pick your region randomly; don’t attempt to aim it at a particular spot.
If there are no stars visible, do not shift to another region. Simply record 0 for the number of stars
visible.
3. Make a total of 8 star counts through the viewing roll
4. Record each of these star counts. (You’ll do the calculations later.)
5. Calculate the number of stars visible to the naked eye by:
Averaging your 8 star counts. (Include any decimal places in your average.)
Multiplying this average by Mr to get the number of stars visible to the naked eye.
Celestial Sphere
Name _______________________________ Pd ____
Questions
1. What are some things that affect the number of stars you can see in the sky?
2. Use the chart below to find the limiting magnitude of your eyes and the telescope.