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Milky Way Activity: REVISED VERSION
Page MW- 9
NAME_______________
NAME__________________
__________________
NAME_______________
NAME__________________
__________________
Part D: Data Reduction

Verify and correct your data in the master table.

A copy of the corrected master table will be distributed to each group
1. Inspect Data from the Entire Class:
(a) Compare between Telescopes: For the same stars, do groups using the 12 inch telescope see
significantly fewer stars than people using the 14 inch? [Note: the limiting magnitude of the 12 inch
is about 0.3 magnitudes less than the 14 inch, and the field of view was somewhat smaller because
of the smaller eyepiece].
(b) Compare Finder to Main Scope: On average, were the star counts bigger or smaller for the
finderscope? Can you provide an explanation for this result? [Hint, it has less aperture, but lower
magnification power].
(c) Compare Your Data to Others: There should be at least one or two stars that you observed that
were seen by others. How do your counts compare? Whose data do you believe is more accurate
(and why)?
Practical Astronomy
Sept2006, W. Pezzaglia
Fall 2006
Feb/06/2016
Milky Way Activity: REVISED VERSION
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2. Bin Data [Data Sheet #4]

Insert Galactic Latitude and Longitude (to nearest degree) for every target star.

Put the data (the counts in the main scope, not the finderscopes) into the “bins” in data sheet
#4. Note, take the absolute values for negative latitudes. Each cell will have several values.

Compute Averages for each column and row.

Using the bottom row (the averages by latitude), make a plot of the average star count as a
function of galactic latitude.
(a) Distribution in Longitude: Along the Milky Way (low latitude) are the star counts similar,
or do they vary with longitude? [If they vary significantly, does it have anything to do with
the “thickness” of the Milky Way at that longitude, or presence of dust lanes?]
(b). Distribution in Latitude: Does your graph show any overall trend (i.e. does it resemble at
all figure 8?). In particular, do you see any evidence of the “critical angle”? Is it roughly
constant at higher latitudes, or does it drop off smoothly?
(c). Are the star counts near minimum at the galactic poles, and maximum in the Milky Way?
Practical Astronomy
Sept2006, W. Pezzaglia
Fall 2006
Feb/06/2016
Milky Way Activity: REVISED VERSION
Page MW- 11
3. Space Penetrating Power
(a) Limiting Magnitude of the Sky (see question B.1)
ms = _________
m = (6.5-ms) =_________
(b) Magnitudes Loss due to sky brightness:
(c) Theoretical Limiting Magnitudes: This is a function of the aperture of the scope. Insert these in
the table below. [Hint: use supplied table, or calculate: m=7.5 + 5 Log(Aperture in cm) ]
(d) Visual Limiting Magnitude: Diminish the Theoretical Limiting Magnitude by the amount loss due
to the sky brightness. Insert these into the table below. [Hint: Visual = Theoretical - m ]
(e) Space Penetration Power: Insert in table the “depth” you can see into the galaxy. Use supplied
table/graph (it assumes stars have absolute magnitude of M=+2.5). There are 2 values:
 No Absorption: [formula is: m=2.5 + 5 Log(D/10) ]
 With Absorption due to dust in galaxy (approximately 1 magnitude/1000 parsecs)
Item
Aperture (mm): A=
Theoretical: m=
Eye
Finderscope
6.4
50
Telescope
6
Limiting
Magnitude
Visual (Actual): m=
Space
Penetrating
Power
(parsecs)
No Absorption: D=
With Absorption: D=
4. Summary: Compare the “space penetrating power” of the finderscope to the telescope.
Approximately how much further does the telescope see? (i.e. what factor increase). How
much further are you seeing in the telescope than by naked eye?]
5. Summary: How much change in the space penetrating power is there when absorption of
light due to the dust in the Milky Way is included?
Practical Astronomy
Sept2006, W. Pezzaglia
Fall 2006
Feb/06/2016
Milky Way Activity: REVISED VERSION
Page MW- 12
6. Thickness of Milky Way
(a) Average Star Count near Galactic Pole:
Np = _____________
(b) Average Star Count near Galactic Equator (Milky Way):
N0 = _____________
(c) Ratio
R= Np /N0 = _____________
R1/3=_____________
(d) Take Cube Root
(e) Multiply by twice the space penetrating power
T = 2D[Np /N0 ]1/3 = _________ Parsecs
of telescope to get galactic thickness
(f) Compare your measurement of the thickness of the Milky Way with the generally
accepted value of around 500 parsecs. Are you close? If not, can you think of what may
have gone wrong? [Note “close” in Galactic Astronomy is a factor of 3]
7. Star Densities in the Milky Way
(a) Eyepiece Used (focal length in mm):
fe = ___________mm
(b) True field (degrees) [44 for 40 mm, 52 for 26 mm]:
 = _______________
(c) Focal Length of telescope (in mm) [see Line B.2c]
f0 = ___________mm
f0/ fe =_____________ 
(d) Magnification Power
 = _______________
(e) Apparent Field in degrees [Divide (b) by (d) ]
(f) Compute Volume in cubic parsecs: V = (/3)D3[tan(/2)]2=______________ parsecs3
(g) Calculate Density of Stars [stars per cubic parsec]
[hint: divide line 4b above by line 5f]
(h) Calculate distance between stars:
= N0/V______________ stars/parsecs3
d = (1/)1/3= ______________ parsecs
(i) Summary: we expect approximately 1 to 5 parsecs between stars. Is your measurement
close to this?
Practical Astronomy
Sept2006, W. Pezzaglia
Fall 2006
Feb/06/2016
Milky Way Activity: REVISED VERSION
Page MW- 13
TABLES
Limiting Magnitude
Item
Aperture
inch
Eye
Finderscope
0.25
2
6
10
12
14
mm
Light
Gathering
Power LGP
6.4
50
152
254
305
356
1
61
567
1575
2268
3087
Limiting
Magnitude
6.5
11.0
13.4
14.5
14.9
15.3
 Light Gathering Power: LGP = [(Aperture in mm)/6.4]2
 Limiting Magnitude:
 Or
Practical Astronomy
m = 2.5 Log(LGP)+6.53
m= 7.5 + 5 Log(Aperture in cm)
Sept2006, W. Pezzaglia
Fall 2006